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 LaNb1−xMoxO4+0.5x(x= 0, 0.08, 0.12, 0.16, 0.20) and excellent oxide-ionic conductivity forx≥ 0.16 (7.0 × 10−3S cm−1at 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 andab initiolattice dynamics calculations provide evidence that the fergusonite-scheelite transition in these materials exhibits characteristics of a first-order transition.