The thermoresponsive behaviour of Nasicon-like CuTi2(PO4)3

Abstract

The synthesis and thermoresponsive behaviour of high purity CuTi2(PO4)3 (CTP) was examined while heating in air and nitrogen at atmospheric pressure. It is known that a spontaneous redox reaction involving the Cu+ cations that are contained in the Ti2(PO4)3− crystal structure converts the CTP phase to the related Cu0.5Ti2(PO4)3 (CTP2) phase when heated in air. However, the extent of this reaction and the effect that it has on the CTP crystal structure has not been established. Using in situ powder X-ray diffraction it was found that during heating in air, the migration of Cu+ cations out of the Ti2(PO4)3− crystal lattice results in the growth of a CuO phase. To quantify the extent of the migration of the Cu+ cations out of the Ti2(PO4)3− crystal lattice, quantitative phase analysis was used to show that the redox reaction converts the CTP phase to the CTP2 phase in the temperature region from 450 to 530 °C. When increasing the temperature above 530 °C it was found that the CTP phase was spontaneously regenerated forming a high temperature form of the parent CTP phase. Previously heat-treated CTP samples were subjected to further heat treatments and produced different quantitative phase behaviour when compared to a fresh CTP sample. Most significantly it was found that the CTP2 phase was prevalent over a broader temperature range. Comparison of the high temperature and parent CTP phases showed differences between the Cu+ site occupancies. Further heat treatment under nitrogen suppressed the redox reaction and stabilized the CTP phase. This allowed the calculation of the coefficients of thermal expansion (CTE) of CTP across a broad temperature range (30–900 °C). Analysis of the CTP lattice constants via Parametric Rietveld refinement showed that the compound exhibited moderate thermal expansion up to 900 °C.