J. Staun Olsen

Post-rutile high-pressure phases in TiO2

The crystal structures of rutile (TiO 2 ) and its high-pressure polymorphs have been studied by X-ray powder diffraction in the pressure range up to 60 GPa. At 12 GPa, rutile transforms to a phase with the baddeleyite (ZrO 2 ) structure. Upon decompression, this phase transforms at 7 GPa to another phase with the α-PbO 2 structure. At ambient conditions, the α-PbO 2 -type phase is 2.1(3)% denser than rutile and the baddeleyite-type phase is 11.3(9)% denser than rutile. In the pressure range of the rutile-to-baddeleyite transition, the difference in density between the two phases is 9.75(15)%. The zero-pressure bulk moduli, as determined from the equation of state, are 230 (20), 260 (30) and 290 (20) GPa for rutile, the α-Pbo 2 -type phase and the baddeleyite-type phase, respectively.

Powder diffraction analysis of cerium dioxide at high pressure

CeO 2 transforms to an orthorhombic PbCl 2 -type structure at a pressure of about 31 GPa. The phase transition is accompanied by a 9.8% volume contraction. The bulk modulus of the low-pressure fluorite-type structure is 236(4) GPa. Comparisons are made with the high-pressure behaviour of UO 2 and ThO 2 .

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{

Bulk moduli and high-pressure phases of the uranium rocksalt structure compounds-I. The monochalcogenides

x = 0.8

A new high-pressure phase of uranium nitride studied by X-ray diffraction and 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.

Actinide compounds under pressure

Abstract The high-pressure crystal structures of the compounds UX, where X = S, Se and Te, have been studied using X-ray diffraction in the pressure range up to about 60 GPa. A rhombohedral distortion is observed for US above 10 GPa. Use and UTe transform to the CsCl structure at about 20 GPa and 9 GPa, respectively. The latter transformations show a considerable hysteresis when releasing the pressure. The scaling behaviour of the bulk modulus has been studied. It is shown that a log-log plot of the bulk modulus versus specific volume for the cubic phases gives a straight line with a slope near - 2.

Electron Hopping and Temperature Dependent Oxidation States of Iron in Ilvaite Studied by Mössbauer Effect

High-pressure X-ray diffraction studies have been performed on UN powder for pressures up to 34 GPa using synchrotron radiation and a diamond anvil cell. For the cubic low-pressure phase the bulk modulus B0 equals 203(6) GPa and its pressure derivative B′0 equals 6.3(6) in good agreement with other data from the literature. The UN material has been found to transform to a new phase, UN III, at 29 GPa. The transformation is first order with a 3.2% decrease in volume. The UN III phase has been indexed according to a face-centred rhombohedral cell with a = 4.657(5) A and α = 85.8(2)° at 34 GPa. The influence of the 5f electrons in the transformation is discussed.

X-ray energy-dispersive powder diffractometry using synchrotron radiation

Abstract An overview of pressure-induced structural phase transitions and compressibility of actinide compounds will be given. Systematic trends in the nature of the high-pressure phases, the transition pressures, the hysteresis to retransformation on pressure release, and the compressibility are observed in the family of AnX compounds of B1 (NaCl) structure type. The dioxides studied up to now form high-pressure phases of PbCl2 type. UX2 compounds of Fe2As type also tend to have PbCl2 type high-pressure phases. The Th3P4 type compounds studied up to now did not transform up to 50 GPa. The same is true for ThOS and UOSe up to about 45 GPa. Comparison with rare earth compounds will be made where possible.

Phase transitions in ReO3 studied by high-pressure X-ray diffraction

The Mossbauer spectrum of ilvaite was measured between 115 K and 898 K, and the energy dispersive X-ray powder pattern was measured between 300 K and 1 123 K. Below 500 K the Mossbauer spectrum varies strongly with temperature while the X-ray spectrum remains unchanged. The results are interpreted by electron exchange between nearly identical Fe-sites in ilvaite.

High pressure x-ray diffraction studies on ThS up to 40 GPa using synchrotron radiation

Abstract The results of a test using synchrotron radiation in X-ray energy-dispersive powder diffractometry are reported and discussed