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MOLCAS/OpenMolcas is an ab initio electronic structure program providing a large set of computational methods from Hartree-Fock and density functional theory to various implementations of multiconfigurational theory. This article provides a comprehensive overview of the main features of the code, specifically reviewing the use of the code in previously reported chemical applications as well as more recent applications including the calculation of magnetic properties from optimized density matrix renormalization group wave functions. ispartof: JOURNAL OF CHEMICAL PHYSICS vol:152 issue:21 ispartof: location:United States status: published

It has always been a spectroscopist's dream to correlate a material's luminescence properties with its microscopic structure, based on reliable structure-property relationships. Electronic structure methods are promising to achieve this goal; yet they are especially challenging in the case of Eu-based materials which are known to feature exceptionally high density of excited states, large spins and severe electron correlation. In this work, state-of-the-art multiconfigurational ab initio embedded-cluster methods are applied to gain a deeper insight into the luminescence mechanisms of Eu2+ and Eu3+-doped phosphors. Regardless of the difficulties, very accurate excitation energies are achieved, reaching 68% prediction intervals of 300 cm(-1), corresponding to an accuracy of 5-10 nm in the visible wavelength range. Complete configurational coordinate curves are obtained, yielding breathing mode vibrational frequencies and equilibrium bond lengths for all excited states. Moreover, electric dipole transition moments and oscillator strengths are used to calculate absorption spectra. Excellent agreement with experiment is found. The ab initio calculations give an unprecedented detailed view of the Eu2+ excited state landscape, allowing for an improved understanding of its structure, including the origin of the so-called 'staircase structure' and the role of ligand covalency in the ligand field and exchange splitting. It is found that more covalent host compounds feature higher exchange splittings due to an increased stabilization of high-spin states by the interaction with virtual LMCT states. It is verified that the equilibrium Eu-ligand bond length contracts upon 4f-5d excitation towards the lowest 5d submanifold and that the bond lengths are directly related to the configurational character of the electronic eigenstate. Moreover, comparing ab initio calculations with crystal field calculations proves that a decoupled model for the Eu2+ excited states is inadequate, making the use of an intermediate coupling scheme compulsory. With this approach, computational design of luminescent materials is getting within reach.

Electron transfer processes between lanthanide activators are crucial for the functional behavior and performance of luminescent materials. Here, a multiconfigurational ab initio study reveals how direct metal-to-metal charge transfer (MMCT) between the Eu2+ luminescence activator and a Ln(3+) co-dopant (Ln(3+) = Ce3+, Pr3+, Nd3+, Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, and Yb3+) systematically dictates the luminescence and optical properties of CaF2. The combination of the structures and energies of the electronic manifolds, the vibrational force constants, and the structural properties of the donor and acceptor in the host determines the predictions of five different behaviors of CaF2:Eu2+, Ln(3+) co-doped materials after MMCT absorption: formation of stable traps, MMCT emission, emission quenching, Ln(3+) emission, and Eu2+ emission.

Wave function theory ab initio embedded cluster calculations show that Ce3+-doped transformed aluminum garnets resulting from Y3Al5O12:Ce3+ after cationic substitutions that enhance the tetragonal field around Ce3+, have their lowest 4f–5d transition red-shifted with respect to the yellow phosphor YAG:Ce3+ used in GaN blue-LED based lighting devices. Red shifts in the range of 1000–3000 cm−1 are found in 33 transformed garnets of the types MIV2MIIIMII2Al3O12:Ce3+, MIV2MIAl5O12:Ce3+, and MIVMIIIMIIAl5O12:Ce3+. This result shapes the design of new red phosphors by enhancing the tetragonal field around Ce3+ via cationic substitutions made in Ce3+-doped aluminum garnets.

An ab initio model potential embedded cluster study on the 4 f 8 and 4 f 7 5d manifolds of the long range compensated, cubic, substitutional Tb3+ defects of Tb-doped BaF2 is presented here. 4f–4f and 4f–5d transition energies and 4f–5d absorption electric dipole oscillator strengths for the dipole allowed transitions are reported and comparisons with emission and excitation experimental spectra are presented. We find good agreement with the energies of the experimental 4f–4f green and blue emissions. The characteristic features of the 4f–5d excitation spectrum (spin allowed vs. spin forbidden transitions) are well reproduced by the calculations. The transition energies from the ground state to the different spin nonet and spin septet levels of the 4 f 7 5d configuration of this ion in the center of the lanthanide series show an error of around 10%, which is in line with previous calculations on other lanthanide ions with lower spin multiplicities and simpler manifolds at the beginning and at the end of the series. The calculation supports a recent assignment of experimental excitation bands between 54,000 and 66,000 cm−1 as 4 f → 5 d ( t 2 g ) transitions. On the other hand, a new assignment is made of the highest band in the excitation spectrum at around 76,000 cm−1: it corresponds to transitions to levels of the 4 f 7 5d configuration with the 4 f 7 subshell in the excited 6P term, rather than to a Tb-bound exciton. Preliminary studies on excitonic states of Tb-doped BaF2 indicate that energy levels that can be characterized as Tb-trapped excitons are found in the same spectral region as the spin forbidden 4 f → 5 d ( t 2 g ) transitions at ground state Tb–F distances, and their energy lowers very much at shorter Tb–F distances.

The effects of the first coordination shell geometry of the trivalent Cerium ion (Ce 3+ ) on its 4 f and 5 d levels in Ce-doped oxides with a D 2 8-fold site, like garnets, are studied with embedded cluster, wave function based ab initio methods. The only deformations of a D 2 CeO 8 moiety that are found to shift the lowest 4 f → 5 d transition to the red (longer wavelengths) are the symmetric Ce–O bond compression and the tetragonal symmetric bond bending. In a first approximation, the lowest 5 d level of Ce 3+ in garnets can be understood as resulting from the cubic E g level with a strong E g × e g Jahn–Teller coupling. These results are analyzed in terms of 5 d − 4 f centroid energy differences and ligand field stabilizations. The splittings of the upper 5 d levels and of the 4 f levels are also discussed.

The optical activity derived from the rich manifolds of excited states of lanthanide-containing materials gives them a key role in the development of a large variety of solid-state devices with high societal demand. The intense experimental research in this complex area finds its theoretical grounds mostly in empirical models, but the consideration of ab initio methods as fruitful theoretical tools to handle such a complexity is steadily growing. These methods do not rely on empirical parameters and are based only on the first principles of quantum mechanics. This chapter is dedicated to give a perspective on their use as basic theoretical tools in the area, which must complement and go together with experiments and empirical models.

