study of disorder in Ni substituted to calcium orthovandates through analysis of low temperature diffraction data

β-calcium orthophosphate and calcium orthovanadate (Ca3(XO4)2, X = P or V), are known to crystallize in R3c space group. A small fraction of Ca atoms can be replaced by other ones, of valences from +1 to +4, without a change of structure. We are interested in studying of the new whitlockite material, Ca10.5-xNix(VO4)7. We already determined structural properties at room temperature. The aim of proposed study is determination of thermal expansion at LT (that is not studied yet for the family of whitlockite-related materials), and atomic displacement variation of different cationic sites. It leads to take a look deeply on type of disorder and modelling of the temperature variation of atomic displacement, separately for each cationic site.