Temperature induced phase transition of CaMn0.5Zr1.5(PO4)3 phosphate

Abstract

In this work we investigated the structural behaviour of a CaMn0.5Zr1.5(PO4)3. Due to the presence of divalent Mn2+ cations this compound can possess interesting luminescence properties. It was recently understood that this phosphate undergoes a temperature induced irreversible phase transition in the range of 800–875°C. It has also been shown that the 3d–3d luminescence of Mn2+ increases 10 fold for the high temperature polymorph. To determine the Mn environment structural investigations of both phases have been performed by the X-ray powder diffraction and Raman spectroscopy methods. The low temperature modification adopts the trigonal NZP structure type with a slightly lower symmetry (space group R32, a=8.7850(2)Å, c=22.6496(7)Å, V=1514.8(1)Å3). The high temperature form in turn has orthorhombic symmetry (space group Pnma, a=6.2350(3)Å, b=6.6281(3)Å, c=14.4731(6)Å, V=598.13(5)Å3). Both structures were solved ab-initio from powder data and structural analysis was performed. In-situ and RT Raman spectra are consistent with the XRD derived structural model. Mn2+ cations occupy different types of positions in these structures and a change in Mn coordination number (6 for LT phase, 7 for HT phase) results in different Mn–O bond lengths. These differences may explain the change in the optical properties between the polymorphs.