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1.
Phys Chem Chem Phys ; 21(30): 16751-16761, 2019 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-31322638

RESUMO

In the context of the decisive role that vibronic interactions play in the functioning of molecular quantum cellular automata, in this article we give a comparative analysis of the two alternative vibronic approaches to the evaluation of the key functional characteristics of molecular cells. Semiclassical Born-Oppenheimer approximation and quantum mechanical evaluations of the vibronic energy pattern, electronic density distributions and cell-cell response function are performed for two-electron square-planar mixed valence molecular cells subjected to the action of a molecular driver. Special emphasis is put on the description of the cell-cell response function, which describes strong non-linearity as a prerequisite for the effective action of quantum cellular automata. Comparison of results obtained within the semiclassical and quantum-mechanical approaches has revealed a drastic difference between the shapes of the cell-cell response functions evaluated within these two approaches in the case of moderate vibronic coupling when the energy levels of the square cell interacting with a weakly polarized driver undergo large tunneling splitting in shallow adiabatic potential minima. In contrast, in the limits of strong vibronic coupling (a double-well adiabatic potential with deep minima) and weak vibronic coupling (a single well adiabatic potential) the adiabatic approximation is shown to describe the cell-cell response function with rather good accuracy.


Assuntos
Nanotecnologia/métodos , Teoria Quântica , Nanotecnologia/instrumentação , Vibração
2.
Phys Chem Chem Phys ; 21(21): 11122-11131, 2019 Jun 07.
Artigo em Inglês | MEDLINE | ID: mdl-31094384

RESUMO

A new vibronic mechanism for the stabilization of pair-delocalized electronic states in trigonal trimeric mixed valence complexes (such as iron-sulfur [Fe3S4]0 proteins) is proposed. Unlike the previously reported mechanism based on the Piepho-Krausz-Schatz model dealing with the local "breathing" vibrations, the present mechanism involves the multicenter vibrations of the triangular complex, which change the metal-metal distances. The former mechanism of the vibronic coupling in combination with the double exchange and Heisenberg type exchange (superexchange) interactions accounts for the existence of the pair-delocalized states of many-electron mixed valence clusters in the intermediate ground spin states. In contrast to the previous model, within which the conditions for the occurrence of pair delocalized states are spin-dependent, the pair delocalization phenomenon in the proposed model is exclusively based on a clear physical mechanism of the interplay between the inter-center vibronic coupling and electron transfer. Therefore, the proposed mechanism demonstrates that the pair delocalization is a more common feature of mixed valence complexes and can appear not only in multi-electron trimers as a result of the interplay between several parameters, but even in one-electron mixed valence trimers in which neither double exchange nor superexchange is operative. The conditions for the pair delocalization in the proposed model are shown to be dependent on the value and sign of the electron transfer parameter as well as on the ratio of the frequencies of the fully symmetric (a1) and doubly degenerate (e) vibrations of a triatomic molecule.

3.
J Comput Chem ; 39(22): 1815-1827, 2018 Aug 15.
Artigo em Inglês | MEDLINE | ID: mdl-30141533

RESUMO

We present a FORTRAN code based on a new powerful and efficient computational approach to solve multidimensional dynamic Jahn-Teller and pseudo Jahn-Teller problems. This symmetry-assisted approach constituting a theoretical core of the program is based on the full exploration of the point symmetry of the electronic and vibrational states. We also report some selected examples of increasing complexity aimed to display the theoretical background as well as the advantages and capabilities of the program to evaluate of the energy pattern, magnetic and optical properties of large multimode vibronic systems. © 2018 Wiley Periodicals, Inc.