In this article, we present a fragment model potential approach for the description of the crystalline environment as an extension of the use of embedding ab initio model potentials (AIMPs). The biggest limitation of the embedding AIMP method is the spherical nature of its model potentials. This poses problems as soon as the method is applied to crystals containing strongly covalently bonded structures with highly nonspherical electron densities. The newly proposed method addresses this problem by keeping the full electron density as its model potential, thus allowing one to group sets of covalently bonded atoms into fragments. The implementation in the MOLCAS 7.0 quantum chemistry package of the new method, which we call the embedding fragment ab inito model potential method (embedding FAIMP), is reported here, together with results of CASSCF/CASPT2 calculations. The developed methodology is applied for two test problems: (i) the investigation of the lowest ligand field states 2A1 and 2B1 of the Cr(V) defect in the YVO4 crystal and (ii) the investigation of the lowest ligand field and ligand–metal charge transfer (LMCT) states at the Mn(II) substitutional impurity doped into CaCO3. Comparison with similar calculations involving AIMPs for all environmental atoms, including those from covalently bounded units, shows that the FAIMP treatment of the YVO4 units surrounding the CrO43− cluster increases the excitation energy 2B1 → 2A1 by ca. 1000 cm−1 at the CASSCF level of calculation. In the case of the Mn(CO3)610− cluster, the FAIMP treatment of the CO32− units of the environment give smaller corrections, of ca. 100 cm−1, for the ligand-field excitation energies, which is explained by the larger ligands of this cluster. However, the correction for the energy of the lowest LMCT transition is found to be ca. 600 cm−1 for the CASSCF and ca. 1300 cm−1 for the CASPT2 calculation. ispartof: Journal of Chemical Theory and Computation vol:4 issue:4 pages:586-594 ispartof: location:United States status: published

The 4 f ↔ 5 d absorption and emission spectra of Ce 3 + -doped YAG ( Y 3 Al 5 O 12 ) are simulated with a quantum chemical ab initio embedded cluster approach applied to Ce 3 + substitutional defects of D 2 local symmetry. The only empirical information used is the structure of the pure host. The simulated absorption spectrum is calculated with overestimations of 2300– 3300 cm - 1 , which became 4600 cm - 1 in the luminescence. The three 4 f → 5 d observed absorptions were assigned and the energies of the two remaining ones, which are hidden by the absorption of the pure host, were predicted. The lowest 5 d → 4 f emission was found to take place from Γ 5 ( 2 2 A ) and the next emission from Γ 5 ( 3 2 B 3 ). Good absorption and emission band shapes and relative intensities are obtained. A large underestimation of the Stokes shift is found, which suggests an underestimation of the relaxation on the emitting state.

The energy levels of the 5f(3) manifold of the free-ion U3+ Up to 30 000 cm(-1) are calculated with two well stabilized ab initio methods based on the two-component Hamiltonians of Cowan-Griffin-Wood-Boring and Douglas-Kroll-Hess. Although the corresponding experimental data do not exist, estimates made out of experiments on the 50 manifold of U3+ in solids are usually referred to as the experimental energies of U3+ in gas phase. On the basis of a discussion on the available data of f-element ions in gas phase and in solids and a discussion on the expected precision of the present calculations, we suggest that the 50 energy levels of free U3+ are higher than the estimates and lower than the present ab initio calculations.

Dopant-to-host electron transfer is calculated using ab initio wavefunction-based embedded cluster methods for Yb/Ca pairs in CaF2 and Yb/Sr pairs in SrF2 crystals to investigate the mechanism of photoconductivity. The results show that, in these crystals, dopant-to-host electron transfer is a two-photon process mediated by the 4fN-15d excited states of Y b2+: these are reached by the first photon excitation; then, they absorb the second photon, which provokes the Y b2+ + Ca2+ (Sr2+) → Y b3+ + Ca+ (Sr+) electron phototransfer. This mechanism applies to all the observed Y b2+ 4f-5d absorption bands with the exception of the first one: Electron transfer cannot occur at the first band wavelengths in CaF2:Y b2+ because the Y b3+-Ca+ states are not reached by the two-photon absorption. In contrast, Yb-to-host electron transfer is possible in SrF2:Y b2+ at the wavelengths of the first 4f-5d absorption band, but the mechanism is different from that described above: first, the two-photon excitation process occurs within the Y b2+ active center, then, non-radiative Yb-to-Sr electron transfer can occur. All of these features allow to interpret consistently available photoconductivity experiments in these materials, including the modulation of the photoconductivity by the absorption spectrum, the differences in photoconductivity thresholds observed in both hosts, and the peculiar photosensitivity observed in the SrF2 host, associated with the lowest 4f-5d band, This work was partly supported by a grant from Ministerio de Economía y Competitividad, Spain (Dirección General de Investigación y Gestión del Plan Nacional de I+D+i, Grant Nos. MAT2011-24586 and MAT2014-54395-P)

The role of structural relaxations on the energy of the lowest $$4f{-}5d$$ transition of $$\hbox {Ce}^{3+}$$ in garnets is studied by means of ab initio calculations. This study completes previous studies on the roles of the interactions of the Cerium impurity with its first neighbors and with the rest of the solid hosts, before the relaxations take place. Periodic boundary conditions density functional theory calculations (DFT) and second-order perturbation theory spin–orbit coupling embedded-cluster wave function theory calculations (WFT) have been performed in the garnets $$\hbox {Y}_{3}\hbox {Al}_{5}\hbox {O}_{12}$$ , $$\hbox {Lu}_{3}\hbox {Al}_{5}\hbox {O}_{12}$$ , $$\hbox {Y}_{3}\hbox {Ga}_{5}\hbox {O}_{12}$$ , $$\hbox {Lu}_{3}\hbox {Ga}_{5}\hbox {O}_{12}$$ , and $$\hbox {Ca}_{3}\hbox {Sc}_{2}\hbox {Si}_{3}\hbox {O}_{12}$$ doped with $$\hbox {Ce}^{3+}$$ . The local relaxation effects on the $$4f{-}5d$$ transition are similar in the WFT and DFT calculations. They produce a blue shift in Al and Ga garnets in which Ce substitutes for smaller Y and Lu cations, which is found to be basically due to the local expansions around the impurity, with only minor contributions from angular relaxations. Atomic relaxations of more distant neighbors enhance the blue shift. Although the embedding effects of the undistorted garnets are known to make the differences between the $$4f{-}5d$$ transition in Al and Ga garnets, we find that the structural relaxations are responsible for the small differences between the $$4f{-}5d$$ transition in $$\hbox {Y}_{3}\hbox {Al}_{5}\hbox {O}_{12}$$ : $$\hbox {Ce}^{3+}$$ and $$\hbox {Lu}_{3}\hbox {Al}_{5}\hbox {O}_{12}$$ : $$\hbox {Ce}^{3+}$$ , and in $$\hbox {Y}_{3}\hbox {Ga}_{5}\hbox {O}_{12}$$ : $$\hbox {Ce}^{3+}$$ and $$\hbox {Lu}_{3}\hbox {Ga}_{5}\hbox{O}_{12}$$ : $$\hbox {Ce}^{3+}$$ .

