Exploring the nature of the fergusonite–scheelite phase transition and ionic conductivity enhancement by Mo6+ doping in LaNbO4

Abstract

A number of metal oxides that crystallise in the scheelite structure type are known to be excellent oxide ion conductors. Here we report the synthesis of a series of materials with general formula LaNb_1−xMo_xO_4+0.5x (x = 0, 0.08, 0.12, 0.16, 0.20) and excellent oxide-ionic conductivity for x ≥ 0.16 (7.0 × 10⁻³ S cm⁻¹ at 800 °C). Bond valence energy landscape analysis showing possible facile oxide ion migration pathways give important insights into the local influence of defects on oxide-ionic conductivity in these phases. We also use variable-temperature powder X-ray diffraction data to present, for the first time for any scheelite-type material, a symmetry distortion mode refinement-based analysis of the phase transition between the scheelite and fergusonite structure types. This structural phase transition is known to have implications for both oxide-ionic conductive and ferroelastic properties. We demonstrate that one particular distortion mode, namely the Γ₂⁺ displacive mode of the Nb atoms, is the most significant structural distortion leading to the symmetry-breaking phase transition from the tetragonal scheelite to the monoclinic fergusonite form of the material. Our diffraction data and ab initio lattice dynamics calculations provide evidence that the fergusonite–scheelite transition in these materials exhibits characteristics of a first-order transition.