Papers by Vladimir S Mironov
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CASPT2 calculations are performed on the dodecahedral and square antiprismatic isomers of the [Mo... more CASPT2 calculations are performed on the dodecahedral and square antiprismatic isomers of the [Mo(CN) 8 ] 4-and [W(CN) 8 ] 4-complexes. The high-energy experimental bands above 40000 cm-1 are assigned to MLCT transitions. The experimental observed trend of the extinction coefficients for the molybdenum and tungsten complex is reproduced by our CASSCF oscillator strengths. All bands below 40000 cm-1 can be ascribed to ligand-field transitions, although small contributions from forbidden MLCT transitions cannot be excluded. In order to account for all experimental bands in the electronic spectrum of these octacyanocomplexes, a dynamic equilibrium in solution between the two isomeric forms must be hypothesized. Spin−orbit coupling effects are found to be more important for the square antiprismatic isomers; in particular, large singlet−triplet mixings are calculated for this isomer of [W(CN) 8 ] 4-. Ligand-field and Racah parameters as well as spin−orbit coupling constants are determined on the basis of the calculated transition energies. The obtained values for these parameters support the recently proposed model for exchange interactions in magnetic clusters and networks containing pentavalent octocyanometalates of molybdenum and tungsten.
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State of the art CASSCF and CASPT2 calculations have been performed to elucidate the nature of fe... more State of the art CASSCF and CASPT2 calculations have been performed to elucidate the nature of ferromagnetism of Co II −NC−W V pairs in the three-dimensional compound [{W V (CN) 2 } 2 {(µ-CN) 4 Co II (H 2 O) 2 } 3 ‚4H 2 O] n , which has been recently synthesized and investigated by a number of experimental techniques (Herrera, J. M.; Bleuzen, A.; Dromzée, Y.; Julve, M.; Lloret, F.; Verdaguer, M. Inorg. Chem. 2003, 42, 7052−7059). In this network, the Co ions are in the high-spin (S) 3 / 2) state, while the single unpaired electron on the W centers occupies the lowest orbital of the d z 2 type of the 5d shell. In agreement with the suggestion made by Herrera et al., we find that the ferromagnetism is due to a certain occupation scheme of the orbitals from the parent octahedral t 2g shell on Co II sites, in which the orbital accommodating the unpaired electron is orthogonal to the d z 2 orbitals of the surrounding W ions. We investigate the stabilization of such an orbital configuration on the Co sites and find that it cannot be achieved in the ground state of isolated mononuclear fragments [Co II (NC) 4 (OH 2) 2 ] 2-for any conformations of the coordinated water molecules and Co−N−C bond angles. On the other hand, it is stabilized by the interaction of the complex with neighboring W ions, which are simulated here by effective potentials. The calculated exchange coupling constants for the Co II −NC−W V binuclear fragments are in reasonable agreement with the measured Curie−Weiss constant for this compound. As additional evidence for the inferred electronic configuration on the Co sites, the ligand-field transitions, the temperature-dependent magnetic susceptibility, and the field-dependent low-temperature magnetization, simulated ab initio for the mononuclear Co fragments, are in agreement with the available data for another compound [W IV {(µ-CN) 4-Co II (H 2 O) 2 } 2 ‚4H 2 O] n containing diamagnetic W and high-spin Co ions in an isostructural environment.
