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

Abstract

In this work we investigated the structural behaviour of a CaMn 0.5 Zr 1.5 (PO 4 ) 3 . Due to the presence of divalent Mn 2 þ 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 Mn 2 þ 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, a¼ 8.7850(2) Å, c¼ 22.6496(7) Å, V¼1514.8(1) Å 3 ). The high temperature form in turn has orthorhombic symmetry (space group Pnma, a ¼6.2350(3) Å, b ¼6.6281(3) Å, c¼ 14.4731(6) Å, V¼ 598.13(5) Å 3 ). 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. Mn 2 þ 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.