S. Heathman

Actinide compounds under pressure

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.

Pressure-induced phase transition in α-Pu

Abstract High purity α-Pu was studied by X-ray diffraction in a diamond anvil cell up to 62 GPa. A structural phase transition occurs around 40 GPa. The high pressure structure was indexed in a hexagonal lattice of space group P63/m, a = 537.77 pm, c = 445.51 pm at 62 GPa, Z = 8. The bulk modulus B0 of α-Pu was determined as 43(2) GPa, with a pressure derivative B0′ = 15(2).

High-pressure phases of plutonium monoselenide studied by X-ray diffraction

Abstract Plutonium monoselenide was studied under high pressure up to 47 GPa, at room temperature, using a diamond anvil cell in an energy dispersive X-ray diffraction facility. At ambient pressure, PuSe has the NaC1-type (B1) structure. The compound has been found to undergo a second-order crystallographic phase transition at around 20 GPa. This phase can be described as a distorted B1 structure, with a rhombohedral symmetry. PuSe transforms to a new phase at around 35 GPa, which can be indexed in the cubic CsCl-type (B2). The volume collapse at this phase transition is 11%. When releasing pressure, we observed a strong hysteresis to the inverse transformation down to 5 GPa. From the pressure-volume relationship, the bulk modulus has been determined to B 0 = 98 GPa and its pressure derivative as B 0 = 2.6. These results are compared to those obtained with other actinide monmictides and monochalcogenides.

Compression study of uranium borides UB2, UB4 and UB12 by synchrotron X-ray diffraction

X-ray diffraction analysis was performed on uranium borides under pressure to study their stability vs. compression as well as to obtain their ambient pressure bulk moduli B0. Their crystalline structures remain stable up to 50 GPa and the mean values of B0 and B01 are 225 GPa and 2.6 for UB2, 181 GPa and 4.8 for UB4, 249 GPa and 3.4 for UB12 respectively.

Synthesis and electronic properties of Th-N films

Abstract Thin layers of Th, ThN, and Th 3 N 4 were synthesized by sputter deposition of Th in an Ar atmosphere with variable nitrogen partial pressure. The phase purity and crystal structure of the nitrides were confirmed by X-ray diffraction. Photoelectron spectroscopy studies demonstrate that ThN is metallic, with a rather high density of 6d states at the Fermi level, while Th 3 N 4 is non-metallic.

Regular paperHigh pressure x-ray diffraction study of UC2 using synchrotron radiation

Abstract UC2 has been studied under the general topic of X-ray diffraction analysis of pressure-induced structural transitions in actinide compounds and the determination of their ambient pressure bulk moduli. At 17.6 GPa UC2 shows a phase transition from a tetragonal to a hexagonal structure. The mean values of the bulk modulus and its pressure derivative are 216 GPa and 2.2 respectively.

High pressure x-ray diffraction study of UC2 using synchrotron radiation

Abstract UC2 has been studied under the general topic of X-ray diffraction analysis of pressure-induced structural transitions in actinide compounds and the determination of their ambient pressure bulk moduli. At 17.6 GPa UC2 shows a phase transition from a tetragonal to a hexagonal structure. The mean values of the bulk modulus and its pressure derivative are 216 GPa and 2.2 respectively.

Structure and nuclear density distribution in the cheralite—CaTh(PO4)2: studies of its behaviour under high pressure (36 GPa)

The crystal structure of the cheralite—CaTh(PO4)2—has been revisited by neutron diffraction and its behaviour under high pressure investigated by X-ray diffraction up to 36xa0GPa. The neutron diffraction data at ambient pressure gave a more accurate determination of the Ca/Th cation position than previous XRD data, taking advantage that the neutron scattering lengths of calcium and thorium are of same order of magnitude. The nuclear density distribution was also determined using the maximum entropy method (MEM) confirming that the two cations are not located at the same position in the unit cell but are slightly displaced from one another along a specific direction in order to minimize the electrostatic repulsion with the surrounding phosphorus atoms. At high pressure, the compound did not show any phase transition or amorphization. From the evolution of the unit-cell volume as a function of the pressure, the zero-pressure bulk modulus B0 and its pressure derivative B0′ have been determined by fitting the experimental compressibility curve to the Birch–Murnaghan equation of state. The results are B0xa0=xa0140(2) GPa and B0′xa0=xa04.4(4) GPa.

A structural study of promethium metal under pressure

Abstract The structural behaviour of Pm metal has been investigated up to 60 GPa of pressure using a Diamond Anvil Cell (DAC) and the energy dispersive X-ray diffraction technique. The room temperature/pressure structural form of Pm is dhcp and it transforms to a fcc phase by 10 GPa. This cubic phase of the metal converts by 18 GPa to a third phase, which has frequently been referred to as representing a distorted fcc structure. This latter form of Pm was retained up to 60 GPa, the maximum pressure studied, but subtle changes in the X-ray spectra between 50 and 60 GPa hinted that an additional structural change could be forthcoming at higher pressures. From the experimental data a bulk modulus (B0) of 38 GPa and a B0′ constant of 1.5 were calculated using the Birch equation. This modulus for Pm is in accord with the moduli reported for the neighboring lanthanide metals.

High pressure X-ray diffraction experiments on NpS and PUS up to 60 GPa

Abstract Neptunium and plutonium monosulfides were studied under high pressure up to ∼60 GPa using a diamond anvil cell in an energy dispersive X-ray diffraction facility. The compounds, of cubic rock salt structure type at ambient pressure, do not show any crystallographic phase transition in the domain of investigation. From the pressure-volume relationship, we determined bulk moduli of 92 and 120 GPa with pressure derivatives of 4.6 and 4.1 for NpS and PUS respectively.