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A microscopic approach to the problem of charge transfer-induced spin transitions in a system of interacting trinuclear Fe-Co-Fe clusters, which takes into account the switching of polarization during the transformation Fe

A crystal containing the heterometallic Cr-ligand-Co cluster with an unpaired electron on the ligand as a structural unit is examined. The developed model which describes the magnetic and polarizability characteristics of this crystal takes into account that the electron residing on the ligand can be transferred to the Co-ion, thus converting the diamagnetic ls-CoIII ion into the paramagnetic hs-CoII one. Since this transformation is accompanied by electron density redistribution and elongation of the Co–N bond lengths, the vibronic interaction of the Co-ion with totally symmetric displacements of the nearest surroundings and cooperative dipole–dipole and electron-deformational interactions are accounted for as well. The exchange interactions between the CrIII ion and the electron localized on the ligand as well as in the CrIII-hs-CoII pair are also included in consideration; the parameters of these interactions are estimated within the framework of the DFT method. Bistability in the magnetic and polarization characteristics is predicted for certain strengths of intra- and intercluster interactions in the crystal under study. Within the framework of the developed approach an explanation of the observed magnetic properties of the [Cr(SS-cth)(Co(RR-cth)(μ-dhbq))](PF6)2Cl crystal is given.

In this article, the vibronic model for an electric field switchable mixed-valence trimer containing two delocalized electrons or holes is proposed and examined. The role of the vibronic coupling on the electric field effects is analyzed by means of the semiclassical adiabatic approach and, alternatively, with the aid of the numerical analysis of the Schrodinger equation with due allowance for the kinetic energy of the ions (dynamic problem). The adiabatic potential landscapes have been calculated by taking into account the influence of the electric field. As the adiabatic approximation has a limited frame of validity, the study of the electric field effects has also been performed within more rigorous quantum mechanical evaluation of the energy levels arising in the combined Jahn–Teller/pseudo-Jahn–Teller two-mode three-level vibronic problem. The vibronic coupling gives rise to a significant effect on the energies of spin levels and charge distributions, considerably facilitating the conditions for the ...

In the present paper a single valence tautomeric cobalt complex placed in a dc electric field is examined. Along with the interaction with the electric field the model employed accounts for intramolecular transfer of the excess electron, vibronic and exchange interactions. To obtain the energy spectrum of the valence tautomeric complex in the electric field, the vibronic problem of the pseudo-Jahn-Teller effect is solved. The vibronic (hybrid) wave-functions and energy levels are used for the calculation of the temperature and field dependence of the molecular magnetic moment, and the polarization and shape of the absorption band in the near infrared range that relates to the light-induced transfer of the electron from the cat ligand to the metal ion. It is predicted that the application of a dc electric field leads to the appearance of electric polarization and significant change in magnetic and spectroscopic properties. A unique possibility to control electrically the magnetic state and optical and polarization characteristics of a valence tautomeric molecule is predicted.

Combined theoretical and experimental studies of the [VO(O2)Hheida]− anion dissolved in water that may serve as a functional model for vanadium haloperoxidase enzymes have been performed. The geometrical structure and absorption and vibrational spectra of this system have been evaluated within the framework of density functional theory (DFT). The obtained theoretical results on the equilibrium structure and optical spectra are in quite good agreement with the experimental data. With the aid of the combination of UV−visible spectroscopy and electronic structure calculations, it has been revealed that, in the apparent absorption spectra of the [VO(O2)Hheida]− anion, the highest in energy band corresponds to a ligand to metal electron excitation, while the band with a maximum at 430 nm arises from the peroxo group. The calculations also reproduce quite well the positions, intensities and the grouping of frequencies in the near-infrared (NIR) spectra. The visualization of the calculated vibrations in the energy range of 400−1100 cm−1 has been presented.

A model is developed to describe the charge transfer-induced spin transition in a crystal containing cyanide-bridged binuclear Co–Fe clusters as a structural element. The model accounts for the electronic states arising from two cluster configurations active in the transition and, namely, from the ground ls-CoIII−ls-FeII and the excited hs-CoII−ls-FeIII ones. The cooperative interaction that originates from the coupling of the acoustic crystalline modes with the molecular vibrations describing the displacements of the nearest ligand surroundings of the metal ions is assumed to be responsible for spin transformation. The problem of interacting clusters is solved within mean field approximation. The role of the order parameters is played by the mean values of the products of the electronic matrices and coordinates of the molecular vibrations in the ground and excited cluster configurations. The developed model provides quite a reasonable description of the observed temperature dependence of the magnetic characteristics of the [{(Tp)Fe(CN)3}{Co-(PY5Me2)}](CF3SO3)⋅2DMF complex. The factors controlling the electron transfer in this complex are elucidated.

