Comparison of the structural behaviour of the low thermal expansion NZP phases MTi₂(PO₄)₃ (M = Li, Na, K)

We present the results of high resolution neutron powder diffraction studies of the NZP phases LiTi2(PO4)(3) (LiTP) and KTi2(PO4)(3) (KTP). The thermal expansion coefficients for both materials are reported and compared to those of NaTi2(PO4)(3). alpha(a) for LiTP is found to decrease from 0.75 to 0.27 x 10(-6)degrees C-1 between 20 and 800 degrees C; alpha(c) is found to be constant at 30.8 x 10(-6)degrees C-1 over this temperature range. alpha(a) for KTP is found to increase from -3.2 to 2.5 x 10(-6)degrees C-1 over the same temperature range and alpha(V) to increase from -0.8 to 12.8 x 10(-6)degrees C-1. The anomalously large value of alpha(c) in LiTP is related to the thermally induced migration of Li+ from MI to MII sites, behaviour which is unique in these systems. Alamo's model for thermal expansivity in this system is applied and parameters reported.