Analysis of the local structure of phosphorus-substituted LAMOX oxide ion conductors

Abstract

The effect of partial substitution of molybdenum by phosphorus on the global and local structural arrangement of the fast oxide-ion conductor La₂Mo₂O₉ (LAMOX) has been studied by X-ray powder diffraction as well as ¹³⁹La and ³¹P solid state NMR. The diffraction patterns show that La₂Mo_2−yP_yO_9−y/2 forms a solid solution at low phosphorus concentrations, and that there is a structural phase transition upon increasing phosphorus concentration. This phase transition is also reflected in ¹³⁹La and ³¹P NMR spectra. The possibility to excite ³¹P multiple-quantum coherences of one of the ³¹P NMR signals gives evidence of an accumulation of phosphorus atoms on neighbouring Mo-type sites already before formation of three-dimensional precipitates. On the basis of our X-ray and NMR results we propose a possible structural arrangement of the compound La₂Mo_2−yP_yO_9−y/2 that explains the experimental observations by crystal twinning.