Unprecedented phase transition sequence in the perovskite Li(0.2)Na(0.8)NbO(3)

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.