4.
Dalton Trans ; 47(34): 11788-11805, 2018 Aug 29.
Artigo em Inglês | MEDLINE | ID: mdl-29855645

RESUMO

In this article, we report our development of a vibronic model for the electric-field control of antiferromagnetic superexchange in the mixed-valence (MV) triferrocenium complex FeIII-FeII-FeIII proposed as a possible candidate for the molecular implementation of a quantum logic gate. Along with the electronic interactions, such as electron transfer between the iron ions in different oxidation degrees and Coulomb repulsion of the extra holes, the proposed model of the triferrocenium complex also takes into account the vibronic coupling as an inherent ingredient of the problem of mixed valency. The latter is described by the conventional Piepho-Krauzs-Shatz (PKS) model adapted to the linear disposition of the redox sites in the trimeric FeIII-FeII-FeIII complex, which gives a clear insight into the degree of delocalization in mixed-valence compounds. By introducing symmetry adapted molecular vibrations composed of the local "breathing" displacements, the three-mode vibronic problem is reduced to a two-mode problem involving interaction with the even and odd molecular vibrations of the linear centrosymmetric complex. The vibronic coupling was shown to play a key role in the degree of localization of the two holes among the three iron centers. This was shown to produce a pronounced influence on the electric-field dependences of the electronic-density distributions and electrically switchable magnetic exchange in the considered linear triferrocenium complex. In particular, it was shown that the vibronic coupling significantly influences the field-induced stepwise transformation FeIII-FeII-FeIII ↔ FeIII-FeIII-FeII, increasing the abruptness of the field dependencies of the singlet-triplet gap and the hole densities, which are of primary importance for the switching function.

5.
Phys Chem Chem Phys ; 19(38): 26098-26106, 2017 Oct 04.
Artigo em Inglês | MEDLINE | ID: mdl-28929159

RESUMO

In this paper, we propose a toy model to describe the magnetic coupling between the localized spins mediated by the itinerant electron in partially delocalized mixed-valence (MV) systems. This minimal model takes into account the key interactions that are common for all such systems, namely, electron transfer in the valence-delocalized moiety and magnetic exchange between the localized spins and the delocalized electrons. The proposed descriptive model is exactly solvable which allows us to qualitatively and quantitatively discuss the main features of the whole class of partially delocalized MV systems. In the case of relatively strong exchange coupling, the combined action of these two interactions is shown to give rise to a specific kind of double exchange coupling termed here as "external core" double exchange. In the opposite case of relatively strong electron transfer, the general Hamiltonian is shown to be reduced to the effective Hamiltonian of indirect exchange of the localized spins. We argue a possibility to efficiently control the magnetic coupling of the localized spins using an external electric field acting on the delocalized part of the system. Finally, we discuss the perspectives of the present model for molecular spintronics and spin qubits.

6.
Inorg Chem ; 56(16): 9547-9554, 2017 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-28771336

RESUMO

As part of the search for systems in which control of quantum entanglement can be achieved, here we consider the paramagnetic mixed valence polyoxometalate K2Na6[GeV14O40]·10H2O in which two electrons are delocalized over the 14 vanadium ions. Applying a homogeneous electric field can induce an antiferromagnetic coupling between the two delocalized electronic spins that behave independently in the absence of the field. On the basis of the proposed theoretical model, we show that the external field can be used to generate controllable quantum entanglement between the two electronic spins traveling over a vanadium network of mixed valence polyoxoanion [GeV14O40]8-. Within a simplified two-level picture of the energy pattern of the electronic pair based on the previous ab initio analysis, we evaluate the temperature and field dependencies of concurrence and thus indicate that the entanglement can be controlled via the temperature, magnitude, and orientation of the electric field with respect to molecular axes of [GeV14O40]8-.

7.
Dalton Trans ; 46(23): 7540-7548, 2017 Jun 13.
Artigo em Inglês | MEDLINE | ID: mdl-28573307

RESUMO

We report a combined experimental characterization and theoretical modeling of the hexa-coordinated high-spin Co(ii) complex cis-[Co(hfac)2(H2O)2] (I). The magnetic static field (DC) data and EPR spectra (measurements were carried out on the powder samples of diluted samples cis-[Co0.02Zn0.98 (hfac)2(H2O)2]) were analyzed with the aid of the parametric Griffith Hamiltonian for the high-spin Co(ii) supported by the ab initio calculations of the crystal field (CF) parameters, g-factors and superexchange parameters between H-bonded Co(ii) ions in the neighboring molecules in a 1D network. This analysis suggests the presence of the easy axis of magnetic anisotropy and also shows the existence of a significant rhombic component. The detected frequency dependent (AC) susceptibility signal shows that complex I exhibits slow paramagnetic relaxation in the applied DC field belonging thus to the class of non-uniaxial field induced single ion magnets with a negative axial component of anisotropy. It is demonstrated that the main contributions to the relaxation come from the direct one-phonon process dominating at low temperatures, while the contribution of the two-phonon Raman process becomes important with increasing temperature.

