Five-Dimensional Incommensurate Structure of the Melilite Electrolyte [CaNd]_2[Ga]_2[Ga2O_7]_2

摘要

Melilite-type gallium oxides are potential intermediate temperature electrolytes for solid oxide fuel cells. Single crystals of [CaNd]_2[Ga]_2[Ga_2O_7]_2 grown using an optical floating zone furnace have been investigated using transmission electron microscopy and powder and single-crystal X-ray diffraction. The anion array topologically conforms to a [(3.5.4.5)~2 , 3.5.3.5] network that contains distorted pentagonal tunnels. The distortion is necessary to achieve space filling and accommodate structural misfit between the layers. Satisfactory bond lengths and angles are obtained through two-dimensional modulation in the tetragonal based plane, leading to five-dimensional symmetry in the superspace group P42_1m(α,α,0)00s(α,α,0)000, α = 0.2319(2), with modulation vectors q_1 = α(a~* + b~* ) and q_2 = α.(-a~* + b~* ). rioth displacive and occupational modulations are found, Through this mechanism, mehlites are primed to accommodate mobile oxygen interstitials, suggesting a rational approach to crystallochemical tailoring that will enhance ionic diffusion and optimize electrolyte performance.