Structural and dielectric studies of the phase behaviour of the topological ferroelectric La1-xNdxTaO4.

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(3n+2) layered perovskites are discussed.