New tetranuclear and octanuclear mixed-valent cobalt(II/III) pivalate clusters, namely, [NaCo4(O2CCMe3)6(HO2CCMe3)2(teaH)2(N3)]·2H2O (in two polymorphic modifications, 1 and 1a) and [Co8(O2CCMe3)10(teaH)4(N3)](Me3CCO2)·MeCN·H2O (2) have been synthesized by ultrasonic treatment of a dinuclear cobalt(II) pivalate precursor with sodium azide and triethanolamine (teaH3) ligand in acetonitrile. The use of Dy(NO3)3·6H2O in a similar reaction led to the precipitation of a tetranuclear [NaCo4(O2CCMe3)4(teaH)2(N3)(NO3)2(H2O)2]·H2O (3) cluster and a heterometallic hexanuclear [Co3Dy3(OH)4(O2CCMe3)6(teaH)3(H2O)3](NO3)2·H2O (4) cluster. Single-crystal X-ray analysis showed that 1 (1a) and 3 consist of a tetranuclear pivalate/teaH3 mixed-ligand cluster [CoII2CoIII2(O2CCMe3)4(teaH)2(N3)]+ decorated with sodium pivalates [Na(O2CCMe3)2(HO2CCMe3)2]− (1 or 1a) or sodium nitrates [Na(NO3)2]− (3) to form a square-pyramidal assembly. In 2, the cationic [Co8(O2CCMe3)10(teaH)4(N3)]+ cluster comprises a mixed-valent {CoII4CoIII4...

We present a new microscopic approach for the description of the effects of an external direct current (dc) electric field in the magnetic, polarizability and spectroscopic characteristics of molecular crystals containing cyanide-bridged Fe-Co clusters as structural units. Besides the interaction with the electric field the model allows for the metal-metal electron transfer, intracluster magnetic exchange and long-range electron-deformational interaction that is taken into account within the mean field approach. It is demonstrated that the external electric field gives an unique possibility of manipulation by the polarizability, magnetic and spectroscopic characteristics of cyanide-bridged Fe-Co clusters.

New exotic phenomena have recently been discovered in oxides of paramagnetic Ir4+ ions, widely known as ‘iridates'. Their remarkable properties originate from concerted effects of the crystal field, magnetic interactions and strong spin-orbit coupling, characteristic of 5d metal ions. Despite numerous experimental reports, the electronic structure of these materials is still challenging to elucidate, and not attainable in the isolated, but chemically inaccessible, [IrO6]8– species (the simplest molecular analogue of the elementary {IrO6}8− fragment present in all iridates). Here, we introduce an alternative approach to circumvent this problem by substituting the oxide ions in [IrO6]8− by isoelectronic fluorides to form the fluorido-iridate: [IrF6]2−. This molecular species has the same electronic ground state as the {IrO6}8− fragment, and thus emerges as an ideal model for iridates. These results may open perspectives for using fluorido-iridates as building-blocks for electronic and magnetic quantum materials synthesized by soft chemistry routes., Iridates are known to exhibit a range of exotic electronic and magnetic behaviours but it is difficult to prepare isolated [IrO6]8− species via soft chemical routes. Here, the authors isolate the isoelectronic [IrF6]2− complex, and assess it as a model and for iridate analogues.

The spin crossover phenomena in Co(II) compounds are in the focus of the present paper. A microscopic theoretical approach for the description of spin transitions in mononuclear Co(II) compounds is suggested. Within the framework of this approach there are taken into account two types of interionic interactions that may be operative in the problem such as the electron-deformational interaction and the cooperative Jahn-Teller interaction arising from the coupling of the low-spin state of the Co(II) ion with the tetragonal vibrations of the nearest surrounding. The different role of these interactions in the spin transformation is demonstrated and discussed. On the basis of developed approach a qualitative and quantitative explanation of the experimental data on the temperature dependence of the magnetic susceptibility for the [Co(pyterpy)2](PF6)2, [Co(pyterpy)2](TCNQ)2⋅DMF⋅MeOH and [Co(pyterpy)2](TCNQ)2⋅MeCN⋅MeOH compounds is given.

A heterometallic hexanuclear mixed valence Co-Dy cluster [Co4Dy2(OH)2(O2CCMe3)8(HO2CCMe3)2(teaH)2(N3)2]·2(EtOH) (1) has been prepared from the reaction of dinuclear Co(II) pivalate with triethanolamine and azide ligands and characterized by elemental analysis IR spectroscopy and X ray crystallography. The metal atoms in the {CoII2CoIII2DyIII2} cluster core are bridged by two µ3 hydroxy groups two azide N3- anions six pivalate residues and two doubly deprotonated teaH2-ligands. A theoretical model has been developed to explain the magnetic behavior of 1. The model takes into account the crystal fields acting on the DyIII ions and the anti and ferromagnetic exchange interactions in the pairs Co-Dy and Co-Co respectively. The Stark structure for the DyIII ions is calculated with due account of the covalence effects within the frames of the exchange charge model of the crystal field. It is demonstrated that at low temperatures the magnetic properties of the cluster are determined by the interaction of the ground state Kramers doublet of the CoII ion with those Stark levels of the DyIII ions for which the energies are smaller than the spin-orbital coupling parameter. At higher temperatures the population of the CoII ion multiplets with the angular momentum values 3/2 and 5/2 leads to the increase of the magnetic susceptibility. The observed temperature dependence of the ?MT product and field dependence of the magnetization are satisfactorily explained within the framework of the suggested model.