8.
Dalton Trans ; 45(42): 16661-16672, 2016 Oct 25.
Artigo em Inglês | MEDLINE | ID: mdl-27778007

RESUMO

In this article we consider two coupled tetrameric mixed-valence (MV) units accommodating electron pairs, which play the role of cells in molecular quantum cellular automata. It is supposed that the Coulombic interaction between instantly localized electrons within the cell markedly inhibits the transfer processes between the redox centers. Under this condition, as well as due to the vibronic localization of the electron pair, the cell can encode binary information, which is controlled by neighboring cells. We show that under certain conditions the two low-lying vibronic spin levels of the cell (ground and first excited states) can be regarded as originating from an effective spin-spin interaction. This is shown to depend on the internal parameters of the cell as well as on the induced polarization. Within this simplified two-level picture we evaluate the quantum entanglement in the system represented by the two electrons in the cell and show how the entanglement within the cell and concurrence can be controlled via polarization of the neighboring cells and temperature.

9.
Inorg Chem ; 55(19): 9696-9706, 2016 Oct 03.
Artigo em Inglês | MEDLINE | ID: mdl-27632142

RESUMO

In this article we report the synthesis and structure of the new Co(II) complex Et4N[CoII(hfac)3] (I) (hfac = hexafluoroacetylacetonate) exhibiting single-ion magnet (SIM) behavior. The performed analysis of the magnetic characteristics based on the complementary experimental techniques such as static and dynamic magnetic measurements, electron paramagnetic resonance spectroscopy in conjunction with the theoretical modeling (parametric Hamiltonian and ab initio calculations) demonstrates that the SIM properties of I arise from the nonuniaxial magnetic anisotropy with strong positive axial and significant rhombic contributions.

10.
J Chem Theory Comput ; 12(8): 3545-60, 2016 Aug 09.
Artigo em Inglês | MEDLINE | ID: mdl-27398679

RESUMO

In this article, we focus on the electron-vibrational problem of the tetrameric mixed-valence (MV) complexes proposed for implementation as four-dot molecular quantum cellular automata (mQCA).1 Although the adiabatic approximation explored in ref 2 is an appropriate tool for the qualitative analysis of the basic characteristics of mQCA, like vibronic trapping of the electrons encoding binary information and cell-cell response, it loses its accuracy providing moderate vibronic coupling and fails in the description of the discrete pattern of the vibronic levels. Therefore, a precise solution of the quantum-mechanical vibronic problem is of primary importance for the evaluation of the shapes of the electron transfer optical absorption bands and quantitative analysis of the main parameters of tetrameric quantum cells. Here, we go beyond the Born-Oppenheimer paradigm and present a solution of the quantum-mechanical pseudo Jahn-Teller (JT) vibronic problem in bielectronic MV species (exemplified by the tetra-ruthenium complexes) based on the recently developed symmetry-assisted approach.3,4 The mathematical approach to the vibronic eigenproblem takes into consideration the point symmetry basis, and therefore, the total matrix of the JT Hamiltonian is blocked to the maximum extent. The submatrices correspond to the irreducible representations (irreps) of the point group. With this tool, we also extend the theory of the mQCA cell beyond the limit of prevailing Coulomb repulsion in the electronic pair (adopted in ref 2), and therefore, the general pseudo-JT problems for spin-singlet ((1)B1g, 2(1)A1g, (1)B2g, (1)Eu) ⊗ (b1g + eu) and spin-triplet states ((3)A2g, (3)B1g, 2(3)Eu) ⊗ (b1g + eu) in a square-planar bielectronic system are solved. The obtained symmetry-adapted electron-vibrational functions are employed for the calculation of the profiles (shape functions) of the charge transfer absorption bands in the tetrameric MV complexes and for the discussion of the magnetic properties.

