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In this contribution, we describe the preparation, by means of a precipitation reaction from aqueous solution at 40 °C, and the structural characterization of nanocrystalline powders of trigonal Tb 1- x Eu x PO 4 · n H 2 O (with x = 0, 0.005, 0.01, 0.05, and 0.1; n tentatively assigned as 0.67) which crystallize in the two possible P 3 1 21 or P 3 2 21 enantiomorphic space groups. While the volume of the crystal lattice is not significantly affected by the Tb 3+ /Eu 3+ substitution, the average crystallite size seems to depend on the Eu 3+ dopant concentration and ranges from 13 to 30 nm. The desired handedness of the crystals has been induced by using, during the synthesis, one of the two possible enantiomers of tartaric acid (l or d). The analysis of the luminescence excitation and emission spectra, together with the decay kinetics of the 5 D 4 Tb 3+ excited state, suggests the presence of a very efficient Tb 3+ → Eu 3+ energy transfer process in the Eu 3+ -doped orthophosphates. Upon excitation of Tb 3+ ions at 368 nm, the enantiomorphic powders grown with l- or d-tartaric acid (i.e., l-TbPO 4 ·0.67H 2 O/d-TbPO 4 ·0.67H 2 O, l-Tb 0.995 Eu 0.005 PO 4 ·0.67H 2 O/d-Tb 0.995 Eu 0.005 PO 4 ·0.67H 2 O, and l-Tb 0.9 Eu 0.1 PO 4 ·0.67H 2 O/d-Tb 0.9 Eu 0.1 PO 4 ·0.67H 2 O) exhibited mirror circularly polarized luminescence signals in the visible spectral region (in the green and/or in the red).

In this work, we investigated from a theoretical point of view the dynamics of the energy transfer process from the ligand to Eu(III) ion for 12 isomeric species originating from six different complexes differing by nature of the ligand and the total charge. The cationic complexes present the general formula [Eu(L)(H 2 O) 2 ] + (where L = bpcd 2- = N , N '-bis(2-pyridylmethyl)- trans -1,2-diaminocyclohexane N , N '-diacetate; bQcd 2- = N , N '-bis(2-quinolinmethyl)- trans -1,2-diaminocyclohexane N , N '-diacetate; and b iso Qcd 2- = N , N '-bis(2-isoquinolinmethyl)- trans- 1,2-diaminocyclohexane N , N '-diacetate), while the neutral complexes present the Eu(L)(H 2 O) 2 formula (where L = PyC3A 3- = N -picolyl- N , N ', N '- trans -1,2-cyclohexylenediaminetriacetate; QC3A 3- = N -quinolyl- N , N ', N '- trans- 1,2-cyclohexylenediaminetriacetate; and iso QC3A 3- = N -isoquinolyl- N , N ', N '- trans- 1,2-cyclohexylenediaminetriacetate). Time-dependent density functional theory (TD-DFT) calculations provided the energy of the ligand excited donor states, distances between donor and acceptor orbitals involved in the energy transfer mechanism ( R L ), spin-orbit coupling matrix elements, and excited-state reorganization energies. The intramolecular energy transfer (IET) rates for both singlet-triplet intersystem crossing and ligand-to-metal (and vice versa) involving a multitude of ligand and Eu(III) levels and the theoretical overall quantum yields (ϕ ovl ) were calculated (the latter for the first time without the introduction of experimental parameters). This was achieved using a blend of DFT, Judd-Ofelt theory, IET theory, and rate equation modeling. Thanks to this study, for each isomeric species, the most efficient IET process feeding the Eu(III) excited state, its related physical mechanism (exchange interaction), and the reasons for a better or worse overall energy transfer efficiency (η sens ) in the different complexes were determined. The spectroscopically measured ϕ ovl values are in good agreement with the ones obtained theoretically in this work.

