L. M. Volkova

New Method to Calculate the Sign and Relative Strength of Magnetic Interactions in Low-Dimensional Systems on the Basis of Structural Data

The connection of strength of magnetic interactions and type ordering the magnetic moments with crystal chemical characteristics in low-dimensional magnets is investigated. The new method to calculate the sign and relative strength of magnetic interactions in low-dimensional systems on the basis of the structural data is proposed. This method allows to estimate magnetic interactions not only inside low-dimensional fragments but also between them, and also to predict the possibility of the occurrence of magnetic phase transitions and anomalies of the magnetic interactions. Moreover, it can be used for search of low-dimensional magnets among the compounds whose crystal structures are known. The possibilities of the method are illustrated in an example of research of magnetic interactions in familiar low-dimensional magnets SrCu2(BO3)2, CaCuGe2O6, CaV4O9, Cu2Te2O5Cl2, Cu2Te2O5Br2, BaCu2Si2O7, BaCu2Ge2O7, BaCuSi2O6, LiCu2O2, and NaCu2O2.

Crystal Chemical Estimation of Possible Exceeding of a Tc of 77 K in Diborides

We establish the empirical correlation for diborides (AB2) between Tc and crystal chemical parameters of a simpler structural fragment—sandwich A2(B2) where the superconductivity appears—like that found in high-Tc cuprate superconductors. Our results suggest that in the absence of vacacies in B2 plane, Tc of diborides can be higher, that is, 77 K, the evaporation temperature of liquid nitrogen. We discuss critical crystal chemistry parameters controlling Tc and ways to achieve higher transition temperatures in diborides, based on this correlation.

Absence of a Competition Between Magnetism and Superconductivity in Layered Nickel Borocarbides: Common Correlations of Tc with Crystal Chemical Parameters for Magnetic and Nonmagnetic Compounds

In the present work, an analysis of the crystal chemical parameters of nickel borocarbides RNi2B2C (R = rare earth) is given. The reasons for the dependence of superconducting transition temperature (Tc) on crystal chemical parameters by two separate curves for magnetic and nonmagnetic R are considered. For all R, a common pattern of dependence of Tc on crystal chemical parameters similar to that existing in layered quasi two-dimensional systems (HTSC cuprates and diborides) is established. The absence of the influence on the Tc of borocarbides of magnetic properties R is also established. On the basis of the correlations found, the radii of a number of rare earths are more precisely defined, and Tc of compounds at various substitutions R are calculated.

Crystal Chemical Concept of Arrangement and Function of Layered Superconducting Materials

The crystal chemical concept of arrangement and function of layered superconducting materials is supposed. The concept is based on the results of our investigation of crystal chemistry of high-temperature superconductors (HTSC) cuprates, diborides AB2, and borocarbides of nickel RNi2B2C. According to these results (1) the main role in appearance of superconductivity played by the structural fragments—sandwiches A2(CuO2) in HTSC cuprates, A2(B2) in diborides, and RB(Ni) in nickel borocarbides but not the separate planes of CuO2, B2, or Ni; (2) correlations between Tc and crystal chemical parameters of these sandwiches have similar character in all three classes of compounds, despite distinction of a nature of their superconductivity. The central idea of the concept consists of following: in contrast to metallic conduction, for which it is enough to provide only concentration and mobility of charge carriers, for occurrence of a superconductivity it is necessary to create in addition a space (channels) for stream of charge carriers, compression of stream of carriers, and focusing to direction on path of motion.

Critical Crystal-Chemical Parameters Controlling Tc of HTSC Cuprates

For all high-temperature superconductive (HTSC) cuprate phases, an empirical relationship has been found between the temperature of the transition to the superconducting state (Tc) and the ratio between the interatomic distances of copper atoms along the diagonal of the CuO2 plane and the sum of “effective” distances from the CuO2 plane to two adjacent cation planes calculated with regard to the charge and size of the cations and doping atoms. The critical parameters controlling Tc are discussed.

On formation of polyatomic mercury cations

This paper analyzes the effect of the increased number of electrons lying on the 6s orbitals of Hg ions on the formation of polyatomic Hgn cations, their structure, and Hg-Hg distances. The critical role in the formation and structural characterstics of Hgn cations is played by the degree of expansion of the Hg5d atomic orbitals; when 6s electrons increase in number, this expansion is enhanced by relativistic effects. Formation of Hgn cations and the observed tendencies are explained in terms of the virial theorem.

Correlation between Tc and the structural parameters of the cation sublattice in the perovskite layer of HgBa2CuO4 +δ and Tl2Ba2CuO4+δ

The superconducting transition temperature, Tc, of HgBa2CuO4 + δ and Tl2Ba2CuO4 + δ was correlated with the interatomic distances in their structures.Tc was found to vary parabolically with the ratio of the Cu-Cu distance along the diagonal direction in the CuO2 plane to the Ba-Ba distance between the Ba layers. The factors determining these interatomic distances are discussed. The structure of the cation sublattice (Ba-Cu) of the perovskite layer is shown to play an important role in the development of superconductivity in these phases.

Investigation of the relationship betweenT c and the Coulomb interaction of copper cations from the neighboring CuO2 layers

Sixty known high-temperature cuprates from different systems are analyzed. It is shown that the temperature of the transition to the superconducting state (Tc) correlates with the Coulomb interaction Vij between the Cu cations from the neighboring CuO2 layers. The dependence Tc(Vij) is plotted as two families of parabolic curves resembling an orthogonal system of confocal parabolas. It follows from this relationship that the optimal doping of CuO2 layers depends on the Cu-Cu distances between the neighboring CuO2 layers. Methods of varying the charges on and the distances between the copper atoms are demonstrated. The potential applications of this relationship for developing the compositions of new high-temperature materials and choosing methods to raise Tc of known high-temperature cuprates are discussed.