11.
Inorg Chem ; 55(7): 3566-75, 2016 Apr 04.
Artigo em Inglês | MEDLINE | ID: mdl-26974224

RESUMO

In view of a wide perspective of 3d-4f complexes in single-molecule magnetism, here we propose an explanation of the magnetic behavior of the two thiacalix[4]arene tetranuclear heterometallic complexes Mn(II)2Gd(III)2 and Co(II)2Eu(III)2. The energy pattern of the Mn(II)2Gd(III)2 complex evaluated in the framework of the isotropic exchange model exhibits a rotational band of the low-lying spin excitations within which the Landé intervals are affected by the biquadratic spin-spin interactions. The nonmonotonic temperature dependence of the χT product observed for the Mn(II)2Gd(III)2 complex is attributed to the competitive influence of the ferromagnetic Mn-Gd and antiferromagnetic Mn-Mn exchange interactions, the latter being stronger (J(Mn, Mn) = -1.6 cm(-1), Js(Mn, Gd) = 0.8 cm(-1), g = 1.97). The model for the Co(II)2Eu(III)2 complex includes uniaxial anisotropy of the seven-coordinate Co(II) ions and an isotropic exchange interaction in the Co(II)2 pair, while the Eu(III) ions are diamagnetic in their ground states. Best-fit analysis of χT versus T showed that the anisotropic contribution (arising from a large zero-field splitting in Co(II) ions) dominates (weak-exchange limit) in the Co(II)2Eu(III)2 complex (D = 20.5 cm(-1), J = -0.4 cm(-1), gCo = 2.22). This complex is concluded to exhibit an easy plane of magnetization (arising from the Co(II) pair). It is shown that the low-lying part of the spectrum can be described by a highly anisotropic effective spin-(1)/2 Hamiltonian that is deduced for the Co(II)2 pair in the weak-exchange limit.

12.
J Chem Phys ; 143(13): 134307, 2015 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-26450314

RESUMO

Our interest in this article is prompted by the vibronic problem of charge polarized states in the four-dot molecular quantum cellular automata (mQCA), a paradigm for nanoelectronics, in which binary information is encoded in charge configuration of the mQCA cell. Here, we report the evaluation of the electronic levels and adiabatic potentials of mixed-valence (MV) tetra-ruthenium (2Ru(ii) + 2Ru(iii)) derivatives (assembled as two coupled Creutz-Taube complexes) for which molecular implementations of quantum cellular automata (QCA) was proposed. The cell based on this molecule includes two holes shared among four spinless sites and correspondingly we employ the model which takes into account the two relevant electron transfer processes (through the side and through the diagonal of the square) as well as the difference in Coulomb energies for different instant positions of localization of the hole pair. The combined Jahn-Teller (JT) and pseudo JT vibronic coupling is treated within the conventional Piepho-Krauzs-Schatz model adapted to a bi-electronic MV species with the square-planar topology. The adiabatic potentials are evaluated for the low lying Coulomb levels in which the antipodal sites are occupied, the case just actual for utilization in mQCA. The conditions for the vibronic self-trapping in spin-singlet and spin-triplet states are revealed in terms of the two actual transfer pathways parameters and the strength of the vibronic coupling. Spin related effects in degrees of the localization which are found for spin-singlet and spin-triplet states are discussed. The polarization of the cell is evaluated and we demonstrate how the partial delocalization caused by the joint action of the vibronic coupling and electron transfer processes influences polarization of a four-dot cell. The results obtained within the adiabatic approach are compared with those based on the numerical solution of the dynamic vibronic problem. Finally, the Coulomb interaction between the cells is considered and the influence of the vibronic coupling on the shape on the non-linear cell-cell response function is revealed.

