A. A. Levin

Causes of modulation and hole conductivity of the high-Tc superconductor Bi2Sr2CaCu2O8+x according to X-ray single-crystal data

The one-dimensionally modulated structure of the high-Tc superconductor Bi2Sr2CaCu2O8+x with a0=5.407(2), b0=5.412(3), c0=30.1771(8) AA, q=0.210(2)b0* has been analysed in the commensurate approximation using the conventional three-dimensional space group Pnnn and utilizing single-crystal X-ray diffraction data. A reliable model (R=0.07, Rw=0.079 for 1011 unique reflections) has been obtained. The extra oxygen atoms have been found inside and between the Bi-O-Bi chains of the BiO layer. The modulation displacement functions of the atoms have been obtained by means of harmonic analysis of the modulation displacements. It has been shown that the topology of Bi-O-Bi chains. The modulation of the structure and the form of the modulation displacement functions can be explained by the shifts of the oxygen atoms in the BiO layer correlated according to the antiferroelectrical law. The cationic vacancies in the Sr sites provide the hole conductivity in the copper-oxygen plane and lead to the formation of the percolation picture in this plane.

The crystal structure of a new 84 K superconductor, Bi4Sr4CaCu3O14+x

Abstract The crystal structure of a new superconductor in the Bi-Sr-Ca-Cu oxide system of nominal composition Bi4Sr4CaCu3O14 has been studied by single crystal X-ray diffraction. A modulation was observed along the b-axis. The structure of the fundamental unit cell has been determined in orthorhombic space group Pbmm, a = 5.411 (2), b = 5.417 (3), c = 27.75 (1) A, Z = 2. The final R-factor is 0.067 for 167 unique reflections collected up to 2θ = 60° with Ag Kα radiation. The essential feature of the structure is the existence of two kinds of copper-oxygen layers with square-pyramidal (CuO3-layer) and square-dipyramidal (CuO4-layer) oxygen coordinations of the Cu atoms. Two perovskite-like blocks [Sr-CuO3-Ca-CuO3-Sr] and [Sr-CuO4-Sr] are alternated by double [BiO]2 layers. The bonds in the BiO planes form chains Bi-O-Bi extending along the b-axis.

Thermal vibrations and the structure of quasi-two-dimensional R2CuO4 crystals (R=La, Pr, Nd, Sm, Eu, and Gd)

Thermal vibrations of ions in R2CuO4 crystals (R=La, Pr, Nd, Sm, Eu, Gd) were studied by x-ray diffractometry. A comparative analysis of thermal displacements of the copper and rare-earth ions permitted a conclusion as to the main interactions responsible for the structural state of the CuO2 sheets and of a crystal as a whole. The structural properties were found to correlate with the magnitude of the ionic radius and with the ground state of the rare-earth ions.

Quantum-chemical modeling of ferroelectric solids with hydrogen-bond networks of different dimensionalities and their deuterated analogs

The application of quantum chemistry methods in the microscopic theory of H/D-bonded ferroelectrics and antiferroelectrics is illustrated using a comparative analysis of low-temperature order-disorder structural phase transitions in zero-dimensional materials of the K3H(SO4)2 family, zero-dimensional crystals of 5-halo derivatives of 9-hydroxyphenalenone (5Hal-9HPO), and three-dimensional crystals of potassium dihydrogen phosphate (KDP) and potassium dideuterium phosphate (DKDP) as examples. In the framework of the Ising model with tunneling, it is demonstrated that (in agreement with experimental data) the transition to the low-temperature ordered phase in zero-dimensional materials is possible only in the case of their deuteration, whereas the quantum paraelectric behavior is characteristic of undeuterated samples. This behavior for KDP crystals is impossible due to the sharp increase in the Ising parameters as compared to zero-dimensional materials. The factors responsible for the increase in these parameters are considered.

Proton-lattice coupling and vibronic effects in KDP-family thermodynamics

Based on the vibronic theory of heteroligand complexes a new model Hamiltonian is proposed for the ferroelectrics of KDP (KH2PO4) family. In local representation three items of this Hamiltonian describe the H-bond protons, the potential energy of lattices oscillators and the coupling of these two subsystems respectively. In Ising form, convenient for numerical calculations, the Hamiltonian offered includes explicitly the characteristics of the electronic structure as well as the orbital vibronic constants of the AO4 tetrahedra. The Ising version of theory is then applied to numerical studying of some questions for KDP thermodynamics.

Pseudospin Hamiltonian Parameters from Quantum Chemical Treatment: K 3 H(SO 4 ) 2 (TKHS) Family

A problem of pure theoretical evaluation of Ising ( J ij ) and tunneling ( z H/D ) parameters in pseudospin Hamiltonians is considered for H-bonded ferroelectrics. The TKHS family is examined as a example. Two methods of J ij evaluation are applied. Various quantum-chemical approaches are probed. The z H/D determination using proton potential energy profiles is also discussed. The obtained parameters allow to explain qualitatively the quantum paraelectric behavior of H-members and the antiferroelectric behavior of D-members of this family even by molecular field approximation. The latter overestimates T c values. The cluster approximation allows to substantially approach the estimated T c to experimental ones.

Indirect non-electrostatic mechanism of the proton-proton interaction and proton-lattice coupling in KH2PO4 ferroelectrics

It has been shown that the mechanism previously proposed for the effective interaction of protons in MH2AO4 ferroelectrics, such as KH2PO4 (KDP), is applicable to the description of the proton-lattice coupling. The expression derived for the total Hamiltonian of the coupled proton-lattice system, along with the single-mode approximation, also allows the consideration of several modes involved in the ferroelectric phase transition. The use of the proposed approach is illustrated by the example of the numerical evaluation of the effect of the proton-lattice coupling on the critical temperature Tc of the ferroelectric phase transition in KDP and deuterated KDP.

X-ray diffraction study of ion thermal vibrations in R2CuO4 (R=Pr and Gd) crystals

An X-ray diffraction study is reported of the symmetry in the spatial distribution of thermal vibrations of Cu2+, Gd3+, and Pr3+ ions in Gd2CuO4 crystals. An analysis of the pattern of the angular thermal-vibration amplitude distribution obtained experimentally at different temperatures allows a conclusion about the character of the local Jahn-Teller effect for the Cu2+ ions, structural phase transitions, and the orbital ground state of the Cu2+ ions.

Structural modulation and hole conductivity of the high-Tc superconductor Bi4Sr4CaCu3O14 + x

Abstract The modulated structure of the high-Tc superconductor Bi4Sr4CaCu3O14 + x has been determined in the commensurate approximation utilizing single-crystal X-ray diffraction data. The modulation displacement functions of the atoms have been obtained by means of harmonic analysis of the modulation displacements. It has been shown that the topology of the BiO layers, the modulation of the structure and the form of the modulation displacement functions can be explained by the shifts of the oxygen atoms in the BiO layers correlated according to the antiferro-electrical law. The vacancies in the Sr positions provide the hole conductivity in the copper-oxygen planes and lead to the formation of the percolation network.

Quantum-Chemical Approach to Zero-Dimensional Antiferroelectrics and Quantum Paraelectrics of the K3H(SO4)2 Family

The application of different quantum-chemical methods to the evaluation of coupling parameters of the Ising model (Jij) is treated for the TKHS family crystals. Two approaches to the Jij parameter evaluation are developed. The use of calculated Jij values for the description of some distinctive features of the TKHS-like materials is discussed. The critical hydrogen concentrations (xc) resulting in the disappearance of the low-temperature structural phase transition for isotopically mixed H-D crystals are examined.