General Considerations.- Transport Properties.- Local Atomic Structure of CMR Manganites and Related Oxides.- Optical Spectroscopic Studies of Metal-Insulator Transitions in Perovskite-Related Oxides.

Localized to Itinerant Electronic Transition in Perovskite Oxides

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.

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

Plutonium monoarsenide was studied under high pressure up to 45 GPa using a diamond anvil cell in an energy dispersive X-ray diffraction facility. The compound, which has the NaCl-type structure at ambient pressure, undergoes a first-order crystallographic phase transition at around 35 GPa. The high-pressure phase could be indexed in the cubic CsCl type. The volume collapse at the phase transition is 9%. From the pressure-volume relationship we determined a bulk modulus of B 0 = 68.6 GPa with a pressure derivative of B ' 0 = 3.3. These results are compared with those obtained with other actinide monoarsenides.

Compression studies on PuAs up to 45 GPa

The rare-earth and actinide based compounds are endowed with several exotic physical and chemical properties due to the presence of f-electrons. These properties exhibit interesting changes under the action of various thermodynamic fields and hence continues to be a subject of extensive research. For instance, under pressure, the nature of f-electrons can be changed from localized to itinerant, leading to a variety of changes in their structural, physical and chemical properties. The present review on the high pressure phase transition behaviour of dialuminides of rare earths and actinides is an outcome of research in our laboratory during the last five years using a unique combination of a Guinier diffractometer and a diamond anvil cell built in-house. To bring out the correlations between the compressibility and structural behaviour with the electronic structure, we have also carried out electronic structure calculation. Further, the usefulness of Villars’ three parameter structure maps in predicting pressure induced structural transitions has been explored and this has been illustrated with the available phase transition data.

Pressure induced phase transition behaviour in f-electron based dialuminides

Uranium is one of the main heat sources in the Earth, as about 25% of the total heat is produced by the radioactive decay of U. The location of U in the deep mantle is then essential for a better understanding of the geodynamics and thermal behavior of the Earth. For the first time, the crystal structure of natural simple dioxide UO 2 uraninite has been studied by X-ray diffraction with synchrotron radiation (ESRF, Grenoble, France), in situ in a laser-heated diamond-anvil cell at pressures and temperatures relevant to the deep Earth’s mantle. Fluorite-type UO 2 displays a new sequence of phase transitions at high P and T , with a cubic modified fluorite Pa 3 observed at 18 GPa, and an orthorhombic Pbca structure from 33 GPa up to 82 GPa. Using a second-order Birch-Murnaghan equation of state, we calculated room-pressure bulk modulus K 0 = 166(7) GPa with pressure derivative K ′ 0 = 4.0 for the Pa 3 structure, and K 0 = 225(8) GPa with K ′ 0 = 4 for the Pbca structure. The expected Pnma cotunnite structure was not observed but is not excluded at pressures higher than 82 GPa. Since UO 2 displays a Pbca structure stable up to 82 GPa and presents a density much higher than the average density of the surrounding mantle, UO 2 could be a host of U in the deep lower mantle.

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Structural characterization of natural UO2 at pressures up to 82 GPa and temperatures up to 2200 K

This chapter is an introduction to the main concepts underlying the present unterstanding of the physical properties of actinide solids.

L. Manes

Papers

Actinide solids 5f dependence of physical properties