HIGH-TEMPERATURE NEUTRON DIFFRACTIONS TUDY OF CAZR4(PO4)6

The lattice thermal expansion of rhombohedral CaZr4(PO4)6 has been determined to be anisotropic with a negative expansion coefficient along the directions in the plane (0001 ). This behaviour is anomalous if compared to the rest of the compounds in the set M(II)M4IV (PO4)6; M(II)=Mg, Ca, Sr, Ba; M(IV)=Ti, Sn, Zr and to the low temperature form of M(I)Sn2(PO4)3 M(I)=Na, K, Rb. High temperature high resolution neutron diffraction shows that the rotations of ZrO6 and PO4 polyhedra are opposite to those in the other cases.