Unprecedented phase transition sequence in the perovskite Li0.2Na0.8NbO3
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We demonstrate the existence of a novel phase transition sequence in the perovskite Li0.2Na0.8NbO3 at elevated temperature. The rare Glazer tilt system, a+a+c-, is observed in both polar and centrosymmetric polymorphs. 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 previously in perovskite crystallography, in both cases in compositions prepared at high pressure.
Dixon , C A L , McNulty , J A , Huband , S , Thomas , P A & Lightfoot , P 2017 , ' Unprecedented phase transition sequence in the perovskite Li 0.2 Na 0.8 NbO 3 ' International Union of Crystallography Journal (IUCrJ) , vol 4 , no. 3 . DOI: 10.1107/S2052252517002226
International Union of Crystallography Journal (IUCrJ)
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Funding for this research was provided by: EPSRC (award No. EP/L505079/1).
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