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Exchange interaction between two lanthanide or actinide ions of f 1 configuration bridged by comm... more Exchange interaction between two lanthanide or actinide ions of f 1 configuration bridged by common diamagnetic ligands is theoretically studied using a modified version of the superexchange theory developed in this paper. Exchange spin Hamiltonians were calculated for the M 2 L 10 and M 2 L 11 dimers serving as models of the 90 • and 180 • f 1 –f 1 superexchange, respectively. Spin–orbit coupling and crystal field splitting of the f 1 configuration (resulting in the 7 ground Kramers doublet and the effective spin S = 1 2 of the metal ion), virtual transfers of electrons of the type 4f 1 (A)–4f 1 (B) → 4f 0 (A)–4f 1 (B)5d 1 (B) via ns(L) and np(L) valent orbitals of the bridging ligands, and exchange pathways in these dimers are considered in detail. The f 1 –f 1 superexchange is found to be extremely anisotropic and very sensitive to the geometry of the dimer. The spin Hamiltonian of the M 2 L 10 dimer is H = J x S x A S x B + J y S y A S y B + J z S z A S z B , where the exchange parameters are rationalized in terms of J πσ and J ππ parameters referring, respectively, to the π–σ and π–π pathways of the 4f(A) → np(L) → 5d(B) electron transfers, J x = 2J πσ − J ππ , J y = J πσ + J ππ and J z = −J πσ + J ππ. The J πσ and J ππ values are analytically expressed through 4f|np and 5d|np overlap integrals, orbital energies and intra-ionic Slater parameters. Exchange interaction between f 1 ions in the M 2 L 11 dimer is described by an antiferromagnetic Ising Hamiltonian H = |J ππ |S z A S z B , where the z axis connects two metal ions. Unusual magnetic properties of MUO 3 (M = Li, Na, K and Rb) and Li 3 UO 4 oxides involving U 5+ (5f 1) ions and BaPrO 3 distorted perovskite are discussed in the light of these theoretical results.
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31 31 Absorption and emission spectra of KY(WO) :Pr and KGd(WO) :Pr crystals are recorded at 77 K... more 31 31 Absorption and emission spectra of KY(WO) :Pr and KGd(WO) :Pr crystals are recorded at 77 K, and the experimental energy 4 2 4 2 31 31 level schemes of Pr ions are determined. Crystal field analysis for Pr ions is carried out using the true C point symmetry. The initial 2 31 set of crystal field parameters was obtained from superposition model calculations. Experimental crystal field energies of KY(WO) :Pr 4 2 31 21 and KGd(WO) :Pr crystals are fitted with the root mean squares deviation s516.4 and 17.5 cm , respectively. The consistency of 4 2 31 31 these sets of crystal field parameter for Pr ions is confirmed from comparative crystal field calculations for Nd ions doped in 21 KY(WO) crystal resulting in s58.2 cm .
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The dependence of the room-temperature magnetic anisotropy of lanthanide complexes on the type of... more The dependence of the room-temperature magnetic anisotropy of lanthanide complexes on the type of the coordination polyhedron and on the nature of the lanthanide ion is quantitatively analyzed in terms of a model approach based on numerical calculations. The aim of this study is to establish general regularities in the variation of the sign and magnitude of the magnetic anisotropy of lanthanide complexes at room-temperature and to estimate its maximal value. Except for some special cases, the variation of the sign of the magnetic anisotropy over the series of isostructural lanthanide complexes is found to obey a general sign rule, according to which CeIII, PrIII, NdIII, SmIII, TbIII, DyIII, and HoIII complexes have one sign of and EuIII, ErIII, TmIII, and YbIII complexes have the opposite sign. Depending on the specific coordination polyhedron, a maximal magnetic anisotropy is observed for TbIII, DyIII, or TmIII complexes, and its absolute value can reach 50 00010 6 cm 3 mol 1 or more. Results of the present study can be helpful for the analysis of the orientational behavior of lanthanide-containing liquid crystals and lanthanide-doped bilayered micelles in an external magnetic field. The use of the Bleaney theory in the quantitative analysis of the magnetic anisotropy of lanthanide compounds is shown to have limitations because of a large ratio between the crystal-field splitting energy of the ground multiplet of the lanthanide ion and the thermal energy at room-temperature.
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A general theoretical model of the origin of the magnetic anisotropy in paramagnetic metal-contai... more A general theoretical model of the origin of the magnetic anisotropy in paramagnetic metal-containing liquid crystals is developed. General relations between the molecular magnetic anisotropy of mesogenic lanthanide complexes and the macroscopic magnetic anisotropy of these liquid crystals in the mesophase are obtained. The net magnetic anisotropy of a real metallomesogen is shown to be the result of a complex interplay between the molecular magnetic anisotropy, orientation of the long molecular axis, and disorder effects. The sign of the magnetic anisotropy depends not only on the anisotropy of the tensor of molecular magnetic susceptibility, but also on the orientation of the long molecular axis of rodlike lanthanide complexes with respect to the principal magnetic axes of the molecular tensor of magnetic susceptibility. The influence of micro-and macroscopic disorder in real liquid crystals is discussed. Numerical parametric calculations were used to rationalize the variation of the magnitude and sign of the magnetic anisotropy in a series of isostructural lanthanide-containing metallomesogens. Experimental magnetic susceptibility and magnetic anisotropy of a series of LnLH 3 NO 3 3 compounds LH is a Schiff base are well reproduced by calculations based on the present model. Limitations of the Bleaney theory of magnetic anisotropy are analyzed.
