Evolution of structure in the incommensurate modulated LaNb1–xWxO4+x/2 (x = 0.04–0.16) oxide ion conductors

Abstract

Hyper-stoichiometric CeNbO4+d phases demonstrate remarkable oxygen diffusivity and provide an interesting structural template for oxygen ion conductors. Previously, we have reported the room temperature structure of the incommensurate modulated LaNb0.88W0.12O4.06, a structural analogue of CeNbO4+d. We have confirmed that it is a pure oxygen ion conductor, with anions diffusing via an interstitialcy mechanism. However, the high temperature structural information for the LaNb1–xWxO4+d (x = 0.04–0.16) family, which is key to understanding the structure–property relationship in oxygen ionic conductors with complex structures at operating conditions, is unreported. In this contribution, we address this question by investigating the high temperature structural evolution of the LaNb1–xWxO4+2/x phases using a combination of thermal analysis, scattering techniques, and 17O and 93Nb nuclear magnetic resonance spectroscopy. We reveal a series of phase transitions between a modulated monoclinic phase, a high temperature modulated tetragonal phase, and a high temperature unmodulated tetragonal phase. These findings are correlated with the ion transport and offer insights into the design of new materials for solid state electrochemical devices.