The ligand-centered radical complex [(CoTPMA)2 -μ-bmtz(.-) ](O3 SCF3 )3 ⋅CH3 CN (bmtz=3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA=tris-(2-pyridylmethyl)amine) has been synthesized from the neutral bmtz precursor. Single-crystal X-ray diffraction studies have confirmed the presence of the ligand-centered radical. The Co(II) complex exhibits slow paramagnetic relaxation in an applied DC field with a barrier to spin reversal of 39 K. This behavior is a result of strong antiferromagnetic metal-radical coupling combined with positive axial and strong rhombic anisotropic contributions from the Co(II) ions.

A new microscopic approach to the problem of cooperative spin crossover in molecular crystals containing polynuclear complexes as structural units has been developed. The cooperative intercluster interaction in the crystal is shown to arise from the coupling of the molecular modes to the acoustic phonons. The interaction between ions, which belong to the same cluster, is also taken into account, that leads to interesting features in the temperature dependence of the magnetic susceptibility, which are not observed in mononuclear molecular crystals. The developed general approach is adapted to the case of tetranuclear square complexes. In the latter, the effect of intracluster interactions on the type of the spin transition is analyzed. The applicability of the developed approach is illustrated by the interpretation of the experimental data on the spin transition in the tetranuclear [Fe(tpa){N(CN)2}]4·(BF4)4·(H2O)2 cluster compound. For this compound, the theory well reproduces the observed two-step spin tr...

New tetranuclear and octanuclear mixed-valent cobalt(II/III) pivalate clusters, namely, [NaCo

The manifestations of an external direct current (dc) electric field on the magnetic and polarizability characteristics of trimeric CuII–CuII–CuIII mixed-valence clusters were examined. The temperature and field dependence of the magnitude of the mean dipole moment as well as of its components were calculated, and the anisotropy of the cluster polarizability was predicted. The low-temperature limit of the cluster magnetic moment is determined by the competition between the tunneling and Heisenberg interactions and the interactions with the external electric field. The transition between two paramagnetic states of the cluster caused by the interaction with the electric field leads to the nontrivial field dependence of the magnetic moment. The external electric field offers a unique possibility to manipulate the polarizability and magnetic characteristics of trimeric mixed-valence clusters.

The structures of different titania, vanadia, and vanadia–titania clusters located on the surface of mesoporous silica SBA-15 are optimized using density functional theory (DFT). The apparent absor...

The manifestations of single ion anisotropy of the Dy III ion in the key physical properties of the [L 2 Ni 2 Dy][ClO 4 ] single molecule magnet have been examined. The crystal field acting on the Dy III ion and the exchange interaction coupling the Dy III and Ni II ions are included in the model. It is shown that the role of the Ni II ions is only to increase the total angular momentum of the cluster via coupling through superexchange with the Dy III ion. The model describes well the observed temperature dependence of the magnetic susceptibility and the field dependence of magnetization.

In this paper we present a microscopic model for the cooperative spin-crossover (SCO) phenomenon in crystals containing cyano-bridged pentanuclear clusters {[MIII(CN)6]2[M′ II(tmphen)2]3} (M/M′ = Co/Fe, 1; Fe/Fe, 2) with a trigonal bipyramidal (TBP) structure. The low-spin to high-spin (ls–hs) transition is considered as a cooperative phenomenon that is driven by the interaction of electronic shells of FeII ions with totally symmetric deformation of the local environment that is extended over the crystal lattice via the acoustic phonon field. Due to proximity of FeII ions in the cluster, the short-range intracluster interaction between these ions via the optical phonon field is included as well. The model takes into consideration also the spin–orbit coupling operating within the cubic 5T2(t24e2) term of the hs-FeII ions. For cluster 2 the Heisenberg-type exchange interaction between the hs-FeII and ls-FeIII ions is taken into account. The competition between short- and long-range interactions is shown to ...

We report the first single-molecule magnet (SMM) to incorporate the [Os(CN)(6)](3-) moiety. The compound (1) has a trimeric, cyanide-bridged Mn(III)-Os(III)-Mn(III) skeleton in which Mn(III) designates a [Mn(5-Brsalen)(MeOH)](+) unit (5-Brsalen=N,N'-ethylenebis(5-bromosalicylideneiminato)). X-ray crystallographic experiments reveal that 1 is isostructural with the Mn(III)-Fe(III)-Mn(III) analogue (2). Both compounds exhibit a frequency-dependent out-of-phase χ''(T) alternating current (ac) susceptibility signal that is suggestive of SMM behaviour. From the Arrhenius expression, the effective barrier for 1 is found to be Δ(eff)/k(B)=19 K (τ(0)=5.0×10(-7) s; k(B)=Boltzmann constant), whereas only the onset (1.5 kHz, 1.8 K) of χ''(T) is observed for 2, thus indicating a higher blocking temperature for 1. The strong spin-orbit coupling present in Os(III) isolates the E'(1g(1/2))(O(h)*) Kramers doublet that exhibits orbital contributions to the single-ion anisotropy. Magnetic susceptibility and inelastic neutron-scattering measurements reveal that substitution of [Fe(CN)(6)](3-) by the [Os(CN)(6)](3-) anion results in larger ferromagnetic, anisotropic exchange interactions going from quasi-Ising exchange interactions in 2 to pure Ising exchange for 1 with J(parallel)(MnOs)=-30.6 cm(-1). The combination of diffuse magnetic orbitals and the Ising-type exchange interaction effectively contributes to a higher blocking temperature. This result is in accordance with theoretical predictions and paves the way for the design of a new generation of SMMs with enhanced SMM properties.