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physica status solidi (b), 1992
ABSTRACT
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We have investigated the
single-molecule magnets [MnIII
2(5-
Brsalen)2ACHTUNGTRENUNG(MeOH)2MIII(C... more We have investigated the
single-molecule magnets [MnIII
2(5-
Brsalen)2ACHTUNGTRENUNG(MeOH)2MIII(CN)6]NEt4
(M=Os (1) and Ru (2); 5-Brsalen=
N,N’-ethylenebis(5-bromosalicylidene)
iminate) by frequency-domain Fourier-
transform terahertz electron paramagnetic
resonance (THz-EPR), inelastic
neutron scattering, and superconducting
quantum interference
device (SQUID) magnetometry. The
combination of all three techniques
allows for the unambiguous experimental
determination of the three-axis anisotropic
magnetic exchange coupling
between MnIII and RuIII or OsIII ions,
respectively. Analysis by means of a
spin-Hamiltonian parameterization
yields excellent agreement with all experimental
data. Furthermore, analytical
calculations show that the observed
exchange anisotropy is due to the bent
geometry encountered in both 1 and 2,
whereas a linear geometry would lead
to an Ising-type exchange coupling.
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CrystEngComm, 2009
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Physica B: Condensed Matter, 2010
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Inorganic Chemistry Communications, 2012
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Single-molecule magnets (SMMs) have been extensively studied in the last 10–15 years due to the i... more Single-molecule magnets (SMMs) have been extensively studied in the last 10–15 years due to the importance of their promising application for future molecular electronics (super-high-density storage of information , quantum computers, molecular spintronics) [1–5]. Single-molecule magnets are paramagnetic molecules capable of fixing magnetization [4]. This effect is observed in high-spin molecules for which the energy spectrum of low-lying spin levels (hereinafter, the spin spectrum) has the following specific features: (a) the ground level is represented by two degenerate quantum states with the magnetic moment projections ± M and (b) the ground level is separated by a considerable energy gap from the excited levels. In particular, such a spectrum is generated due to splitting of the ground spin multiplet S in a zero magnetic field for high-spin molecules with axial symmetry and negative magnetic anisotropy D. Since the splitting of the (2 S + 1)-fold degenerate ground spin multiplet S is given by D (M s is the spin projection), a double-well potential is formed at D < 0, its minima corresponding to the quantum states with spin projections M s = + S and M s = – S [4]. These minima are separated by the energy barrier U , which enters into the exponent of the Arrhenius temperature dependence τ = τ 0 exp of the paramag-netic relaxation time τ of a cluster. Formally, the magnetic anisotropy D is determined by the relationship U = | D | S 2. The barrier U = | D | S 2 blocks tunneling transitions between the spin states M s = + S and M s = – S and, thus, fixes the magnetization of the molecule.
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31 31 Absorption and emission spectra of KY(WO) :Pr and KGd(WO) :Pr crystals are recorded at 77 K... more 31 31 Absorption and emission spectra of KY(WO) :Pr and KGd(WO) :Pr crystals are recorded at 77 K, and the experimental energy 4 2 4 2 31 31 level schemes of Pr ions are determined. Crystal field analysis for Pr ions is carried out using the true C point symmetry. The initial 2 31 set of crystal field parameters was obtained from superposition model calculations. Experimental crystal field energies of KY(WO) :Pr 4 2 31 21 and KGd(WO) :Pr crystals are fitted with the root mean squares deviation s516.4 and 17.5 cm , respectively. The consistency of 4 2 31 31 these sets of crystal field parameter for Pr ions is confirmed from comparative crystal field calculations for Nd ions doped in 21 KY(WO) crystal resulting in s58.2 cm .
