Structural and dielectric studies of the phase behaviour of the topological ferroelectric La_1-xNd_xTaO₄

The layered perovskite LaTaO₄ 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_0.90Nd_0.10TaO₄ 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 LaTaO₄, we have not succeeded in obtaining ferroelectric P–E hysteresis behaviour. Structural relationships in the wider family of A_nB_nX_3n+2 layered perovskites are discussed.