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Structure and nuclear density distribution in the cheralite—CaTh(PO4)2: studies of its behaviour under high pressure (36?GPa) |
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| Authors: | Philippe E Raison Stephen Heathman Gilles Wallez Carmen E Zvoriste Denis Bykov Ga?l Ménard Emmanuelle Suard Karin Popa Nicolas Dacheux Rudy J M Konings Roberto Caciuffo |
| Institution: | 1. European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125, Karlsruhe, Germany 2. Chimie-ParisTech, Laboratoire de Chimie de la Matière Condensée de Paris, UPMC Univ. Paris 06, UMR CNRS 7574, 75231, Paris Cedex 05, France 3. Institute Laue-Langevin, Rue Jules Horowitz, BP 156, 38042, Grenoble cedex 9, France 4. Department of Chemistry, “Al.I. Cuza” University, 11-Carol I Blvd., 700506, Iasi, Romania 5. Institut de Chimie Séparative de Marcoule, UMR CNRS 5257, 30207, Bagnols sur Cèze, France |
| Abstract: | 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 36?GPa. 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 B0?=?140(2) GPa and B 0 ′ ?=?4.4(4) GPa. |
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