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The competition of the superexchange interaction and the cooperative Jahn –Teller (JT) effect in ... more The competition of the superexchange interaction and the cooperative Jahn –Teller (JT) effect in the first-order magnetic and structural phase transition in UO 2 is analyzed. The effective spin Hamiltonian of the superexchange interaction between the neighboring U 4þ ions in the cubic crystal lattice of UO 2 is calculated for the first time in terms of a specially adapted kinetic exchange model. The 5f 2 –5f 2 superexchange interaction is shown to be essentially non-Heisenberg: the effective spin Hamiltonian is anisotropic and contains large biquadratic terms, which formally correspond to a quadrupole –quadrupole interaction. The strength of the JT effect in UO 2 is estimated from the calculations of the linear vibronic coupling constants between the local e g
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Substitution of the organic cation X in the 1D polymer, (X) 2 [Mn(acacen)Fe(CN) 6 ], leads to an ... more Substitution of the organic cation X in the 1D polymer, (X) 2 [Mn(acacen)Fe(CN) 6 ], leads to an essential change in magnetic behavior. Due to the presence of more voluminous Ph 4 P + cations, the polyanion has a more geometrically distorted chain skeleton and, as a consequence, enhanced single chain magnet (SCM) characteristics compared to those for Et 4 N +. The Arrhenius relaxation energy barriers, the exchange interaction constant and the zero-field splitting anisotropy of Mn III are determined from the analysis of magnetic measurements. The discussion is supported with ligand field calculations for [Fe(CN) 6 ] 3− that unveils the significant anisotropy of Fe magnetic moments.
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Unusual spin coupling between Mo III and Mn II cyano-bridged ions in bimetallic molecular magnets... more Unusual spin coupling between Mo III and Mn II cyano-bridged ions in bimetallic molecular magnets based on the [Mo III (CN)7] 4-heptacyanometalate is analyzed in terms of the superexchange theory. Due to the orbital degeneracy and strong spin-orbit coupling on Mo III , the ground state of the pentagonal-bipyramidal [Mo III (CN)7] 4-complex corresponds to an anisotropic Kramers doublet. Using a specially adapted kinetic exchange model we have shown that the Mo III-CN-Mn II superexchange interaction is extremely anisotropic: it is described by an Ising-like spin Hamiltonian JS Mo z S Mn z for the apical pairs and by the JzS Mo z S Mn z + Jxy(S Mo x S Mn x + S Mo y S Mn y) spin Hamiltonian for the equatorial pairs (in the latter case Jz and Jxy can have opposite signs). This anisotropy resulted from an interplay of several Ising-like (S Mo z S Mn z) and isotropic (SMoSMn) ferro-and antiferromagnetic contributions originating from metal-to-metal electron transfers through the π and σ orbitals of the cyano bridges. The Mo III-CN-Mn II exchange anisotropy is distinct from the anisotropy of the g-tensor of [Mo III (CN)7] 4-; moreover, there is no correlation between the exchange anisotropy and g-tensor anisotropy. We indicate that highly anisotropic spin-spin couplings (such as the Ising-like JS Mo z S Mn z) combined with large exchange parameters represent a very important source of the global magnetic anisotropy of polyatomic molecular magnetic clusters. Since the total spin of such clusters is no longer a good quantum number, the spin spectrum pattern can differ considerably from the conventional scheme described by the zero-field splitting of the isotropic spin of the ground state. As a result, the spin reorientation barrier of the magnetic cluster may be considerably larger. This finding opens a new way in the strategy of designing single-molecule magnets (SMM) with unusually high blocking temperatures. The use of orbitally degenerate complexes with a strong spin-orbit coupling (such as [Mo III (CN)7] 4-or its 5d analogues) as building blocks is therefore very promising for these purposes.
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The reaction of potassium dimethylmalonate (K 2 Me 2 Mal) and cobalt(II) pivalate [Co(Piv) 2 ] n ... more The reaction of potassium dimethylmalonate (K 2 Me 2 Mal) and cobalt(II) pivalate [Co(Piv) 2 ] n under various conditions resulted in {[K 2 Co(H 2 O-jO)(l-H 2 O)(l 6-Me 2 Mal)(l 5-Me 2 Mal)]Á2H 2 O} n (1) and {[K 6 Co 36 (H 2 O-jO) 22 (l-H 2 O) 6 (l 3-OH) 20 (l 4-HMe 2 Mal-j 2 O,O 0) 2 (l 6-Me 2 Mal-j 2 O,O 0) 2 (l 5-Me 2 Mal-j 2 O,O 0) 8 (l 4-Me 2 Mal-j 2 O,O 0) 12 (l 4-Me 2 Mal) 6 ]Á58H 2 O} n (2) (where Me 2 Mal 2À is the dimethylmalonate dianion). Coordination polymers 1 and 2 were characterized by X-ray diffraction and magnetochemical studies. Analysis of the magnetic behavior indicates that 1 is characterized by an extremely high anisotropy of magnetic susceptibility and very weak spin coupling between Co II centers through malonate groups; compound 2 contains a highly symmetric, spherical-like Co 36 metal core that exhibits low magnetic anisotropy and antiferromagnetic interactions between Co II centers. Theoretical aspects of anisotropic magnetic properties of orbitally-degenerate Co II ions in polynuclear cobalt(II) complexes are discussed.
