Alicja Waskowska

CuMn2O4: properties and the high-pressure induced Jahn-Teller phase transition

Single crystal x-ray diffraction, x-ray photoelectron spectroscopy and magnetic susceptibility measurements at normal pressure have shown that, in spite of two Jahn-Teller active ions in CuMn2O4, the crystal is cubic with partly inverse spinel structure, the inversion parameter being \mbox{

Influence of nickel substitution on the cation distribution and magnetic properties of ZnCr2Se4

x = 0.8

The bulk modulus of cubic spinel selenides: an experimental and theoretical study

}. The cation configuration at normal pressure was determined as Cu0.2+Mn2+0.8[Cu2+0.8Mn3+0.2Mn4+1.0]O4. The high-pressure behaviour of the crystal was investigated up to 30 GPa using the energy dispersive x-ray diffraction technique and synchrotron radiation. A first-order phase transition connected with a tetragonal distortion takes place at Pc = 12.5 GPa, the c/a ratio being 0.94 at P = 30 GPa. The high-pressure phase has been described in terms of ligand field theory and explained by the changes to the valence and electronic configuration of the metal ions, leading to the formula Cu2+0.2Mn3+0.8[Cu2+0.8Mn3+1.2]O4. The electron configuration of the tetrahedrally coordinated Cu2+ and Mn3+ is (e4)t5 and e2t2, respectively. On the other hand, the electron configuration of Cu2+ located at octahedral sites is (t62g)e3g. While six electrons with antiparallely aligned spins occupy the triplet (t62g), three electrons on the orbital eg can be distributed in two ways (double degeneracy): (dx2-y2)1(dz2)2 and (dx2-y2)2(dz2)1. The first alternative leads to an axially elongated octahedron; the second one causes flattening of the octahedron. The contraction of the c axis indicates, that in the high-pressure phase the second configuration with unpaired electron on the dz2 orbital occurs. A similar effect of the octahedral contraction brings the orbital degeneracy of Mn3+ with the t32ge1g distribution. It follows that at high pressure the ligand field forces the two metals to take the valences that they show in the parent oxides CuO and Mn2O3.

Temperature- and pressure-induced lattice distortion in CdCr2−xGaxSe4 (x = 0, 0.06, and 0.12)

Abstract Single crystals of ZnCr2Se4 spinel doped with nickel were prepared by chemical vapour transport in closed silica tubes using ZnSe and NiSe with CrCl3 as the transport agent. The chemical compositions of three crystals have been determined by X-ray diffraction for the system Zn–Ni–Cr–Se to determine the cation distribution and to study an influence of the Ni ions on the magnetic properties of the system. Based on the structural data the formula describing cation distribution in the system is: Zn[Cr2−xNix]Se4. For x values equal to 0.03, 0.05 and 0.10, the observed symmetry was cubic, space group Fd-3m. The site occupation factors resulting from the structure refinement showed Ni2+ substituted for the Cr3+ ions on octahedral sites. The experimental data for the saturation magnetization on the concentration of nickel in the external magnetic fields have been analyzed. The nickel substitution modified the saturation magnetic moments, which decrease with increasing Ni concentration (comparatively to the parent ZnCr2Se4).

Crystal lattice dilatation in Cd-Cr-Se spinels with gallium admixtures

It is argued that mainly the selenium sublattice determines the overall compressibility of the cubic spinel selenides, AB2Se4, and that the bulk modulus for these compounds is about 100 GPa. The hypothesis is supported by experiments using high-pressure X-ray diffraction and synchrotron radiation, and by first-principles calculations using density functional theory.

High-pressure x-ray diffraction and Raman spectroscopic studies of the tetragonal spinel CoFe 2 O 4

Structural changes in the cubic spinels CdCr2−xGaxSe4 have been studied by means of single-crystal x-ray diffraction at low temperature and energy-dispersive diffraction in a diamond-anvil cell at high pressure. In stoichiometric samples (x = 0), a spontaneous magnetostriction reduces the thermal expansion coefficient from 6.7 × 10−6 K−1 in the paramagnetic phase to 2.2 × 10−6 K−1 in the ferromagnetic phase (TC = 130 K). In the samples with Ga3+ admixtures (x = 0.06 and 0.12), a slight structural distortion causes an order–disorder-type phase transition at Td ≈ 285 K connected with changes in the electronic configuration of the Jahn–Teller-active Cr cations. The magnetostriction is apparently not very sensitive to the Ga3+ admixtures in the present concentration range. At high pressure the cubic unit cell transforms to a tetragonal one with c/a = 0.91. The Jahn–Teller effect is combined with the rocking motions of corner-sharing metal coordination polyhedra, altering their shape. The transition pressure is about 10 GPa. The bulk modulus of the cubic phase is B0 = 103 GPa.

High-pressure phase transition and properties of spinel ZnMn2O4

CRYSTAL LATTICE DILATATION IN Cd_Cr_Se SPINELS WITH GALLIUM ADMIXTURES A. Waskowska L. Gerward J. Staun_Olsen 3 E. Malicka Institute For Low Temperature And Structure Research, Polish Academy of Sciences WROCLAW 50 422 POLAND Department of Physics, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark N. Bohr Institute, Oersted Laboratory, University of Copenhagen, 2100 Copenhagen, Denmark Institute of Chemistry, Silesian University in Katowice, 40 007 Katowice, Poland