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

Abstract

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. α_a for LiTP is found to decrease from 0.75 to 0.27 × 10^–6 °C^–1 between 20 and 800 °C; α_c is found to be constant at 30.8 × 10^–6 °C^–1 over this temperature range. α_a for KTP is found to increase from –3.2 to 2.5 × 10^–6 °C^–1 over the same temperature range and α_V to increase from –0.8 to 12.8 × 10^–6 °C^–1. The anomalously large value of α_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.