To explain the single-molecule magnet behavior of the mononuclear complex [(tpaMes)Fe](-) we have developed a model that takes into account the trigonal ligand field splitting of the atomic (5)D term of the Fe(II) ion, and the spin-orbital splitting and mixing of the ligand field terms. The ground ligand field term is shown to be the orbital doublet (5)E possessing an unquenched orbital angular momentum. We demonstrate that the splitting of this term cannot be described by the conventional zero-field splitting Hamiltonian proving thus the irrelevance of the spin-Hamiltonian formalism in the present case. The first-order orbital angular momentum is shown to lead to the strong magnetic anisotropy with the trigonal axis being the easy axis of the magnetization.

This article is a part of our efforts to control the magnetic anisotropy in cyanide-based exchange-coupled systems with the eventual goal to obtain single-molecule magnets with higher blocking temperatures. We give the theoretical interpretation of the magnetic properties of the new pentanuclear complex {[Ni(II)(tmphen)(2)](3)[Os(III)(CN)(6)](2)} x 6 CH(3)CN (Ni(II)(3)Os(III)(2) cluster). Because the system contains the heavy Os(III) ions, spin-orbit coupling considerably exceeds the contributions from the low-symmetry crystal field and exchange coupling. The magnetic properties of the Ni(II)(3)Os(III)(2) cluster are described in the framework of a highly anisotropic pseudo-spin Hamiltonian that corresponds to the limit of strong spin-orbital coupling and takes into account the complex molecular structure. The model provides a good fit to the experimental data and allows the conclusion that the trigonal axis of the bipyramidal Ni(II)(3)Os(III)(2) cluster is a hard axis of magnetization. This explains the fact that in contrast with the isostructural trigonal bipyramidal Mn(III)(2)Mn(II)(3) cluster, the Ni(II)(3)Os(III)(2) system does not exhibit the single-molecule magnetic behavior.

In the present article, we proffer an explanation for the origin of single molecule magnetism in [CuIILTbIII(hfac)2]2, a member of the novel class of mixed transition−lanthanide metal clusters. The theoretical model takes into account the crystal field acting on the TbIII ion as well as the exchange interaction between the TbIII and CuII ions and provides a good account of the low-lying energy levels, rigorously measured by inelastic neutron scattering and magnetic data. We demonstate that the single molecule magnet (SMM) behavior is not a sole consequence of the low-lying levels of the lanthanide metal ion but a property of the tetranuclear cluster itself. The energy levels are shown to increase with the decrease of the mean value of the Z-projection of the total angular momentum of the cluster, thus forming a barrier for magnetization reversal that is wholly consistent with the observed SMM behavior. On the basis of this study, recommendations are formulated on how the SMM properties for nd−4f clusters ...

We report a model for the explanation of the single molecule magnet behavior of the [Cu II LTb III (hfac) 2 ] 2 cluster. The model takes into account the crystal field acting on the Tb III ion and the exchange interaction between Tb III and Cu II ions. The energies of the low-lying levels are shown to increase with the decrease of the mean value of the Z-projection of the total angular momentum of the cluster, thus forming a barrier for magnetization reversal that is in accordance with the experimental evidence.

The electronic structures of the compounds K[(5-Brsalen)2(H2O)2-Mn2MIII(CN)6]·2H2O (MIII = CoIII, CrIII, FeIII) have been determined by inelastic neutron scattering (INS) and magnetic susceptibility studies, revealing the manganese(III) single-ion anisotropy and exchange interactions that define the low-lying states of the Mn−MIII−Mn trimeric units. Despite the presence of an antiferromagnetic intertrimer interaction, the experimental evidence supports the classification of both the Cr(III) and Fe(III) compounds as single-molecule magnets. The value of 17(2) cm−1 established from AC susceptibility measurements for a spin-reversal barrier of K[(5-Brsalen)2(H2O)2-Mn2Cr(CN)6]·2H2O may be readily rationalized in terms of the energy level diagram determined directly by INS. AC susceptibility measurements on samples of K[(5-Brsalen)2(H2O)2-Mn2Fe(CN)6]·2H2O are contrary to those previously reported, exhibiting but the onset of peaks below temperatures of 1.8 K at oscillating frequencies in the range of 100−800 H...

