The influence of the 6s2 configuration of Bi3+ on the structures of A′BiNb2O7 (A′ = Rb, Na, Li) layered perovskite oxides

Abstract

Solid state compounds which exhibit non-centrosymmetric crystal structures are of great interest due to the physical properties they can exhibit. The ‘hybrid improper’ mechanism – in which two non-polar distortion modes couple to, and stabilize, a further polar distortion mode, yielding an acentric crystal structure – offers opportunities to prepare a range of novel non-centrosymmetric solids, but examples of compounds exhibiting acentric crystal structures stabilized by this mechanism are still relatively rare. Here we describe a series of bismuth-containing layered perovskite oxide phases, RbBiNb₂O₇, LiBiNb₂O₇ and NaBiNb₂O₇, which have structural frameworks compatible with hybrid-improper ferroelectricity, but also contain Bi³⁺ cations which are often observed to stabilize acentric crystal structures due to their 6s² electronic configurations. Neutron powder diffraction analysis reveals that RbBiNb₂O₇ and LiBiNb₂O₇ adopt polar crystal structures (space groups I2cm and B2cm respectively), compatible with stabilization by a trilinear coupling of non-polar and polar modes. The Bi³⁺ cations present are observed to enhance the magnitude of the polar distortions of these phases, but are not the primary driver for the acentric structure, as evidenced by the observation that replacing the Bi³⁺ cations with Nd³⁺ cations does not change the structural symmetry of the compounds. In contrast the non-centrosymmetric, but non-polar structure of NaBiNb₂O₇ (space group P2₁2₁2₁) differs significantly from the centrosymmetric structure of NaNdNb₂O₇, which is attributed to a second-order Jahn-Teller distortion associated with the presence of the Bi³⁺ cations.