O. E. Kvyatkovskii

Quantum Effects in Incipient and Low-Temperature Ferroelectrics (A Review)

AbstractThe paper discusses the theory of quantum effects in incipient and low-temperature ferroelectrics and its application to the description of some phenomena observed in SrTiO3, KTaO3, and solid solutions based on these compounds. Two approaches are considered, namely, the Barrett theory in the framework of the single-ion (one-sublattice) Devonshire-Slater model and the Rechester and Khmel’nitski

Optical and structural properties of fullerene films doped with cadmium telluride

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Ab initio calculations of neutral and charged impurity centers of manganese and chromium in strontium titanate

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Laser controlled magnetism in hydrogenated fullerene films

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Cluster ab initio calculations for the C60F24, C60Cl24, and C60Br24 halofullerenes

-Shneerson theories developed in terms of a soft ferroelectric (polar TO) mode dressed by its anharmonic interaction with zero-point and thermal lattice vibrations. Consideration is given to the specific features of an isoelectronic-impurity-induced ferroelectric phase transition in incipient ferroelectrics, the theory of an isotopic-impurity-induced ferroelectric phase transition in incipient ferroelectrics, the theory of the isotopic Curie-temperature shift in low-temperature ferroelectrics, and the electric-field effect on the dielectric properties of incipient and low-temperature ferroelectrics.

Polarization mechanism of ferroelectric lattice instability in crystals

Cluster calculations of the local adiabatic potential for an impurity atom in position A in Sr1−xAxTiO3 (A=Mg, Ca, Ba, Pb, Cd, Zn), as well as for the Nb and O atoms in the Ta-O-Nb chain in KTa1−xNbxO3, were carried out in the nonempirical Hartree-Fock-Roothaan MO-LCAO formalism. For comparison, similar calculations of the local adiabatic potential were performed for a sublattice-A atom in the ATiO3 cubic perovskites (A=Ca, Sr, Ba, Pb), for K and Ta atoms in KTaO3, and for Li in K1−xLixTaO3. The calculations revealed that in all the cases considered, except the Zn, Mg, and Li impurities, the impurity atoms move in single-well potentials and that the corresponding solid solutions are displacive ferroelectrics. Zn in Sr1−xZnxTiO3 and Mg in Sr1−xMgxTiO3 were found to occupy off-center positions, as does the Li atom in K1−xLixTaO3; i.e., they move in a multiwell local potential. An explanation is proposed for the first-order Raman scattering observed in the paraelectric phase of the above solid solutions with central impurities. The critical concentration xc for the displacive KTa1−xNbxO3 and Sr1−xAxTiO3 solid solutions was calculated in the virtual-crystal approximation within the soft ferroelectric mode theory. The values of xc thus obtained agree with the available experimental data.

Magnetic transition in the polymerized fullerene matrix

Composite thin-film structures based on C60/CdTe were prepared by discrete vacuum evaporation in a quasi-closed volume and by vacuum evaporation from a Knudsen cell. The synthesized fullerene samples contained cadmium telluride with a content ranging from 1 to 50 wt %. The morphology and composition of the films were controlled using scanning electron microscopy and energy-dispersive X-ray analysis. The optimum geometry, the total energy, and the spectrum of excited states of the fullerene-cadmium telluride molecular complex were calculated by quantum-chemical methods. The spectral dependences of the photo-luminescence, Raman scattering, extinction coefficient, and refractive index were measured for different compositions. It was found that, in contrast to the spectra of pure fullerene, the luminescence and absorption spectra of the fullerene doped with cadmium telluride exhibit an additional peak in the wavelength range of 600–620 nm. These data were interpreted as the existence of dipole-allowed transitions in the spectrum of excited singlet states of the fullerene due to the interaction with cadmium telluride. The composite films had an increased resistance to degradation under the action of oxygen and water vapor.

Ab initio calculations of supramolecular complexes of fullerene C60 with CdTe and CdS

This paper presents the results of ab initio calculations of the equilibrium geometry, the electronic structure, and the spin and charge densities for neutral and negatively charged defects produced by the Mn and Cr impurities in the B position of the SrTiO3 structure. It has been shown that, in both cases, the neutral defect is an acceptor center, while the singly charged defect is a donor center. It has been found that doubly charged defects are polar, have the symmetry C4v, and reside in the ionic configurations 5Mn3+ + 3Ti3+ and 4Cr3+ + 3Ti3+, respectively. In each case, there is a pair of almost energy-degenerate electronic states (4B1 and 6B1 for Mn and 3A1 and 5A1 for Cr), which differ only in the direction of the spin of the electron polaron localized at one of the neighboring titanium atoms. For the manganese impurity, the energy of the polar state 6B1 is only 0.174 eV lower than that of the state 6A1g (Oh) with the Mn2+ ion in the high-spin state. A new mechanism of dielectric relaxation in STO: Mn has been proposed.