13.
J Chem Phys ; 143(8): 084502, 2015 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-26328851

RESUMO

In this article, we present a new microscopic theoretical approach to the description of spin crossover in molecular crystals. The spin crossover crystals under consideration are composed of molecular fragments formed by the spin-crossover metal ion and its nearest ligand surrounding and exhibiting well defined localized (molecular) vibrations. As distinguished from the previous models of this phenomenon, the developed approach takes into account the interaction of spin-crossover ions not only with the phonons but also a strong coupling of the electronic shells with molecular modes. This leads to an effective coupling of the local modes with phonons which is shown to be responsible for the cooperative spin transition accompanied by the structural reorganization. The transition is characterized by the two order parameters representing the mean values of the products of electronic diagonal matrices and the coordinates of the local modes for the high- and low-spin states of the spin crossover complex. Finally, we demonstrate that the approach provides a reasonable explanation of the observed spin transition in the [Fe(ptz)6](BF4)2 crystal. The theory well reproduces the observed abrupt low-spin → high-spin transition and the temperature dependence of the high-spin fraction in a wide temperature range as well as the pronounced hysteresis loop. At the same time within the limiting approximations adopted in the developed model, the evaluated high-spin fraction vs. T shows that the cooperative spin-lattice transition proves to be incomplete in the sense that the high-spin fraction does not reach its maximum value at high temperature.

14.
J Chem Phys ; 139(4): 044304, 2013 Jul 28.
Artigo em Inglês | MEDLINE | ID: mdl-23901977

RESUMO

We propose a microscopic analytical approach to the description of the low-temperature dissipative intracluster electron transfer dynamics in centrosymmetric one-electron mixed-valence (MV) dimers. The dissipative system (bath) is supposed to consist of the acoustic phonons of the crystal surrounding that are coupled to the delocalized electron(s) of a MV dimer. Although the concept of the bath is the spin-boson model is more generic, the present consideration is relevant, for example, to a MV bi-center impurity in an ionic crystal. The model allows us to develop an approximate microscopic approach within which the relaxation processes are explicitly taken into account without additional assumption regarding spectral function of the bath. It is assumed that initially the extra electron is localized on a certain center and then the time-dependent localization probability (averaged value of the electron dipole moment) is evaluated with the emphasis on the damping of the amplitude of the Rabi oscillations. The approach assumes the following conditions: (i) the vibrational spectrum of the crystal does not show the presence of local modes; (ii) the itinerant electron is weakly coupled to the long-waves acoustic phonons which is peculiar to fully delocalized Robin and Day class III MV systems; (iii) the Debye energy ℏωD exceeds the electronic resonance energy gap 2ß (ß is the electron transfer parameter). We have demonstrated that the dissipation in this case is super-ohmic with the low-frequency spectral function J(ω) ∝ ω(5). The time dependences of the localization probabilities show nearly picosecond damped oscillations. The longitudinal relaxation time T1 has been shown to be two times shorter than the decoherence time T2 thus giving the upper bound for T2, T2 ≤ 2T1.

15.
Chemphyschem ; 13(11): 2662-5, 2012 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-22689507

RESUMO

Multiferroic molecules for spintronics: In a many-electron mixed-valence dimer with dominant double exchange, as compared with antiferromagnetic superexchange, the electric field is shown to induce a spin crossover from the ferromagnetic spin state to the antiferromagnetic one. This leads to a sharp decrease in the magnetic moment of the molecule and a simultaneous stepwise increase in the electric dipole (see figure).

