Polymorphism, negative thermal expansion and electrical properties of SrNb₂O₆

A combination of variable temperature X-ray diffraction, thermal analysis, and electrical measurements by fixed frequency dielectric spectroscopy, impedance spectroscopy, and electro-motive force measurements are used to establish the structure-electrical properties of SrNb₂O₆ ceramics prepared by the conventional solid state reaction technique. A polymorphic phase transition from a monoclinic, m, (space group P2₁/c) to an orthorhombic, o, (space group, Pmcn that is isomorphic with Pnma, CaTa₂O₆) cell occurs at ∼420 °C, accompanied by a negative thermal expansion effect (NTE) along the a-axis. The NTE in the monoclinic phase is driven primarily by a buckling of Nb₂O₁₀ dimers linked by corners sharing anions along the b-axis combined with a switch in their canting arrangement along the c-axis across the m- to o-transition. SrNb₂O₆ ceramics show no evidence of ferroelectricity; however, they display dielectric behaviour below ∼300 °C with m-SrNb₂O₆ exhibiting a room temperature permittivity of ∼30 that approximately doubles on transformation to o-SrNb₂O₆. There is significant leakage conductivity associated with adsorption of moisture near room temperature and mixed oxide-ion and p-type electronic conductivity at >300 °C. Both of these space charge related mechanisms are possible origins of the anomalous behaviour in the dielectric spectroscopy measurements that give rise to the apparent ferroelectric behaviour and mask the intrinsic electrical properties of SrNb₂O₆.