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A new highly anisotropic coordination hetero-bimetallic polymer [Mn III (Schiff-base)] 3 [Re IV (... more A new highly anisotropic coordination hetero-bimetallic polymer [Mn III (Schiff-base)] 3 [Re IV (CN) 7 ] was synthesized and characterized structurally and magnetically. The single crystal X-ray analysis has revealed that this is the first framework among the complexes composed of homoleptic cyanometallate and Mn III complex of the tetradentate Schiff base ligand. A formation of 3D assembly is possible due to both the pentagonal bipyrimidal geometry of the cyanome-tallate unit and suitable size of constituents: [Re(CN) 7 ] 3− and [Mn III (acacen)] + , where acacen = N,N′-ethylenebis-(acetylacetoneiminato). The powder and crystal magnetic studies show that the compound undergoes an antiferromag-netic ordering of a complicated character below Neel temperature of 13 K, and exhibits a metamagnetic behavior and strong magnetic anisotropy similar to those observed in related 3D Mn II −[Mo(CN) 7 ] 4− systems. Unusual magnetic properties of [Mn III (acacen)] 3 [Re IV (CN) 7 ] (1) originate from an interplay of Re−Mn anisotropic spin coupling and ZFS effect of Mn III ions with a noncollinear orientation of the local magnetic axes in the cyano-bridged 3D network. A theoretical model of anisotropic spin coupling between orbitally degenerate [Re IV (CN) 7 ] 3− complexes and Mn III ions is developed, and specific microscopic mechanisms of highly anisotropic spin coupling in Re IV −CN−Mn III linkages in complex 1 are analyzed in detail.
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Papers by Vladimir S Mironov
single-molecule magnets [MnIII
2(5-
Brsalen)2ACHTUNGTRENUNG(MeOH)2MIII(CN)6]NEt4
(M=Os (1) and Ru (2); 5-Brsalen=
N,N’-ethylenebis(5-bromosalicylidene)
iminate) by frequency-domain Fourier-
transform terahertz electron paramagnetic
resonance (THz-EPR), inelastic
neutron scattering, and superconducting
quantum interference
device (SQUID) magnetometry. The
combination of all three techniques
allows for the unambiguous experimental
determination of the three-axis anisotropic
magnetic exchange coupling
between MnIII and RuIII or OsIII ions,
respectively. Analysis by means of a
spin-Hamiltonian parameterization
yields excellent agreement with all experimental
data. Furthermore, analytical
calculations show that the observed
exchange anisotropy is due to the bent
geometry encountered in both 1 and 2,
whereas a linear geometry would lead
to an Ising-type exchange coupling.
single-molecule magnets [MnIII
2(5-
Brsalen)2ACHTUNGTRENUNG(MeOH)2MIII(CN)6]NEt4
(M=Os (1) and Ru (2); 5-Brsalen=
N,N’-ethylenebis(5-bromosalicylidene)
iminate) by frequency-domain Fourier-
transform terahertz electron paramagnetic
resonance (THz-EPR), inelastic
neutron scattering, and superconducting
quantum interference
device (SQUID) magnetometry. The
combination of all three techniques
allows for the unambiguous experimental
determination of the three-axis anisotropic
magnetic exchange coupling
between MnIII and RuIII or OsIII ions,
respectively. Analysis by means of a
spin-Hamiltonian parameterization
yields excellent agreement with all experimental
data. Furthermore, analytical
calculations show that the observed
exchange anisotropy is due to the bent
geometry encountered in both 1 and 2,
whereas a linear geometry would lead
to an Ising-type exchange coupling.