16.
Chem Soc Rev ; 40(6): 3130-56, 2011 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-21431145

RESUMO

In this critical review we review the problem of exchange interactions in polynuclear metal complexes involving orbitally degenerate metal ions. The key feature of these systems is that, in general, they carry an unquenched orbital angular momentum that manifests itself in all their magnetic properties. Thus, interest in degenerate systems involves fundamental problems related to basic models in magnetism. In particular, the conventional Heisenberg-Dirac-Van Vleck model becomes inapplicable even as an approximation. In the first part we attempt to answer two key questions, namely which theoretical tools are to be used in the case of degeneracy, and how these tools can be employed. We demonstrate that the exchange interaction between orbitally degenerate metal ions can be described by the so-called orbitally-dependent exchange Hamiltonian. This approach has shown to reveal an anomalously strong magnetic anisotropy that can be considered as the main physical manifestation of the unquenched orbital angular momentum in magnetic systems. Along with the exchange coupling, a set of other interactions (such as crystal field effects, spin-orbit and Zeeman coupling), which are specific for the degenerate systems, need to be considered. All these features will be discussed in detail using a pseudo-spin-1/2 Hamiltonian approach. In the second part, the described theoretical background will be used to account for the magnetic properties of several magnetic metal clusters and low-dimensional systems: (i) the dinuclear face-sharing unit [Ti(2)Cl(9)](3-), which exhibits a large magnetic anisotropy; (ii) the rare-earth compounds Cs(3)Yb(2)Cl(9) and Cs(3)Yb(2)Br(9), which, surprisingly, exhibit a full magnetic isotropy; (iii) a zig-zag Co(II) chain exhibiting unusual combination of single-chain magnet behavior and antiferromagnetic exchange coupling; (iv) a trigonal bipyramidal Ni(3)Os(2) complex; (v) various Co(II) clusters encapsulated by polyoxometalate ligands. In the two last examples a pseudospin-1/2 Hamiltonian approach is applied to account for the presence of exchange anisotropy (150 references).

17.
Dalton Trans ; (1): 21-36, 2010 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-20023927

RESUMO

The structural versatility characterizing polyoxometalate chemistry, in combination with the option to deliberately use well-defined building blocks, serves as the foundation for the generation of a large family of magnetic clusters, frequently comprising highly symmetric spin arrays. If the spin centers are coupled by antiferromagnetic exchange, some of these systems exhibit spin frustration, which can result in novel magnetic properties of purely molecular origins. We discuss here the magnetic properties of selected nanosized polyoxometalate clusters featuring spin triangles as their magnetic 'building blocks' or fragments. This includes unique porous Keplerate clusters of the type {(Mo)Mo(5)}(12)M(30) (M = Fe(III), Cr(III), V(IV)) with the spin centers defining a regular icosidodecahedron and the {V(15)As(6)}-type cluster sphere containing a single equilateral spin triangle; these species are widely discussed and studied in the literature for their role in materials science as molecular representations of Kagomé lattices and in relation to quantum computing, respectively. Exhibiting fascinating and unique structural features, these magnetic molecules allow the study of the implications of frustrated spin ordering. Furthermore, this perspective covers the impact of spin frustration on the degeneracy of the ground state and related problems, namely strong magnetic anisotropy and the interplay of antisymmetric exchange and structural Jahn-Teller effects.


Assuntos
Compostos de Tungstênio/química , Magnetismo , Modelos Moleculares , Estrutura Molecular , Nanoestruturas/química , Teoria Quântica
18.
J Phys Chem A ; 113(25): 6886-90, 2009 Jun 25.
Artigo em Inglês | MEDLINE | ID: mdl-19496596

RESUMO

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.


Assuntos
Níquel/química , Nitrilos/química , Compostos Organometálicos/química , Osmio/química , Anisotropia , Magnetismo , Modelos Moleculares , Temperatura
19.
Appl Opt ; 48(1): 17-21, 2009 Jan 01.
Artigo em Inglês | MEDLINE | ID: mdl-19107166

RESUMO

The terbium-activated yttrium tantalite (YTaO(4):Tb(3+)) phosphor is of great interest due to the interesting spectroscopic properties of rare earth ions in crystals and also practical use in x-ray imaging. Using the group-theoretical approach, we analyze the selection rules for the transition between Stark components of Tb(3+) in symmetry of the actual crystal field and the polarization for the allowed transitions. The luminescence upon UV, vacuum-ultraviolet (VUV), and x-ray excitation is presented and discussed. The YTaO(4):Tb(3+) phosphors are found to be efficient VUV-excited luminescent materials that could be used not only in x-ray intensifying screens, but also in mercury-free fluorescent lamps or plasma display panels.

20.
J Am Chem Soc ; 130(44): 14729-38, 2008 Nov 05.
Artigo em Inglês | MEDLINE | ID: mdl-18839950

RESUMO

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.

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