Ligand and solvent effects on the bond length shift experienced by complexes of lanthanide ions upon 4 f → 5 d excitation, addressed by means of ab initio embedded cluster calculations, are presented on the clusters ( CeF 6 ) 3 - , ( CeCl 6 ) 3 - , and ( CeBr 6 ) 3 - in solid elpasolites, in liquid acetonitrile and in vacuo. Previous predictions of bond length shortening upon 4 f → 5 d ( t 2 g ) excitation seem to be general and, in particular, chloride and bromide compounds in liquid solution are predicted to be good candidates for excited-state EXAFS measurements of the distortion signs. A quantitative analysis of contributions to the bond length shifts is presented, which shows the importance of ligand field effects and points out insufficiencies in the Judd–Morrison model proposed to account for 4 f → 5 d transitions in crystals.

In this paper we present the results of spin–orbit relativistic ab initio model potential embedded cluster calculations of the 5f26d1 excited manifold of (UCl6)3− embedded in a reliable representation of the Cs2NaYCl6 elpasolite host. They are aimed at interpreting the 5f3→5f26d1 absorption bands reported by Karbowiak et al. [J. Chem. Phys. 108, 10181 (1998).] An excellent agreement is found between the calculated energies of the absorption transitions from the ground state 5f3 1 Γ8u(4I9/2) and the experimental data, which supports a detailed interpretation of the electronic nature of the absorption spectrum in the energy region 14 000–23 000 cm−1. In particular, the three unidentified electronic origins that had been experimentally detected are now assigned, and the observed bands are interpreted as having multiple electronic origins. From the structural point of view, the excited states of the 5f26d1 manifold are classified in two sets of main configuration 5f26d(t2g)1 and 5f26d(eg)1 with bond distances...

An analysis is made of the transferability of frozen core potentials of neutral lanthanide and actinide elements to 4f/5f and 5d/6d states of their respective 3+ and 4+ ions. A good description of the orbital spin—orbit coupling constants ζ4f/ζ5f and ζ5d/ζ6d of the Ln3+/An4+ ions and of the 4f → 5d/5f ζ 6d transition energies is achieved by using [Kr]/[Xe, 4f] cores. The corresponding relativistic core ab initio model potentials (AIMPs) based on Cowan—Griffin atomic calculations, together with optimized Gaussian valence basis sets, are produced for the lanthanide elements Ce to Lu and for the actinide elements Th to Lr. Results are given for the Ce3+ and Pa4+ free ions and in Ce3+-and Pr3+-doped Cs2NaYCl6 and Pa4+-and U4+-doped Cs2ZrCl6 materials.

An ab initio model potential (AIMP) has been developed for the lanthanide series from Ce to Lu. The corresponding (14s10p9d8f) valence basis sets were optimized by means of the minimization of the valence SCF total energies. The relativistic effect was considered with the scalar relativistic third-order Douglas–Kroll approximation. The applicability of the present AIMP, and the valence basis sets, was tested for the lanthanide atoms, and a monoxide, CeO. Both the atomic and molecular AIMP calculations showed excellent agreement with all-electron calculations. The present AIMP scheme has proved to work well in the calculations of whole series of the lanthanides.

In this article we present the results of ab initio model potential (AIMP) embedded cluster calculations of the electronic structure of (CrO 4 ) 3 - in the lattice of YVO 4 , in order to study the factors which determine the relative energy position of the two lowest lying electronic states of the Cr(V) ion as a dopant in this host lattice: 2 A 1 and 2 B 1 . Electron paramagnetic resonance (EPR) and optical spectroscopy studies have reported these two states to be in reversed order from what should be expected following crystal field theory (CFT) considerations, taking into account the elongated distortion of the (VO 4 ) 3 - cluster characteristic of the YVO 4 lattice. The relative order predicted by CFT ( 2 B 1 < 2 A 1 ) is found when self-consistent field (SCF) (CrO 4 ) 3 - cluster calculations are done in vacuo. When AIMP embedded cluster calculations are performed using large multiconfigurational expansions for the 2 A 1 and 2 B 1 wavefunctions, the 2 A 1 state is found to be the ground state, in agreement with experimental evidence; the energy difference with respect to the 2 B 1 i state is calculated to be 1650 cm - 1 . We have analyzed the results produced using different embeddings and different wavefunction expansions and we have reached the conclusion that embedding effects [both direct and indirect, through the local distortion produced by the Cr(V) impurity] and covalency (beyond SCF molecular orbital formation) are responsible for the energy reversal. Our results do not support previous interpretations of the energy reversal in terms of unusually strong interactions between the Cr(V) impurity and the two Y 3 + cations in the second coordination sphere of the Cr(V).

Ab initio embedded cluster calculations on Ce(3+)-doped Y3Al5O12, Lu3Al5O12, Gd3Al5O12, Y3Ga5O12, Lu3Ga5O12, and Gd3Ga5O12, which do not make use of any adjustable parameter, support recent assignments of the seventh 4f level of Ce(3+) in garnets [Przybylińska et al., Appl. Phys. Lett., 2013, 102, 241112] and that the splitting of the 4f shell of Ce(3+) in these materials is slightly smaller than 4000 cm(-1) and much larger than the 2000-2500 cm(-1) commonly assumed in analyses of 5d → 4f emission bands. Why this wrong assumption has been working well so far is explained by the fact that the intensity of the emission to the seventh level of the 4f(1) configuration is found to be only one hundredth of the integrated intensity of the emissions to the other six levels, which group themselves into two sets of three individual levels separated by 2000-2500 cm(-1). The effective field splitting and the spin-orbit coupling splitting are found to be of the same size. From a strong field coupling point of view, the first six levels result from the interactions between (2)T(2u) and (2)T(1u) cubic levels and the higher, isolated seventh level comes directly from the cubic (2)A(2u). From a weak field coupling point of view, the first three levels result from the splitting of (2)F(5/2), the second three levels from the splitting of (2)F(7/2) and the seventh level from a strong, cubic field driven interaction between (2)F(7/2) and (2)F(5/2) components [Herrmann et al., J. Appl. Phys., 1966, 37, 1312].

