Low thermal expansion materials:: a comparison of the structural behaviour of La0.33Ti2(PO4)3, Sr0.5Ti2(PO4)3 and NaTi2(PO4)3

Variable temperature neutron powder diffraction data have been used to examine the structural mechanism of unusual thermal expansion behaviour in La0.33Ti2(PO4)(3) (LaTP). These data are compared to those we have recently reported for the related systems NaTi2(PO4)(3) (NaTP) and Sr0.5Ti2(PO4)(3) (SrTP), which differ in the extent of occupation of the extra-Framework MI (La, Sr, Na) cation sites. The root cause of the unusual behaviour is ascribed to the differing expansivities of the occupied and vacant MI sites. This leads to cooperative rotations of TiO6 and PO4 polyhedra, which can be used to quantify and rationalise the overall anisotropic thermal expansion behaviour. (C) 1999 Elsevier Science Ltd. All rights reserved.