Eu 3+ (1 mol %)-doped Ca 2 LnSbO 6 (replacing Ln 3+ ; Ln = Lu, Y, Gd, and La) and Ca 2 EuSbO 6 were synthesized and structurally characterized by means of X-ray powder diffraction. The Eu 3+ luminescence spectroscopy of the doped samples and of Ca 2 EuSbO 6 has been carefully investigated upon collection of the excitation/emission spectra and luminescence decay curves of the main excited states. Surprisingly, apart from the dominant red emission from 5 D 0 , all the doped samples show an uncommon blue and green emission contribution from 5 D J ( J = 1, 2, and 3). This is made possible thanks to both multiphonon and cross-relaxation mechanism inefficiencies. However, the emission from 5 D 3 is more efficient and the decay kinetics of the 5 D J ( J = 0, 1, and 2) levels is slower in the case of Y- and Lu-based doped samples. This evidence can find a possible explanation in the crystal chemistry of this family of double perovskites: our structural investigation suggests an uneven distribution of the Eu 3+ dopant ions in Ca 2 YSbO 6 and Ca 2 LuSbO 6 hosts of the general A 2 BB'O 6 formula. The luminescent center is mainly located in the A crystal site, and on average, the Eu-Eu distances are longer than in the case of the Gd- and La-based matrix. These longer distances can further reduce the efficiency of the cross-relaxation mechanism and, consequently, the radiative transitions are more efficient. The slower depopulation of Eu 3+ 5 D 2 and 5 D 1 levels in Ca 2 YSbO 6 and Ca 2 LuSbO 6 hosts is reflected in the longer rise observed in the 5 D 1 and 5 D 0 decay curves, respectively. Finally, in Ca 2 EuSbO 6 , the high Eu 3+ concentration gives rise to an efficient cross-relaxation within the subset of the lanthanide ions so that no emission from 5 D J ( J = 1, 2, and 3) is possible and the 5 D 0 decay kinetics is faster than for the doped samples.

In the pursuit of a systematic characterization of rare-earth vanadates under compression, in this work we present a multifaceted study of the phase behavior of zircon-type orthovanadate PrVO 4 under high-pressure conditions, up to 24 GPa. We have found that PrVO 4 undergoes a zircon to monazite transition at around 6 GPa, confirming previous results found by Raman experiments. A second transition takes place above 14 GPa, to a BaWO 4 -II type structure. The zircon to monazite structural sequence is an irreversible first-order transition, accompanied by a volume collapse of about 9.6%. The monazite phase is thus a metastable polymorph of PrVO 4 . The monazite-BaWO 4 -II transition is found instead to be reversible and occurs with a similar volume change. Here we report and discuss the axial and bulk compressibility of all phases. We also compare our results with those for other rare-earth orthovanadates. Finally, by means of optical-absorption experiments and resistivity measurements, we determined the effect of pressure on the electronic properties of PrVO 4 . We found that the zircon-monazite transition produces a collapse of the band gap and an abrupt decrease in the resistivity. The physical reasons for this behavior are discussed. Density functional theory simulations support our conclusions.

We have studied the high-pressure behavior of FeVO 4 by means of single-crystal X-ray diffraction (XRD) and density functional theory (DFT) calculations. We have found that the structural sequence of FeVO 4 is different from that previously assumed. In particular, we have discovered a new high-pressure phase at 2.11(4) GPa (FeVO 4 -I'), which was not detected by previous powder XRD studies. We have determined that FeVO 4 , under compression (at room temperature), first transforms at 2.11(4) GPa from the ambient-pressure triclinic structure (FeVO 4 -I) to a second previously unknown triclinic structure (FeVO 4 -I'), which experiences a subsequent phase transition at 4.80(4) GPa to a monoclinic structure (FeVO 4 -II'), which was also previously detected in powder XRD experiments. Single-crystal XRD has enabled these novel findings as well as an accurate determination of the crystal structure of FeVO 4 polymorphs under high-pressure conditions. The crystal structure of all polymorphs has been accurately solved at all measured pressures. The pressure dependence of the unit-cell parameters and polyhedral coordination have been obtained and are discussed. The room-temperature equation of state and the principal axes of the isothermal compressibility tensor of FeVO 4 -I and FeVO 4 -I' have also been determined. The structural phase transition observed here between these two triclinic structures at 2.11(4) GPa implies abrupt coordination polyhedra modifications, including coordination number changes. DFT calculations support the conclusions extracted from our experiments.

