Abstract
The perovskite Li(0.2)Na(0.8)NbO(3) 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 P4(2)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 P4(2)mc to P4(2)/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 P4(2)mc and P4(2)/nmc phases have each been seen only once and twice each, respectively, in perovskite crystallography, in each case in compositions prepared at high pressure.