In this paper we present the results of spin–orbit relativistic ab initio model potential embedded cluster calculations on (PaCl6)2− embedded in a reliable representation of the Cs2ZrCl6 host. Totally symmetric local distortions and vibrational frequencies are calculated for all the states of the 5f1 and 6d1 manifolds, as well as the corresponding 5f↔6d transition energies and the shape of the 5f(Γ8u)←6d(Γ8g) fluorescence band. An excellent overall agreement with available experimental data is observed which allows us to conclude that the quality of the spin–orbit operators used is very high for actinide elements, as was already known for transition metal and lanthanide elements. Furthermore, it is concluded that the structural and spectroscopic information produced here is very reliable and that the 6d(Γ8g′) state is around 10 000 cm−1 higher in energy than it was thought; our calculations suggest a value of 30 000 cm−1 for the 10Dq parameter of Pa4+ in Cs2ZrCl6, which would be compatible with the lower ...

When alkali halide crystals are doped with ns 2 impurities such as Tl + , dimer centers as well as monomer centers are formed. In this paper, we present the results of ab initio model potential (AIMP) embedded cluster calculations of the geometric structure of the centers formed in Tl + -doped KCl. Using the AIMP method, which is able to bring into an ab initio cluster calculation all the main interactions with a frozen lattice (classical Madelung and short-range Coulomb, quantum mechanical exchange and orthogonality) as well as the effects of polarization and relaxation of the lattice ions external to the cluster, we have optimized the geometry of clusters representing the monomer and the D 2 h and D 4 h dimer defects, in their electronic ground state. We have found that the main result is an expansion of the clusters with respect to the perfect lattice geometry, in line with the larger ionic radius of Tl + compared to K + . As it was observed in other point defects, the lattice relaxation effects result in an enhancement of the distortions calculated with a frozen lattice. Even though the structures of the present defects are important ingredients for the detailed understanding of ns 2 defects in halides, and in particular of their spectroscopy, no direct measurements of them exist. Furthermore, the measurements are expected to be very difficult to perform, which leads to the relevance of the present calculations. The reliability of the results lies in the systematically good performance that the present method observes in transition metal doped ionic crystals.

The reaction of SnMe2Br2 with Et4NBr, in a 1:2 molar ratio, yields (Et4N)2[SnMe2Br4] (1) in CHCl3/hexane mixtures but (Et4N)[SnMe2Br3] (2) in water. This remarkable solvent dependence is explained by means of a thermochemical cycle that includes the lattice enthalpies of both compounds, the solvation enthalpies of the Et4N+ and Br- ions, and the gas-phase dissociation enthalpy of [SnMe2Br4]2- into [SnMe2Br3]- and Br-. Both compounds have been characterized in the solid state by IR, Raman, and 119Sn Mossbauer and MAS NMR spectroscopy and in solution by 1H, 13C, and 119Sn NMR spectroscopy. The X-ray crystal structures of 1 and (Me4N)[SnMe2Br3] (3) are reported. The crystallographic study of 3 provides the first X-ray crystal structure containing an [SnR2Br3]- anion. The structures of SnMe2X2, [SnMe2X3]-, trans-[SnMe2X4]2- (X = F, Cl, Br, I), cis-[SnR2Cl4]2- (R = Me, Et), SnEt2Cl2, [SnEt2Cl3]-, and trans-[SnEt2Cl4]2- have been optimized, at the SCF level, by ab initio MO methods. The gas-phase formation of [...

Wave function-based embedded cluster ab initio calculations have been carried out in order to study the 4f(2) and 4f5d energy levels of the cubic substitutional defect of Pr-doped CaF2. The 4f(2) → 4f5d absorption spectrum and 4f5d → 4f(2) emission spectrum have been calculated. The 4f(2 1)S0 level is found to be immersed in the 4f5d(eg) manifold, so that no quantum cutting from (1)S0 can occur and only strong 4f5d(eg) → 4f(2) emission is predicted, which supports previous assumptions made in order to explain results in CaF2:Pr(3+). The details of the 4f(2) → 4f5d(eg) and 4f(2) → 4f5d(t2g) bands of the absorption spectrum are interpreted and assignments are made. The lowest level of the 4f5d(eg) configuration is found to have 80% of singlet character, in opposition to Hund's Rules, and the issue is discussed in detail. The comparison between the experimental 4f5d(eg) → 4f(2) high resolution emission spectrum of the cubic site of CaF2:Pr(3+) and the calculated emission spectra from the two lowest 4f5d(eg) states 1T2u((1)T2u) and 1Eu(1(3)T2u) suggests the possibility that the experimental emission of the cubic Pr defect of CaF2:Pr(3+) is in fact a multiple emission.

The ab initio model potential (AIMP) method has been proven to produce effective one-electron operators that accurately represent the embedding effects of the cations and anions that constitute crystalline lattices like the KMgF{sub 3}, KZnF{sub 3}, and CsCaF{sub 3} cubic fluoroperovskites. The combination of these quantum mechanical embedding potentials with highly sophisticated molecular quantum chemical calculations of the CrF{sub 6}{sup 3{minus}} defect cluster enables theoretical study of the structure and spectroscopy of promising laser materials like the Cr{sup 3+}-doped KMgF{sub 3}, KZnF{sub 3}, and CsCaF{sub 3} crystals at the level of quality attainable in molecular quantum chemistry. The combination of the theoretical results with available experimental data [electron paramagnetic resonance (EPR), electron spin resonance (ESR), absorption, and emission spectra at ambient and high pressures] can contribute to clarifying the electronic structure of these systems, where the difficulty associated with the existence of cubic, trigonal, and tetragonal Cr{sup 3+} defects contributing to the spectra. The results of AIMP embedded-cluster studies of the cubic defects are presented here as one more useful and independent source of information that serves to clarify the divergent assignments and to provide new spectroscopic information, which shows that the laser emission of Cr{sub 3+} defects in allmore » three crystals should be free from excited state absorption losses if the pumping process is done through selective excitation to the {sup 4}T{sub 2g} laser level, below the {sup 2}E{sub g} higher excited state. Since the Cr{sup 3+} substitutional impurities create an excess positive charge, the effects of lattice site relaxation and dipole polarization on the local structure have been investigated and are presented here.« less

