Unprecedented phase transition sequence in the perovskite Li0.2Na0.8NbO3

Abstract

The perovskite Li0.2Na0.8NbO3 is shown, by powder neutron diffraction, to display a unique sequence of phase transitions at elevated temperature. The ambient temperature polar phase (rhombohedral, space group R3c) transforms via a first-order transition to a polar tetragonal phase (space group P42 mc) in the region 150-300°C; these two phases correspond to Glazer tilt systems a - a - a - and a + a + c -, respectively. At 500°C a ferroelectric-paraelectric transition takes place from P42 mc to P42/nmc, retaining the a + a + c - tilt. Transformation to a single-Tilt system, a 0 a 0 c + (space group P4/mbm), occurs at 750°C, with the final transition to the aristotype cubic phase at 850°C. The P42 mc and P42/nmc phases have each been seen only once and twice each, respectively, in perovskite crystallography, in each case in compositions prepared at high pressure.