Experimental and theoretical investigations on the polymorphism and metastability of BiPO4

Abstract

In this work we report the metastability and the energetics of the phase transitions of three different polymorphs of BiPO₄, namely trigonal (Phase-I, space group P3₁21), monoclinic monazite-type (Phase-II, space group P2₁/n) and SbPO₄-type monoclinic (Phase-III, space group P2₁/m) from ambient and non-ambient temperature powder XRD and neutron diffraction studies as well as ab initio density functional theory (DFT) calculations. The symmetry ambiguity between P2₁ and P2₁/m of the high temperature polymorph of BiPO₄ has been resolved by a neutron diffraction study. The structure and vibrational properties of these polymorphs of the three polymorphs have also been reported in detail. Total energy calculations have been used to understand the experimentally observed metastable behavior of trigonal and monazite-type BiPO₄. Interestingly, all of the three phases were found to coexist after heating a single phasic trigonal BiPO₄ to 773 K. The irreversible nature of these phase transitions has been explained by the concepts of the interplay of the structural distortion, molar volume and total energy.