Ab initio model potential (AIMP) embedded cluster calculations of the absorption spectrum of Cs2GeF6:Mn4+ show unusually large dicrepancies with very accurate experimental data in electronic transition energies that involve an effective single excitation within the octahedral Mn(3d3) manifold: ; good results are obtained only for lower, intraconfigurational excitations that can be approximately described as metal transitions. The large discrepancies are in clear contrast with previous (good) results obtained for similar materials with the same methods; their origins have been investigated here. Peculiar to the Cs2GeF6:Mn4+ material, as compared to V2+ and Cr3+ isoelectronic impurity ions in similar fluoride hosts, are a shorter metal–ligand distance, larger crystal field, and higher oxidation state. Consequently, it has been considered that the usual, truncated configuration interaction treatments might be failing in describing the proper role of ligand-to-metal charge transfer and non-negligible F–F bonding interactions in the tight MnF62− cluster. Complete and restricted active space self-consistent-field (CASSCF, RASSCF), and averaged coupled pair aproximation (ACPF) calculations have been done in the embedded MnF62− cluster in an attempt to investigate these factors. The effects of the dn and charge transfer (CT) configuration interactions on the wave functions and energies of the ground and excited states are found to be very state dependent. In particular, the inclusion of different CT configurations in the multiconfigurational space leads to very large variations on the calculated transition energies. These large differential nondynamic interactions appear to be peculiar to this intermediate oxidation state transition metal system and suggests that dynamic correlation treatments based on larger multireferences than those that are nowadays computationally feasible are needed. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 623–635, 2000

A simple, approximate method of simultaneously handling the 5s and 6s valence orbitals of the lanthanide elements in Wood-Boring based relativistic ab initio core model potential calculations (WB-AIMP), which had previously been formulated with the restriction of having only one valence orbital with a given l, is presented. Its applicability is shown in spin-orbit configuration interaction calculations on Ce and spin-free calculations on CeO. The good quality of the Wood-Boring spin-orbit operators of Ce is shown and a recommended contraction of its valence basis set is established. These results support the production of WB-AIMP data for the lanthanide series.

In this paper we present the results of an ab initio model potential embedded-cluster study of the ground and lowest excited states of defects in the elpasolites, K 2 NaScF 6 , Cs 2 NaYCl 6 and Cs 2 NaYBr 6 ; complete active space SCF and averaged coupled-pair functional calculations are performed on (VF 6 ) 3− , (VCl 6 ) 3− , and (VBr 6 ) 3− clusters embedded in ab initio model potential representations of the surrounding lattices K 2 NaScF 6 , Cs 2 NaYCl 6 and Cs 2 NaYBr 6 . The geometry optimizations of the ground and low-lying excited states in the totally symmetric stretch and in the Jahn–Teller active e g vibrational modes of the (VX 6 ) 3− clusters (X=F, Cl, Br) show notable vibronic coupling only in the spin triplet excited state 3 T 2 g . The calculated electronic transition energies (ground state absorptions, emissions and excited state absorptions) compare very well with experimental measurements, when available. The question of the competition of the excited-state absorptions with the potential vibronic laser emission has been adressed.

In this paper we present an ab initio model-potential embedded-cluster study of the spin doublet excited states of ${\mathrm{V}}^{2+}$-doped ${\mathrm{KMgF}}_{3}$, ${\mathrm{KZnF}}_{3}$, and ${\mathrm{CsCaF}}_{3}$ fluoroperovskites, which includes intracluster electron correlation and quantum-mechanical lattice effects. The discrepancies of the calculated ground-state absorptions to the spin doublets with available experiments are systematic and amount to some $1000 {\mathrm{cm}}^{\ensuremath{-}1},$ leaving room for improvement of the treatment of valence electron correlation. The calculated excited-state absorption spectra originating in ${}^{2}{E}_{g}$ and ${}^{2}{T}_{1g}$ show that the broad ${}^{2}{E}_{g},$${}^{2}{\stackrel{\ensuremath{\rightarrow}}{{T}_{1g}}}^{2}{A}_{1g}$ absorption bands considerably overlap the potential ${}^{4}{\stackrel{\ensuremath{\rightarrow}}{{T}_{2g}}}^{4}{A}_{2g}$ laser emission, thus establishing a mechanism for laser loss that had not been considered so far in these low-field materials. The results obtained in these ${\mathrm{V}}^{2+}$-doped fluoroperovskites, together with those in other ${d}^{3}$-doped low-field crystals, point out that the observed excited-state absorption spectra may correspond to either a single absorbing excited state or to the superposition of electronic transitions originating in all the different stable excited states lying below the energies used for pumping. Each of these excited states becomes a channel for absorptions that may result in multiple laser loss mechanisms. The separate study of each of these channels appears to be feasible using ab initio embedded-cluster methods. The conditions for the occurrence of single ${(}^{4}{T}_{2g})$ versus multiple ${(}^{4}{T}_{2g},$${}^{2}{E}_{g},$${}^{2}{T}_{1g})$ excited-state absorption, and, therefore, single versus multiple loss mechanisms, are discussed in this paper in terms of the energy barrier to ${}^{2}{E}_{g},$${}^{2}{\stackrel{\ensuremath{\rightarrow}}{{T}_{1g}}}^{4}{T}_{2g}$ nonradiative decay.

The spin-orbit-CI WB-AIMP method[Chem. Phys. Lett., 147 (1988) 597; J. Chem. Phys., 102 (1995) 8078] has been recently proposed, monitored and applied, as a means to include spin-orbit relativistic effects in molecular and solid state effective core potential calculations. Necessary ingredients of the method are the valence spin-orbit model potentials and the spin-orbit-corrected valence basis sets, which are available in the literature for FI, TlRn, Ni and Pt; in this work, we obtain and compile the ones of all the remaining elements of the Periodic Table from boron to barium.

When Ce(3+) is doped in a garnet, it substitutes for some cations. The effect of the substitution on the optically active 4f and 5d levels of Ce(3+) can be analyzed in terms of an undistorted substitution followed by a structural relaxation, but, whereas the contribution of the undistorted substitution can be predicted/calculated using the crystallographic structure of the pure garnet, which is at hand, that of the structural relaxation demands the detailed local structure around the Ce(3+) impurity, which is hard to know. Hence the importance of knowing the role of the undistorted substitution. What we study in this paper is the role of the unrelaxed host effects on the 4f and 5d levels of Ce(3+)-doped garnets (i.e. the interactions between Ce and the second and more distant neighbors). When they are added to the (previously studied) unrelaxed first-neighbor effects, they give the contributions of the undistorted substitutions. We performed spin-orbit coupling, relativistic, embedded cluster, wave function based ab initio calculations on the (CeO8)(13-) cluster under the effects of the embedding potentials of 21 selected garnets of Si, Al, and Ga, which have been obtained in this work, using experimental unrelaxed structures of the pure garnets. The calculations reveal that the unrelaxed host effect: (1) produces a red shift of the first 4f → 5d transition and (2) is an important contribution to this transition which plays a very important role in differentiating the values it has in different garnet families. The unrelaxed host effect is found to be responsible for the low value of the first 4f → 5d transition in Ce(3+)-doped Lu2CaMg2Si3O12. It is analyzed in terms of 5d and 4f centroid energy contributions and crystal field splitting contributions in the 21 doped garnets. The effect on the splittings of the 4f and 5d levels is also discussed. The undistorted host approximation is found to give reasonable comparison with experiments, so that it represents a relatively fast way to provide reliable ab initio information on the 4f and 5d levels of Ce(3+) in garnets.