In this article we report for the first time experimental details concerning the synthesis and full characterization (including the single-crystal X-ray structure) of the spin-canted zigzag-chain compound [Co(H2L)(H2O)]infinity [L = 4-Me-C6H4-CH2N(CPO3H2)2], which contains antiferromagnetically coupled, highly magnetically anisotropic Co(II) ions with unquenched orbital angular momenta, and we also propose a new model to explain the single-chain magnet behavior of this compound. The model takes into account (1) the tetragonal crystal field and the spin-orbit interaction acting on each Co(II) ion, (2) the antiferromagnetic Heisenberg exchange between neighboring Co(II) ions, and (3) the tilting of the tetragonal axes of the neighboring Co units in the zigzag structure. We show that the tilting of the anisotropy axes gives rise to spin canting and consequently to a nonvanishing magnetization for the compound. In the case of a strong tetragonal field that stabilizes the orbital doublet of Co(II), the effective pseudo-spin-1/2 Hamiltonian describing the interaction between the Co ions in their ground Kramers doublet states is shown to be of the Ising type. An analytical expression for the static magnetic susceptibility of the infinite spin-canted chain is obtained. The model provides an excellent fit to the experimental data on both the static and dynamic magnetic properties of the chain.

The luminescence properties of the ZnAl 2(1− х ) Cr 2 х S 4 single crystals with different chromium content are investigated. The emission spectra of the samples with a small amount of chromium ( х ⩽ 0.005) consist of the R-line ( E R = 1.61 eV) and its phonon side band. At increasing of chromium concentration two new U-bands centered at Е U(a) = 1.45 eV and Е U(b) = 1.3 eV appear in the emission spectra. It is assumed, that the increase of the new broad bands intensity with increasing of chromium concentration takes place due to the formation of nonequivalent Cr +3 -ions occupying octahedral sites, in the first cationic coordination sphere of which one or two aluminum ions are replaced by chromium ions.

We report a model for the explanation of the single molecule magnet behavior of the [CuIILTbIII(hfac)2]2 cluster. The model takes into account the crystal field acting on the TbIII –ion and the exchange interaction between the TbIII and CuII ions. The energies of the low-lying levels are shown to increase with the decrease of the mean value of the z-projection of the total angular momentum of the cluster, thus forming a barrier for magnetization reversal that is in accordance with the experimental evidence.

We study the effects of the external direct current (dc) electric field in the magnetic and polarizability characteristics of trimericCu(II)-Cu(II)-Cu(III) mixed valence clusters. We calculate the temperature and field dependence of the magnitude of the mean dipole moment as well as of its components. We predict the anisotropy of the cluster polarizability. The magnetic moment at the low temperature limit is determined by the competition between the tunnel and Heisenberg interactions and the interaction with the external electric field.It is demonstrated that the external electric field gives an unique possibility of manipulation by the polarizability and magnetic characteristics of trimeric mixed-valence clusters.

The experimental and theoretical study of a family of linear trimeric [NEt4] {[Mn(L)]2[M(CN)6]} (M = Fe, Ru, Os) compounds is reported. All three complexes demonstrate slow relaxation of magnetization at low temperatures. The theoretical model for the explanation of the magnetic behavior (the magnetic susceptibility as a function of temperature and magnetization as a function of the applied magnetic field) includes the spin-orbital interaction acting within the ground 2 T 1 multiplet of the FeIII, RuIII or OsIII ion, the axial crystal field that splits this multiplet into an orbital singlet and orbital doublet, the zero-field splitting for the MnIII ions, the isotropic exchange interaction in the M-Mn pairs as well as the intercluster interaction taken within the mean field approximation. The proposed model provides a good agreement between the observed and calculated magnetic behavior. The low-lying energy levels form the energy barriers for magnetization reversal for all examined compounds. The calculated heights of these barriers are close to the observed ones.

The experimental and theoretical study of two mononuclear dysprosium-based complexes is reported. Both complexes demonstrate slow relaxation of magnetization at low temperatures. The Hamiltonian for explanation of the magnetic behavior (the magnetic susceptibility as a function of temperature and magnetization as a function of the applied magnetic field) includes the crystal field of the nearest ligands of the dysprosium ion and the interaction with the external magnetic field. The crystal field parameters are evaluated in the framework of the exchange charge model, taking account for covalence effects. The proposed model provides a good agreement between the observed and calculated magnetic behavior. It is demonstrated that the low-lying energy levels for both studied complexes form the energy barriers for magnetization reversal that agrees with the observed single-ion magnetic behavior.

A theoretical model is developed for the explanation of the magnetic properties of the [MnCl] 4 [Re(triphos)(CN) 3 ] 4 single molecule magnet, which represents a 3d–5d molecular cube compressed along one of the C 3 axes. In the model, the strong local trigonal crystal field on each Re(II) ion is assumed to split the ground cubic 2 T 2 ( t 2 5 ) term into the states 2 A 1 and 2 E in such a way that 2 E term proves to be the ground one. The ground Kramers doublet that appears under the spin–orbit splitting of the 2 E term is shown to be fully anisotropic ( g ⊥ = 0) and the magnetic superexchange (mediated by cyanide bridge) between Re and Mn ions is approximated by the Ising-like Hamiltonian. In this case, the magnetic anisotropy of the trigonally distorted cube represents a first order effect that is totally incorporated in the effective Ising-like Hamiltonian, including the pseudo-spin-1/2 operator for the Re(II) ion and the operator of the true spin-5/2 for the Mn(II) ion. The developed pseudo-spin-1/2 formalism is applied to the explanation of the DC magnetic susceptibility and magnetization data, and the set of the best fit parameters is found. The temperature dependence of the DC magnetic susceptibility and the field dependence of the magnetization calculated with this set of parameters are in accord with the experimental data being at the same time compatible with the existence of the barrier for magnetization reversal. Finally, the applicability of the quantum–classical spin approach for the calculation of the magnetic properties of large 3d–5d clusters is discussed.