We present a structural and optical characterization of magnetoelastic zircon-type TmVO 4 at ambient pressure and under high pressure. The properties under high pressure have been determined experimentally under hydrostatic conditions and theoretically using density functional theory. By powder X-ray diffraction we show that TmVO 4 undergoes a first-order irreversible phase transition to a scheelite structure above 6 GPa. We have also determined (from powder and single-crystal X-ray diffraction) the bulk moduli of both phases and found that their compressibilities are anisotropic. The band gap of TmVO 4 is found to be E g = 3.7(2) eV. Under compression the band gap opens linearly, until it undergoes a huge collapse following the structural phase transition (Δ E g = 1.15 eV). Ab initio structural and free energy calculations support our findings. Moreover, calculations of the band structure and density of states reveal that for both zircon and scheelite TmVO 4 the band gap is entirely determined by the V 3d and O 2p states of the VO 4 3- ion. The behavior of the band gap can thus be understood entirely in terms of the structural modifications of the VO 4 units under compression. Additionally, we have calculated the evolution of the infrared and Raman phonons of both phases upon compression. The presence of soft modes is related to the dynamic instability of the low-pressure phase and to the phase transition.

The zircon to scheelite phase boundary of ErVO 4 has been studied by high-pressure and high-temperature powder and single-crystal X-ray diffraction. This study has allowed us to delimit the best synthesis conditions of its scheelite-type phase, determine the ambient-temperature equation of state of the zircon and scheelite-type structures, and obtain the thermal equation of state of the zircon-type polymorph. The results obtained with powder samples indicate that zircon-type ErVO 4 transforms to scheelite at 8.2 GPa and 293 K and at 7.5 GPa and 693 K. The analyses yield bulk moduli K 0 of 158(13) GPa for the zircon phase and 158(17) GPa for the scheelite phase, with a temperature derivative of d K 0 /d T = -[3.8(2)] × 10 -3 GPa K -1 and a volumetric thermal expansion of α 0 = [0.9(2)] × 10 -5 K -1 for the zircon phase according to the Berman model. The results are compared with those of other zircon-type vanadates, raising the need for careful experiments with highly crystalline scheelite to obtain reliable bulk moduli of this phase. Finally, we have performed single-crystal diffraction experiments from 110 to 395 K, and the obtained volumetric thermal expansion (α 0 ) for zircon-type ErVO 4 in the 300-395 K range is [1.4(2)] × 10 -5 K -1 , in good agreement with previous data and with our experimental value given from the thermal equation of state fit within the limits of uncertainty.

A new chiral complex {[EuL(tta) 2 (H 2 O)]CF 3 SO 3 ; L = N, N'-bis(2-pyridylmethylidene)-1,2-( R, R + S, S)-cyclohexanediamine; tta = 2-thenoyltrifluoroacetyl-acetonate} has been synthesized and characterized from a structural and spectroscopic point of view. The molecular structure in the solid state shows the presence of one chiral L, two tta, and one water molecules bound to the metal center. L and tta molecules can efficiently harvest and transfer to Eu(III) the UV light absorbed in the 250-400 nm range. The forced electric-dipole 5 D 0 → 7 F 2 emission band dominates the Eu(III) emission spectra recorded in the solid state and in solution of acetonitrile or methanol and the calculated intrinsic quantum yield of the metal ion is around 40-50%. The light emitted by the enantiopure complex shows a sizable degree of polarization with a maximum value of the emission dissymmetry factor ( g lum ) equal to 0.2 in methanol solution. If compared with the complex in the solid state or in acetonitrile solution, then the first coordination sphere of Eu(III) when the complex is dissolved in methanol is characterized by the presence of one CH 3 OH molecule instead of water. This fact is related to different Eu(III) CPL signatures in the two solvents.

A detailed investigation of the overall crystal structure, and in particular of the local structure around the cations in M 2 La 3 Sb 3 O 14 (M = Mg, Ca) was accomplished using X-ray diffraction, steady state luminescence spectroscopy and decay kinetics, and state of the art density functional calculations. The computational tool was also used to investigate the structure of Mn 2 La 3 Sb 3 O 14 . The Eu 3+ dopant ion was employed as an optical probe of the local symmetry at the cationic sites. The use of these complementary techniques shows that the antimonates under investigation belong to the rhombohedral pyrochlore family with space group R3̅ m (No. 166), but while Mg 2 La 3 Sb 3 O 14 and Mn 2 La 3 Sb 3 O 14 show an ordered cationic configuration, the Ca 2+ and La 3+ of Ca 2 La 3 Sb 3 O 14 are disordered because of their similar ionic radii. In both the Mg- and the Ca-based compounds, the Eu 3+ ions formally occupy centrosymmetric sites, but in the case of Ca 2 La 3 Sb 3 O 14 the presence of disorder in the outer coordination spheres removes the local inversion symmetry in these sites. This has a strong influence on the Eu 3+ luminescence spectrum and on the radiative decay rate of the 5 D 0 emitting level.