Optical excitation of ions or molecules typically leads to an expansion of the equilibrium bond lengths in the excited electronic state. However, for 4f(n-1)5d(1) excited states in lanthanide ions both expansion and contraction relative to the 4f(n) ground state have been reported, depending on the crystal field and nature of the 5d state. To probe the equilibrium distance offset between different 4f(n-1)5d(1) excited states, we report excited state excitation (ESE) spectra for Tm(2+) doped in CsCaBr3 and CsCaCl3 using two-color excited state excitation spectroscopy. The ESE spectra reveal sharp lines at low energies, confirming a similar distance offset for 4f(n-1)5d(t2g)(1) states. At higher energies, broader bands are observed, which indicate the presence of excited states with a different offset. On the basis of ab initio embedded-cluster calculations, the broad bands are assigned to two-photon d-d absorption from the excited state. In this work, we demonstrate that ESE is a powerful spectroscopic tool, giving access to information which cannot be obtained through regular one-photon spectroscopy.

The Wood-Boring-based ab initio model potential method, proposed as a means of including spin-orbit interactions on top of the mass-velocity and Darwin scalar relativistic effects considered by the former Cowan-Griffin-based ab initio model potential method, is applied to the calculation of the (5d, 6s) 10 manifold of atomic platinum, which is recognized to be a difficult task for ab initio methods. The results, which are compared with experiment and with four-component Dirac-Hartree-Fock plus configuration-interaction calculations, are of the quality required of a relativistic effective core potential method which includes spin-orbit effects.

Ab initio embedded cluster calculations have been performed on $$\hbox{Pr}^{3+}$$ -doped $$\hbox{Lu}_2\hbox{O}_3$$ , in order to investigate the mechanism responsible for the highly efficient $$^3P_0\rightarrow^1D_2$$ non-radiative relaxation experimentally observed. $$(\hbox{PrO}_6)^{9-}$$ embedded clusters representing the C 2 and S 6 substitutional sites of Pr3+:Lu2O3 have been studied using wave function-based methods. It is found that an outward relaxation of the first coordination sphere around the impurity takes place upon doping. At the relaxed geometry of the lowest spin triplet 4f5d state, all the 4f5d states lie much higher in energy than all 4f 2 states (except the 1S multiplet). This result is in opposition to the interpretation of intersystem crossing through a low-lying 4f5d excited state of Pr3+ as the mechanism for the fast non-radiative 3 P 0 → 1 D 2 relaxation proposed in the literature. Absorption to the lowest spin triplet 4f5d state is calculated to be around 4,800 cm−1 higher for the C 2 site than for the S 6 site, supporting the assignment of bands in the excitation spectrum previously reported.

In this paper, the differential effects brought about by the MgO, CaO, and SrO family of hosts on the topology of the energy surface corresponding to the ground state of the clusters (CuO6)10− and (AgO6)10− along the a1g and eg,θ vibrational modes, are studied by means of the ab initio embedding model potential method (AIEMP) at the restricted open‐shell Hartree–Fock level. For all six defect crystals, the equilibrium geometries, Jahn–Teller energies, vibrational frequencies, and relative energies between D4h elongated and compressed structures (energy barriers) have been calculated using (a) two different definitions of the defect cluster, namely (MeO6)10− and (MeO6M6)2+ (Me=Cu, Ag; M=Mg, Ca, Sr), (b) two different cluster LCAO expansions (including or not impurity second neighbor components), and (c) relativistic Cowan–Griffin ab initio (core and environment) model potentials. The results show the importance of using basis set functions located at lattice sites next to the (CuO6)10− and (AgO6)10− clusters in order to mimic the results of the larger clusters (CuO6M6)2+ and (AgO6M6)2+. The net relativistic effects calculated for SrO:Cu2+, Ag2+ and the individual contributions due to the impurity and to the environmental Sr2+ ions have been analyzed, all of them being rather small. The use of a simple point‐charge model for the MgO, CaO, and SrO crystalline environments (Madelung potential) is shown to be inadequate and to lead to meaningless results, in agreement to what has been found in previous studies of bulk and surface defects in other ionic crystals.Given that the Ag2+ impurity in SrO, CaO, and MgO hosts is accepted to be an exceptionally clear example of transition from static to dynamic Jahn–Teller effect at low temperature, the reproduction of this trend can be considered to be a demanding test for an embedding method like the AIEMP: A joint analysis of our calculations and the available EPR experimental results is presented which reveals an agreement in the conclusion that the environmental effects promote stronger Jahn–Teller coupling from MgO to SrO. Also, the inadequacy of the strong linear Jahn–Teller coupling approximation for MgO:Cu2+ is suggested to be responsible for the present incomplete understanding of the features of its EPR spectrum, in contrast with the other systems.

In this paper we present the results of an ab initio study of the pressure effects on the 4A2g ground state and the 4T2g and 2Eg excited states of Cr3+‐doped K2NaGaF6. Complete active space SCF (CASSCF) and averaged coupled‐pair functional (ACPF) calculations are performed on a CrF63− cluster embedded in an ab initio model potential (AIMP) representation of the K2NaGaF6 lattice at ambient and high pressure. The results are in close agreement with the experimentally measured pressure shifts of vibrational frequencies and emission spectra, which demonstrates the ability of the ab initio embedding model potential method to accurately model hydrostatic pressure experiments in this type of defect crystals. It is also shown that if only the standard Madelung embedding potential is used, none of the observed effects of pressure are reproduced.

The absorption and emission spectra of Yb(2+)-doped SrCl(2) have been calculated on the basis of ab initio quantum chemical calculations which consider recently found, unexpected excited states with double-well energy curves and complex electronic structure, resulting from avoided crossings between Yb-trapped excitons and Yb impurity states, which influence prominent spectral features. The root mean square deviation and largest absolute error of the calculated energy levels are 394 and -826 cm(-1), respectively. The YbCl(8) moiety breathing mode vibrational frequencies and bond lengths of the lowest states are consistent with observed vibrational progressions and energy shifts induced by uniaxial compression. Photoionization is predicted above 49,000 cm(-1) as a consequence of the spin-orbit induced spreading of the Yb-trapped exciton character in the upper part of the spectrum and three new emission bands are predicted with origins at about 33,800, 36,400, and 43,600 cm(-1). The electron correlation methods used overestimate the relative stabilization of the 4f(14) ground state and this leads to a constant error of the whole absorption spectrum of about 3500 cm(-1) (23%-7%). Although this energy shift is customarily considered an adjustable parameter, it is a nonparametric, direct product in an ab initio route which shows the limitations on the proper representation of differential correlation between the 4f(N) and 4f(N-1)5d (or similar) configurations and the need for theoretical improvement.

