Exploring the unique electrical properties of metastable BiPO4 through switchable phase transitions

Metastable materials usually possess unique properties. How to acquire these properties is still a great challenge. In this work, we explored the electrical properties of metastable BiPO4 through switchable phase transitions. A metastable monoclinic phase (denoted as HTMP) was synthesized by a heat-treatment over a hexagonal phase (HP). It is found that there is a reversible phase transformation between HTMP and a low-temperature monoclinic phase (LTMP). Namely, HTMP gradually transformed to LTMP by a simple hand-grinding or ball milling, while LTMP transformed back to HTMP upon a heat treatment. Accompanying the transformation from HTMP to LTMP, the morphology varied from the cobblestone-like to spherical-like, and particle sizes changed from micrometre scale to several tens of nanometres, as followed by a decrease in the symmetry of tetragonal PO43- groups from C-s to C-1. The reversible transformation was understood by taking into account several structural factors like the arrangement of PO4 tetrahedra and BiO8 polyhedra, unit cell volume (V/Z), and the symmetry of PO4. Finally, the electrical properties of the metastable HTMP were successfully acquired through a pellet-pressing technique. The temperature dependence of bulk conductivity indicates that the conductivities deviated from the Arrhenius law, but followed a simple temperature dependence, T-1/4, obeying a Mott variable range hopping conduction mechanism. The corresponding characteristic temperature of the bulk conduction for HTMP is estimated to be 2.3 x 10(9) K. These findings are fundamentally important, as they enable one to acquire the properties of many other metastable phases, meanwhile they pave a way for solving the controversies presented in metastable systems.