S. Steenstrup

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{

High pressure x-ray diffraction experiments on US using synchrotron radiation

x = 0.8

A spectrometer for X-ray energy-dispersive diffraction using synchrotron radiation

}. 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.

High pressure studies of the high temperature superconductors RBa2Cu3O9−δ with R: Y, Eu and Ho up to 60 GPa

Abstract High pressure X-ray diffraction studies (up to 40 GPa) were performed on US using synchrotron radiation and a diamond anvil cell. The measured value of 92 GPa for the bulk modulus B0 is in reasonable agreement with calculations. The high pressure behaviour indicates a phase transformation to US III at about 15 GPa. The transformation is a smooth deformation process, which starts with a tetragonal structure ( a tetr = a cub 2 1 2 ; ctetr = 2acub) and continues with an orthorhombic structure (a = 375(3) pm; b = 345(3) pm; c = 1069(24) pm) at 35 GPa; it is second order within experimental error and it should involve some contributions from uranium f electrons.

High-pressure structural studies of uranium and thorium compounds with the rocksalt structure

Describes a white-beam X-ray energy-dispersive diffractometer built for Hasylab in Hamburg, FRG, using the synchrotron radiation from the electron storage ring DORIS. The following features of the instrument are discussed: horizontal or vertical scattering plane, collimators, sample environment, remote control of the goniometer, data acquisition, energy-sensitive detectors using small-area and large-area detector crystals, modes of operation, powder and single crystal diffraction. An example is given from a high-pressure study of YbH2 using a diamond anvil cell.

X-ray Diffraction Study on Pressure-Induced Phase Transformation in Nanocrystalline GaAs

The crystal structure of the superconducting compounds RBa2Cu3O9−δ where R is one of the rare earth metals Ho, Eu or Y, has been studied at room-temperature at pressures up to 60 GPa by synchrotron X-ray powder diffraction in a diamond anvil cell using the energy dispersive method At increasing pressure the compounds show semilar behavior with a bulk modulus in the range 140–176 GPa and a phase transition from an orthorhombic to a tetragonal structure at around 20 GPa. The transition is without a volume change and shows no hysteresis.

X-ray difffraction studies on samarium up to one megabar pressure

Abstract High-pressure X-ray diffraction studies have been performed on uranium and thorium rocksalt-structure compounds. Bulk modulus B 0 and its pressure derivative B 0 ′ have been determined. Pressure-induced first order phase transformations have been found for UC and UN, second order transitions for US, UP and ThS. No transition is found for ThC and ThN up to 40–45 GPa.

High pressure studies up to 50GPa of CuO

We have shown that the onset and transition pressures of the GaAs I M II transition are 17 GPa and 20 GPa, respectively, for both bulk and nanophase material. The observed gradual change in resistivity of nanophase GaAs at the semiconductor-to-metal transition is explained by the two-component model developed by Gleiter.

A New High-Pressure Phase and the Equation of State of YbH2

Abstract High—pressure crystal structure studies have been performed on Sm up to 100 GPa using synchrotron x-radiation and a diamond anvil cell. The structural sequence Sm-dhcp-fcc-dist.fcc has been confirmed. There is no evidence of any volume collapse. The bulk modulus and its pressure derivative have been determined (B0 = 30.7 GPa, B0’ = 2.5).

X-ray Diffraction Study of Zn3 (PO4) 24H2O and Zn/Ca Phosphates at High Pressure and Temperature

Abstract Copper oxide has been studied at high pressure up to 50 GPa. A monoclinic structure was compatible with the measurements at all pressures, and no phase change was observed. A bulk modulus, B0, = 98 GPa, and its pressure derivative B′0 = 5.6 was obtained.