In this paper we present the results of a systematic comparison between the values of the bond lengths, bond angles, and vibrational frequencies of group IV dihydrides and tetrahydrides (XH2, XH4, X=Si,Ge,Sn,Pb), and monoxides (XO, X=Ge,Sn,Pb), as well as the XH4→XH2+H2 reaction energies, calculated with the spin‐free (Cowan–Griffin based) quasirelativistic ab initio core model potential method (AIMP) and the all‐electron Dirac–Hartree–Fock method (DHF), using basis sets of similar quality in their valence part. The deviations between the AIMP and DHF results on the absolute values of the properties and on the sizes of the relativistic effects are of the order expected for an effective core potential method and follow the expected tendency of importance of the spin–orbit effects. The quality of the AIMP results is shown to be consistent going down the group IV of the Periodic Table.

Ab initio model potential calculations on X − , X, and X + (XF, Cl, Br, and I) using CISD wavefunctions have been performed in order to test a recent conclusion on the irrelevance of the valence orbital internal nodes in the values of atomic valence correlation energies. The results show that the ab initio model potential orbitals, showing the correct nodal structure, provide valence correlation energies much closer to the corresponding all-electron values than nodeless pseudo-orbitals do.

Multiple spontaneous 4f(13)5d(1) --4f(14) emissions are predicted in Yb(2+)-doped CsCaBr(3) crystals by ab initio quantum chemical calculations. Four emission bands are found at 23,900, 26,600, 34,600, and 43,900 cm(-1) that should be experimentally observable at low temperatures. The first, third, and fourth bands are slow, electric dipole forbidden emissions that can be described as spin-forbidden. The second band is a fast, electric dipole-allowed emission that cannot be described as spin-allowed, but as spin-enabled; its radiative emission lifetime is 400 ns. Large energy gaps (23 900, 4600, 4000 cm(-1), respectively), relative to the maximum local phonon energies calculated (around 185 cm(-1)), are found below the emitting levels of the slow bands, which indicates that these states should be significantly stable and multiphonon relaxation to the lower states should be negligible. A smaller gap (2600 cm(-1)) separates the states of the fast band, which should result in a temperature dependent competition between radiative and nonradiative decay. Differential correlation between 4f-4f and 4f-5d pairs, splitting of the 5d shell by interactions with the host, and spin-orbit effects within the 4f(13) subshell, are found to be responsible for the existence of the gaps, which, in turn, split the absorption spectrum into four groups of separate bands, three of which could lie below the host absorption threshold. The quantum chemical methods employed make use of explicit wave functions expanded in terms of flexible basis sets, multiconfigurational self-consistent-field and multireference second-order perturbation methods to account for nondynamic and dynamic electron correlation, scalar and relativistic terms in the (YbBr(6))(4-) defect cluster Hamiltonian, and quantum mechanical embedding potentials to represent the host crystal.

An improvement in the method of production of embedding ab initio model potentials (AIMP) for embedded cluster calculations in ionic solids is proposed and applied to the oxides CeAlO(3), CeO(2), and UO(2). The improvement affects the calculation of one of the AIMP components, the Pauli repulsion operator, which prevents the cluster electrons from collapsing onto the occupied orbitals of the host in embedded cluster calculations and, so, their over occupancy. The linear constants involved in such operator are proposed to be obtained in embedded cluster calculations in the perfect host, with the requirement that local structures calculated with working embedded clusters of relatively small size agree with those calculated with reference embedded clusters of much larger size.

In this paper we present the results of a theoretical study of the local geometry distortions produced by Cr+ and Cr3+ ions in fluorite, by means of the ab initio environment model potential method [J. Chem. Phys. 89, 5739 (1988)]. The valence energy and wave function of the (CrF8)q−(q=7,5) clusters embedded in the CaF2 lattice represented by 118 ab initio model potential ions plus 750 point‐charge ions are calculated using a short CI, which includes all states related to the dn ionic configuration. A geometry optimization is performed for CaF2: (CrF8)7− within the cubic point symmetry and for CaF2: (CrF8)5− within the D3d point symmetry; the conclusions of analyses of the multimode Jahn–Teller coupling T1⊗(e+τ2+τ’2) are used to address the geometry optimization, which is completed by a final unrestricted search. In an attempt to study all the structures proposed along previous experimental studies, the D3d structure corresponding to an octahedral cluster plus two axially displaced fluorines (octahedron+2...

In this paper, we present the results of ab initio model potential embedded-cluster calculations of ${\mathrm{Sm}}^{2+}$ impurities in $\mathrm{Sr}{\mathrm{F}}_{2}$ in order to study the behavior of the electronic transitions of the dopant ion under high hydrostatic pressure. We find that the impurity-ligand bond length shortens upon $f\ensuremath{\rightarrow}d({e}_{g})$ excitation and, as a consequence, the $f\ensuremath{\rightarrow}d({e}_{g})$ transition energy decreases with increasing applied pressure. On the other hand, the bond lengths do not appreciably change upon $f\ensuremath{\rightarrow}f$ excitation and the energies of the $f\ensuremath{\rightarrow}f$ transitions are almost constant with pressure. These trends are in agreement with spectroscopic measurements under pressure in the title material, which gives credit to the computed bond length changes upon excitation, in contradiction with the widespread assumption of bond length lengthening upon $f\ensuremath{\rightarrow}d$ excitations. Spectroscopic experiments under high pressure are shown to be able to provide the sign of bond length changes in electronic transitions, constituting a simpler alternative to difficult excited-state x-ray absorption fine structure experiments.

Excitons trapped at impurity centers in highly ionic crystals were first described by McClure and Pedrini [Phys. Rev. B 32, 8465 (1985)] as excited states consisting of a bound electron-hole pair with the hole localized on the impurity and the electron on nearby lattice sites, and a very short impurity-ligand bond length. In this work the authors present a detailed microscopic characterization of impurity-trapped excitons in U(4+)-doped Cs(2)GeF(6). Their electronic structure has been studied by means of relativistic ab initio model potential embedded cluster calculations on (UF(6))(2-) and (UF(6)Cs(8))(6+) clusters embedded in Cs(2)GeF(6), in combination with correlation methods based on multireference wave functions. The local geometry of the impurity-trapped excitons, their potential energy curves, and their multielectronic wave functions have been obtained as direct, nonempirical results of the methods. The calculated excited states appear to be significantly delocalized outside the UF(6) volume and their U-F bond length turns out to be very short, closer to that of a pentavalent uranium defect than to that of a tetravalent uranium defect. The wave functions of these excited states show a dominant U 5f(1)7s(1) configuration character. This result has never been anticipated by simpler models and reveals the unprecedented ability of diffuse orbitals of f-element impurities to act as electron traps in ionic crystals.

