The crystal structure of the cheralite - CaTh(PO4)2 - has been revisited by neutron diffraction and its behaviour under very high pressure investigated by X-ray diffraction up to 38 GPa. The neutron diffraction data at ambient pressure gave a more accurate determination of the Ca/Th cations 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 specific directions 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 in agreement with other monazite-type compounds. From the evolution of the unit-cell volume as a function of the pressure, the zero-pressure bulk modulus B0 and its pressure derivative B'0 have been determined by fitting the experimental compressibility curve to the Birch-Murnaghan equation of state. The results are B0 = 131(4) GPa and B'0 = 6.1(4) GPa. Keywords: actinides, phosphates, cheralite, monazite, neutron diffraction, Rietveld analysis, maximum entropy method.

RAISON Philippe;  HEATHMAN Stephen;  WALLEZ G;  ZVORISTE Carmen Elena;  BYKOV Denis;  MENARD Gael;  SUARD E.;  POPA Karin;  DACHEUX Nicolas;  KONINGS Rudy;  CACIUFFO Roberto; 

2013-01-08

SPRINGER

JRC67332

0342-1791,   

https://publications.jrc.ec.europa.eu/repository/handle/JRC67332,   

10.1007/s00269-012-0522-z,