The effects of vibronic interaction in the formation of the barrier for magnetization reversal in the cyanide-based binuclear Mn(III)–CN–Mn(II) clusters containing octahedrally coordinated Mn(III) and Mn(II) ions were examined. Along with the orbitally dependent superexchange between the Mn(II) and Mn(III) ions the model includes an axial component of the crystal field, spin–orbit coupling and vibronic interaction operating within the ground 3 T 1 ( t 2 4 ) term of the low-spin Mn(III) ion. The vibronic coupling parameters for the Mn(III) ion in the octahedral surrounding of six carbon ions have been evaluated in the framework of the exchange charge model of the crystal field accounting for the exchange and covalence effects. The dominating role of the tetragonal vibrations for the 3 T 1 term of a single Mn(III) ion was demonstrated. The ability of the investigated complex to act as a Single-Molecule Magnet was analyzed as a function of both the temperature and the strength of vibronic coupling with the tetragonal vibrations. Very strong vibronic interaction strongly facilitates tunneling through the barrier, i.e., suppresses the barrier for magnetization reversal and prevents from long life time of magnetization.

The present contribution is aimed at the elaboration of the model of magnetic relaxation in the cyano-bridged pentanuclear Mn(III) 2 Mn(II) 3 cluster that belongs to a new family of single molecule magnets (SMM) containing ions with unquenched orbital angular momenta. We proceed from the energy pattern of the cluster formed by the trigonal component of the crystal field acting on the ground-state cubic terms 4 T 1 ( t 2 4 ) of the Mn(III)-ions, spin–orbital interaction and Heisenberg exchange between Mn(II) and Mn(III) ions. The ground state of the cluster possesses the total angular momentum projection ∣ M J ∣ = 15/2, while the energies of the excited states increase with decreasing ∣ M J ∣ values, thus giving rise to a barrier for the reversal of magnetization. The monophonon transitions between the states ∣ M J 〉 and ∣ M J ± 1〉, ∣ M J ± 2〉 induced by electron–vibrational interaction are shown to be allowed. The rates of all possible transitions between the states with 1/2 M J ∣ T n M J ( t ) of the ∣ M J 〉 states of the Mn(III) 2 Mn(II) 3 clusters. The relaxation time is shown to diminish from 10 12 s to 10 s with decrease in temperature from 1 K to 3 K for the cluster [Mn(III)(CN) 6 ] 2 [Mn(II)(tmphen) 2 ] 3 (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) with the trigonal crystal field parameter Δ = −251 cm −1 . The obtained values of the relaxation time are in qualitative agreement with the temperature dependence of the ac susceptibilities observed for this SMM. In order to reveal the possibility of enhancing the relaxation time of magnetization in the family of clusters Mn(III) 2 Mn(II) 3 we vary the trigonal crystal field parameter ∣ Δ ∣( Δ Δ ∣ leads to a considerable growth of the relaxation time.

In this article, we report the theoretical evaluation of the vibronic constants for the spinel-type crystals ZnAl2S4, ZnAl2Se4, CdAl2Se4, and CdIn2S4 doped with the transition metal ions Cr3+, Cr4+, Ti2+, and Ti3+. In these crystals the ions Cr3+, Ti3+ and Cr4+, Ti2+ occupy positions with octahedral and tetrahedral surrounding correspondingly. In the case of a tetrahedral complex the interaction with full symmetric A1, tetragonal E and two trigonal vibrations T 2 ( 1 ) and T 2 ( 2 ) is considered, while for the octahedral cluster along with the A1 and E vibrations only one trigonal vibrational mode is taken into account. The examined semiconductor systems are mainly covalent and application of the crystal field point charge model loses proper exactity. For this reason, we employ the exchange charge model for the crystal field that accounts for the covalence effects and provides relatively simple expressions for the crystal field and vibronic parameters keeping at the same time a reasonable level of accuracy. The vibronic coupling constants are numerically calculated and for each case an appropriate multi-mode Jahn–Teller vibronic problem is found out.

A microscopic model of nonequilibrium light-induced phase transitions in spin crossover systems based on the solution of the equations for the density operator is developed. The conditions of light-induced and thermally induced bistability are revealed. The results are compared with those obtained in the framework of the Glauber's approach. The qualitative explanation of experimental data on a series of spin crossover systems is given.