The 5f--6d(t(2g)) absorption spectrum of U(3+)-doped Cs(2)NaYCl(6) is simulated with a quantum chemical ab initio embedded-cluster approach applied to U(3+) substitutional defects of O(h) local symmetry. The first-principles results help to provide a detailed interpretation of the very rich experimental absorption spectrum of this material between 14 000 and 25 000 cm(-1). Also, the effects of high pressures up to 26 kbars on the absorption spectrum are predicted, the most relevant feature being a redshift of around 21 cm(-1)/kbar, which is the fingerprint of a bond length shortening upon 5f--6d(t(2g)) excitation.

It has been experimentally determined that Cs2ZrCl6:Os4+ shows luminescence and up-converted luminescence from the highest t(2g) (4) excited level 2 A1g(1A1g), whereas Cs2GeF6:Os4+ 2 A1g(1A1g) does not luminescence at all. Ab initio quantum chemical calculations on these materials are presented here and show that the variation of the energy gap between the t2g 4 and t2g 3 eg 1 manifolds with F to Cl chemical substitution is a key factor to interpret the experimental findings. This energy gap is calculated to be some 1500 cm(-1) (approximately 2nua1g) in the fluoride host, whereas it is about 3300 cm(-1) (approximately 9nua1g) in the chloride host. The calculated values for the ground state totally symmetric vibrational frequency nu(a1g) are 626 cm(-1) (Cs2GeF6:Os4+) and 355 cm(-1) (Cs2ZrCl6:Os4+), in good agreement with the available experimental data. Geometrical structure of (OsX6)2- clusters (X=F,Cl) embedded in Cs2GeF6 and Cs2ZrCl6 lattices is calculated as well. New assignments for some spectral features based in the results of our calculations are proposed.

The existence of intervalence charge transfer (IVCT) luminescence is reported. It is shown that the so called anomalous luminescence of Ce-doped elpasolite Cs2LiLuCl6, which is characterized mainly by a very large Stokes shift and a very large band width, corresponds to an IVCT emission in Ce3+-Ce4+ pairs, from the 5deg orbital of Ce3+ to 4f orbitals of Ce4+. Its Stokes shift is the sum of the large reorganization energies of the Ce4+ and Ce3+ centers formed after the fixed-nuclei electron transfer and it is equal to the energy of the IVCT absorption commonly found in mixed-valence compounds, which is predicted to exist in this material and to be slightly larger than 10000 cm-1. The large band width is the consequence of the large offset between the minima of the Ce3+-Ce4+ and Ce4+-Ce3+ pairs along the electron transfer reaction coordinate. This offset is approximately 2*sqrt(3) times the difference of Ce-Cl equilibrium distances in the Ce3+ and Ce4+ centers. It is shown that the energies of the peaks and the widths of IVCT absorption and emission bands can be calculated ab initio with reasonable accuracy from diabatic energy surfaces of the ground and excited states and that these can be obtained, in turn, from independent calculations on the donor and acceptor active centers. We obtained the energies of the Ce3+ and Ce4+ active centers of Ce-doped Cs2LiLuCl6 by means of state-of-the-art wave-function-theory spin-orbit coupling relativistic calculations on the donor cluster (CeCl6Li6Cs8)11+ and the acceptor cluster (CeCl6Li6Cs8)12+ embedded in a quantum mechanical embedding potential of the host. The calculations provide interpretations of unexplained experimental observations as due to higher energy IVCT absorptions, and allow reinterpreting others. The existence of another IVCT emission of lower energy, at around 14000-16000 cm-1 less than the 5dt2g emission, is also predicted., Comment: Accepted for publication in J. Chem. Phys

The experimental determination of the electronic energy levels for Ce(3+) in some chloroelpasolite hosts for both the ground 4f(1) and the excited 5d(1) configurations is described. High-resolution f-f absorption and f-(2)T(2g) d absorption and emission spectra have been recorded at low temperatures for Ce(3+) diluted into various hexachloroelpasolite lattices. A fluorescence spectrum at approximately 50 000 cm(-1) is tentatively assigned to the emission from the highest 5d crystal field level, (2)E(g), of a Ce(3+) impurity in Cs(2)NaErCl(6), enabling the values of all the energy levels of both the 4f(1) and 5d(1) configurations to be given for Ce(3+) in elpasolite hosts. Vibronic structure superimposed on the electronic transitions is analyzed in terms of a simple configurational coordinate model involving the ground and excited configurations. It is found that the difference in the Ce-Cl bond length between the 4f(1) and 5d(1) configurations is approximately 0.04 A. Ab initio model potential calculations on the (CeCl(6))(3-) cluster embedded in a reliable representation of the Cs(2)NaYCl(6) host support these conclusions.

The lanthanide–chlorine bond length, Re, and the frequency of the symmetric stretching mode, νa1g, of the (LnCl6)3− octahedral defect clusters embedded in Cs2NaYCl6 have been calculated for all 14 Ce3+ to Lu3+ impurities in their ground 4fn electronic state using wave-function-based ab initio methods of solid state quantum chemistry which include relativistic effects and electron correlation within the (LnCl6)3− defect clusters and quantum mechanical interactions between the (LnCl6)3− electronic group and the Cs2NaYCl6 embedding host ions. The bond distance values obtained provide useful data to improve the Judd–Morrison model of the 4f→5d energy separation by explicitly including the local distortions the Ln3+ ions produce in the Cs2NaYCl6 host. The values of the structural parameters Re and νa1g, and their variation across the series have also been studied using simpler models of the embedding host (in vacuo and Madelung embeddings), which has revealed that host effects, particularly those associated ...

In this paper we present the results of ab initio model potential embedded cluster average coupled pair functional calculations on (VCl6)3− embedded in a reliable representation of the Cs2NaYCl6 host. They are aimed at complementing recent high pressure, room temperature, broad band luminescence studies with detailed theoretical data. In particular, we study the effects of high hidrostatic pressure (i) on the structure of V3+ substitutional defects in four low lying electronic states (a3T1g, 3T2g, 1T2g, and b3T1g), (ii) on the individual a1g and eg force constants and equilibrium displacements, and (iii) on the zero-phonon and Franck–Condon energy differences between them. It is shown that only the eg equilibrum displacements between different electronic sates are large and pressure dependent, while the a1g ones are small and essentially pressure independent. A value of κ=4×10−3 kbar−1 is suggested for the isothermal bulk modulus of the Cs2NaYCl6 elpasolite, because it allows for an excellent match betwee...