We studied the electronic and vibrational properties of monazite-type SrCrO 4 under compression. The study extended the pressure range of previous studies from 26 to 58 GPa. The existence of two previously reported phase transitions was confirmed at 9 and 14 GPa, and two new phase transitions were found at 35 and 48 GPa. These transitions involve several changes in the vibrational and transport properties with the new high-pressure phases having a conductivity lower than that of the previously known phases. No evidence of chemical decomposition or metallization of SrCrO 4 was detected. A tentative explanation for the reported observations is discussed.

The crystal structures, lattice vibrations, and electronic band structures of PbCrO4, PbSeO4, SrCrO4, and SrSeO4 were studied by ab initio calculations, Raman spectroscopy, X-ray diffraction, and optical-absorption measurements. Calculations properly describe the crystal structures of the four compounds, which are isomorphic to the monazite structure and were confirmed by X-ray diffraction. Information is also obtained on the Raman- and IR-active phonons, with all of the vibrational modes assigned. In addition, the band structures and electronic densities of states of the four compounds were determined. All are indirect-gap semiconductors. In particular, chromates are found to have band gaps smaller than 2.5 eV and selenates higher than 4.3 eV. In the chromates (selenates), the upper part of the valence band is dominated by O 2p states and the lower part of the conduction band is composed primarily of electronic states associated with the Cr 3d and O 2p (Se 4s and O 2p) states. Calculations also show that the band gap of PbCrO4 (PbSeO4) is smaller than the band gap of SrCrO4 (SrSeO4). This phenomenon is caused by Pb states, which, to some extent, also contribute to the top of the valence band and the bottom of the conduction band. The agreement between experiments and calculations is quite good; however, the band gaps are underestimated by calculations, with the exception of the bang gap of SrCrO4, for which theory and calculations agree. Calculations also provide predictions of the bulk modulus of the studied compounds.

Single crystals of CaWO(4) and CaMoO(4) doped with Tb(3+) have been grown by the flux growth method. Their luminescence properties have been investigated in the 10-600 K temperature range under different experimental conditions. In spite of very similar spectra at low temperature upon excitation at 365 nm, the crystals show a very different behavior as the temperature is raised or the excitation wavelength is changed. These differences have been accounted for on the basis of models that take into consideration the position of the energy levels of the rare earth relative to the bandgap of the host material.

The electronic structure of a series of thiophenolate-capped ionic/neutral clusters ([Zn(SPh)(4)](2)(-) (1); [Zn(4)(&mgr;(2)-SPh)(6)(SPh)(4)](2)(-) (2); Zn(10)(&mgr;(3)-S)(4)(&mgr;(2)-SPh)(12) (3); and [Zn(10)(&mgr;(3)-S)(4)(&mgr;(2)-SPh)(12)(SPh)(4)](4)(-) (4), Ph = phenyl), indicated as supertetrahedral fragments and possible molecular models of cubic ZnS, has been investigated by coupling density functional calculations to UV electronic and X-ray photoelectron (XP) spectroscopy. Theoretical outcomes indicate that, on passing from the tetrametallic to the decametallic clusters, there is a modification in the nature of the outermost occupied and lowermost unoccupied molecular orbitals. Actually, both in 1 and in 2 the frontier orbitals are delocalized and mainly composed of the S 3p pairs strongly mixed with the Ph pi levels (the HOMOs) and of the linear combinations of Ph pi orbitals, the LUMOs. At variance to that, in 3 and 4 both the HOMO and LUMO are highly localized, the former on &mgr;(3)-S atoms occupying C(3)(v)() coordinatively unsaturated tetrahedral positions and the latter on peripheral Zn atoms. The nature of the electronic levels involved in the UV absorption bands is discussed, and the agreement between theory and experiment is satisfactory. Neither experimental nor theoretical electronic excitation energies are influenced by the cluster size. Moreover, XPS data match quite well variations of the Zn and S gross atomic charges along the series. The different Zn-S bonding scheme characterizing terminal, &mgr;(2)-bridging, and &mgr;(3)-pyramidal S atoms allows a rationalization of the cluster behavior in solution. Along the investigated series, the only species reasonably mimicking both the structural arrangement and the electronic structure of the solid ZnS is Zn(10)(&mgr;(3)-S)(4)(&mgr;(2)-SPh)(12), which can be considered a molecular model of ZnS nonpolar surfaces.