Structural and dielectric studies of the phase behaviour of the topological ferroelectric La1−xNdxTaO4Electronic supplementary information (ESI) available. See DOI: 10.1039/c4dt03721a

The layered perovskite LaTaO 4 has been prepared in its polar orthorhombic polymorphic form at ambient temperature. Although no structural phase transition is observed in the temperature interval 25° C < T < 500 °C, a very large axial thermal contraction effect is seen, which can be ascribed to an anomalous buckling of the perovskite octahedral layer. The non-polar monoclinic polymorph can be stabilised at ambient temperature by Nd-doping. A composition La 0.90 Nd 0.10 TaO 4 shows a first-order monoclinic-orthorhombic (non-polar to polar) transition in the region 250° C < T < 350 °C. Dielectric responses are observed at both the above structural events but, despite the 'topological ferroelectric' nature of orthorhombic LaTaO 4 , we have not succeeded in obtaining ferroelectric P - E hysteresis behaviour. Structural relationships in the wider family of A n B n X 3 n +2 layered perovskites are discussed. Phase behaviour in the layered perovskite LaTaO 4 has been analysed, including an unusual tilt-induced axial thermal contraction effect.