Static and Dynamic Charge Inhomogeneity and Crystal-field Fluctuations for 4f Ions in High-Jc Cuprates

The main mechanism in homogeneous broadening and relaxation of crystal-field excitations for R-ions in cuprates is believed to be provided by the fluctuations of crystalline electric field induced by a static and dynamic charge inhomogeneity generic to doped cuprates. Such an inhomogeneity is assumed to be the result of topological phase separation. We address the generalized granular model as one of the model scenarios for describing the static and dynamic charge inhomogeneity in cuprates. The charge subsystem is believed to be similar to that of a Wigner crystal with melting transition and phononlike positional excitation modes. We consider a simple model of charge inhomogeneity that allows us to elucidate the main universal features of the density of CF states and the respective inhomogeneous broadening. The formal description of R-ion relaxation mainly coincides with that of the recently suggested magnetoelastic mechanism by Lovesey and Staub. © 2004 MAIK "Nauka/Interperiodica".ACKNOWLEDGMENTS This paper was supported in part by INTAS (grant no. 01-0654), the CRDF (grant no. REC-005), RME (grant nos. 02.-3.4-392 and UR.01.01.062), and the Russian Foundation for Basic Research (project no. 04-02-96077). A. S. M. has benefited from stimulating discussions with A. T. Boothroyd, A. Mirmel-stein, and J. Mesot

Mixed-valence hydrated iron fluoridotitanate: Synthesis, optics and calorimetry

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Mixed-valence hydrated fluoridotitanate with idealized formula FeTiF6·6H2O is the main product of fluoride processing of natural mineral ilmenite with hydrofluoric acid. Its single crystals were grown and optical and calorimetric experiments were carried out in the temperature range 200–350 K. Charge re-distribution, Fe2+Ti4+ to Fe3+Ti3+, in the compound studied was evident in XPS spectra. A first order ferroelastic phase transition occurs at temperatures T0↓ = 271.5 K, T0↑ = 274 ÷ 275.5 K with the symmetry change P3‾↔ P1‾. The structural transformation is accompanied by the appearance of a very weak optical anisotropy in the slice (001), a small anomaly of the heat capacity (ΔS = 1.5 J/mol K = 0.2R), and positive baric coefficients dT0/dp ≈ 30 ± 10 K/GPa. © 2020 Elsevier Lt

Sequence of phase transitions in (NH4)3SiF7

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Single crystals of silicon double salt (NH4)(3)SiF7 = (NH4)(2)SiF6 center dot NH4F = (NH4)(3)[SiF6]F were grown and studied by the methods of polarization optics, X-ray diffraction and calorimetry. A sequence of symmetry transformations with the temperature change was established: P4/mbm (Z = 2) (G(1)) Pbam (Z = 4) (G(2)) P2(1)/c (Z = 4) (G(3)) P (1) over bar (Z = 4) (G(4)) P2(1)/c (Z = 8) (G(5)). Crystal structures of different phases were determined. The experimental data were also interpreted by a group-theoretical analysis of the complete condensate of order parameters taking into account critical and noncritical atomic displacements. Strengthening of the N-H center dot center dot center dot F hydrogen bonds can be a driving force of the observed phase transitions

Sequence of phase transitions in (NH4)3SiF7

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Single crystals of silicon double salt (NH4)(3)SiF7 = (NH4)(2)SiF6 center dot NH4F = (NH4)(3)[SiF6]F were grown and studied by the methods of polarization optics, X-ray diffraction and calorimetry. A sequence of symmetry transformations with the temperature change was established: P4/mbm (Z = 2) (G(1)) Pbam (Z = 4) (G(2)) P2(1)/c (Z = 4) (G(3)) P (1) over bar (Z = 4) (G(4)) P2(1)/c (Z = 8) (G(5)). Crystal structures of different phases were determined. The experimental data were also interpreted by a group-theoretical analysis of the complete condensate of order parameters taking into account critical and noncritical atomic displacements. Strengthening of the N-H center dot center dot center dot F hydrogen bonds can be a driving force of the observed phase transitions