The aim of this communication is to probe the possibility of increasing the barrier for reversal of magnetization in the family of new cyano-bridged pentanuclear Mn(III)2Mn(II)3 clusters in which single molecule magnet behavior has been recently discovered. In this context, we analyze the global magnetic anisotropy arising from the unquenched orbital angular momenta of ground terms (3)T1(t2(4)) of the two apical Mn(III) ions. The model takes into account the trigonal component of the crystal field, spin-orbit interaction in (3)T1(t2(4)), and an isotropic exchange interaction between Mn(III) and Mn(II) ions. The height of the barrier is shown to be sensitive to the change of the trigonal field stabilizing orbital doublet (3)E, which carries the first-order orbital magnetic contribution and enhances with an increase of the trigonal field. This result is expected to be useful for the more rational design of new cyano-bridged SMMs with high blocking temperatures.

A theoretical study of the vibronic Jahn-Teller interaction in a series of II–VI crystals doped with Cr 2+ ion is reported. The vibronic coupling parameters for the tetrahedrally coordinated Cr 2+ ion (high-spin d 4 configuration) are evaluated in the framework of the exchange charge model of the crystal field accounting for the exchange and covalence effects. The calculations demonstrate the leading role of the trigonal vibrations for the 5 T 2 term although the contribution of tetragonal modes is also important; the full symmetric and tetragonal vibrations for the 5 E term significantly contribute to the phonon broadening of the absorption and emission bands related to the 5 E ↔ 5 T 2 transition in Cr 2+ ion.

In this article we report the study of the vibronic Jahn–Teller interaction in CdSe crystals doped with Cr2+ ions. The vibronic coupling constants are evaluated in the framework of the exchange charge model of the crystal field accounting for the exchange and covalence effects. The calculations demonstrate the leading role of the trigonal vibrations for the 5T2 term although the contribution of tetragonal modes is also important; the full symmetric and tetragonal vibrations for the 5E term significantly contribute to the formation of the optical band 5E ↔ 5T2 for CdSe:Cr2+ system.

An explanation is presented for the unusual magnetic behavior of two Re(II) compounds, [Re(triphos)(CH3CN)3][BF4]2 and [Et4N][Re(triphos)(CN)3], for which magnetic susceptibility data obtained on powder samples (2-300 K) suggest unusually strong temperature-independent paramagnetism ( TIP ) (1.4-1.8) 10 -3 cm 3 mol -1 ) and small low-temperature effective magnetic moments. A model is developed based on the jj-coupling scheme appropriate for description of the 5d shell of a Re(II) ion in a crystal field. The model accounts for a cubic crystal field, strong spin-orbit coupling, and a trigonal component of the crystal field produced by the ligand field acting on the Re(II) ions. The last two interactions act within the truncated basis containing eight lowest, strong cubic crystal field terms and result in the stabilization of two closely spaced Kramers doublets originating mainly from the 2 T2(t2 ) term. Efficient mixing of these Kramers doublets with those arising from 6 A1(t 2 e 2 ) and 4 T1[t 2 ( 3 T1)e] terms is shown to result in the small low-temperature effective magnetic moments and anomalously strong temperature-independent paramagnetism, in accordance with the observed magnetic behavior. The model perfectly reproduces the T vs T dependences over a wide temperature range (2-300 K), and the energy pattern obtained with the set of the best fit parameters provides a qualitative explanation for the observed light absorption and diffuse reflectance in a wide spectral area (200 -2600 nm). The theoretical consideration predicts extremely strong anisotropy of the magnetic susceptibility and g-factors for both compounds with a C3 easy axis of magnetization.

A new model for the photochromic effect in sodium nitroprusside Na2[Fe(CN)5(NO)]·2H2O based on the concept of the pseudo-Jahn–Teller effect is proposed. The model takes into account the electron transfer from the Fe2+ ion to the π* orbitals of the NO-ligand as well as the vibronic mixing of three electronic states of the Fe NO fragment through the non-symmetric and full-symmetric modes. The problem is solved within the adiabatic approximation. Under certain conditions the lower sheet of the adiabatic potential is shown to possess three minima with the increasing energies that correspond to the N-bound, sideways bound and O-bound NO group. The barriers between the minima are estimated and the excited minima are attributed to the MS2 and MS1 metastable states observed experimentally.

The model employed takes into account the main physical factors responsible for the temperature transformations of optical spectra and magnetic properties: intramolecular transfer of the excess electron, vibronic and exchange interactions. For a single valence tautomeric complex the vibronic problem of pseudo-Jahn-Teller effect is solved. The magnetic moment is shown to increase gradually from 1.73μB to 4.34μB. The spectrum of the isolated complex in the near infrared range is revealed to represent a band related to the light induced transfer of an electron from the catecholate ligand to the metal ion. The suggested model is in qualitative agreement with the experimental data.

In this article, we present a theoretical microscopic approach to describe the magnetic and spectroscopic behavior of multifunctional hybrid materials which demonstrate spin crossover and ferromagnetic ordering. The low-spin to high-spin transition is considered as a cooperative phenomenon that is driven by the interaction of the electronic shells of the Fe ions with the full symmetric deformation of the local surrounding that is extended over the crystal lattice via the acoustic phonon field. The proposed model is applied to the analysis of the series [Fe(III)(sal2-trien)] [Mn(II)Cr(III)(ox)3]·solv, in short 1·solv, where solv = CH2Cl2, CH2Br2, and CHBr3.

