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

• Published in: IUCrJ Vol. 4; no. 3; pp. 215 - 222
• Main Authors: Dixon, Charlotte A L, McNulty, Jason A, Huband, Steven, Thomas, Pamela A, Lightfoot, Philip
• Format: Journal Article
• Language: English
• Published: Chester International Union of Crystallography 01.05.2017
• Subjects: ferroelectric; perovskite; Perovskites; powder neutron diffraction; Research Papers
• ISSN: 2052-2525; 2052-2525
• DOI: 10.1107/S2052252517002226

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 P42mc) 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 P42mc to P42/nmc, retaining the a+a+c− tilt. Transformation to a single-tilt system, a0a0c+ (space group P4/mbm), occurs at 750°C, with the final transition to the aristotype cubic phase at 850°C. The P42mc 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.