The energy states of tetrameric iron clusters with d6-d6-d6-d5 and d5-d5-d5-d6 electronic configurations, which form the metal frameworks of the [Fe4S4]+ and [Fe4S4]3+ iron-sulfur proteins, are calculated using a general approach. It is revealed that the structural distortions of these systems significantly affect the composition of the magnetic states. The effect of the isotropic Heisenberg exchange is considered.

To identify the origin of the $XY$ spin dimensionality in the bilayered system ${$Cu${}_{4}$(tetrenH${}_{5}$)[W(CN)${}_{8}$]${}_{4}\ifmmode\cdot\else\textperiodcentered\fi{}$7.2H${}_{2}$O${}}_{n}$ (WCuT) we use a combination of single-crystal experiments (bulk magnetization, neutron flipping ratio, electron magnetic resonance, neutron diffraction) and theoretical modeling (exchange-charge model of the crystal field, dipolar energy, and density functional calculations). Our experiments show that the magnetic properties of WCuT are anisotropic and two-dimensional correlations build up below 70 K. The hard anisotropy axis is perpendicular to the layers ($b$ axis) and a small anisotropy within the $ac$ layers is present. Modeling of the crystal field validates treatment of tungsten and copper as spin $S=\frac{1}{2}$ ions with anisotropic $g$ values. The local magnetic anisotropy results from the common action of the crystal field and spin-orbit coupling and is along the $c$ axis for both ions. Density functional calculations identify the origin of the ferromagnetic exchange in different energies and symmetries of the tungsten- and copper-dominated orbitals and anticipate different exchange couplings across the apical (along the $b$ axis) and equatorial (in the $ac$ plane) Cu-CN-W bridges due to difference in the hybridization efficiency. Calculation of the dipolar energy for various spin configurations suggests that dipolar interactions play a decisive role in the $ac$-planar anisotropy in this system. We propose that the effective $XY$ spin dimensionality in WCuT is established by a combination of the axial local anisotropy of the W and Cu ions and the long-range magnetic dipolar interactions on the bilayered square lattice.

9 pages; International audience; In this report, we present combined experimental and theoretical studies of the trinuclear {[L2Co2Tb]- [NO3]} (LH3 = (S)P[N(Me)NCHC6H3-2-OH-3- OMe]3) complex exhibiting single molecule magnet properties. Our theoretical model includes the axial crystal fields acting on the TbIII and CoII ions, the spin−orbit interaction operating within the 4T1 ground term of the CoII ion, and the ferromagnetic exchange between Tb and Co magnetic centers. We demonstrate for the [L2Co2Tb]+ complex that the strong negative single-ion anisotropy of both TbIII and CoII ions is the reason for the observed slow relaxation of magnetization. The elaborated model describes perfectly the experimental magnetic properties of the complex as a function of temperature and the external magnetic field.

New exotic phenomena have recently been discovered in oxides of paramagnetic Ir(4+) ions, widely known as 'iridates'. Their remarkable properties originate from concerted effects of the crystal field, magnetic interactions and strong spin-orbit coupling, characteristic of 5d metal ions. Despite numerous experimental reports, the electronic structure of these materials is still challenging to elucidate, and not attainable in the isolated, but chemically inaccessible, [IrO6](8-) species (the simplest molecular analogue of the elementary {IrO6}(8-) fragment present in all iridates). Here, we introduce an alternative approach to circumvent this problem by substituting the oxide ions in [IrO6](8-) by isoelectronic fluorides to form the fluorido-iridate: [IrF6](2-). This molecular species has the same electronic ground state as the {IrO6}(8-) fragment, and thus emerges as an ideal model for iridates. These results may open perspectives for using fluorido-iridates as building-blocks for electronic and magnetic quantum materials synthesized by soft chemistry routes.

The present work is aimed at the elaboration of the model of magnetic properties and magnetic relaxation in the mononuclear [Pc(2)Tb](-)TBA(+) complex that displays single-molecule magnet properties. We calculate the Stark structure of the ground (7)F(6) term of the Tb(3+) ion in the exchange charge model of the crystal field, taking account for covalence effects. The ground Stark level of the complex possesses the maximum value of the total angular momentum projection, while the energies of the excited Stark levels increase with decreasing |M(J)| values, thus giving rise to a barrier for the reversal of magnetization. The one-phonon transitions between the Stark levels of the Tb(3+) ion induced by electron-vibrational interaction are shown to lead to magnetization relaxation in the [Pc(2)Tb](-)TBA(+) complex. The rates of all possible transitions between the low-lying Stark levels are calculated in the temperature range 14 KT40 K. With the purpose of calculation of the temperature dependence of the relaxation time of magnetization, we solve the set of master equations for the populations of the Stark levels. The relaxation time is shown to diminish from 3.2 × 10(-2) s to 1.52 × 10(-4) s as the temperature increases from 27 K to 40 K. The obtained values of the relaxation time are in satisfactory agreement with the observed ones. The developed model also provides satisfactory description of the dc-magnetic data and paramagnetic shifts.