Incommensurate Modulated Structures in the Ta2O5 - Al2O3 System

Abstract

Members of the (1 – x)Ta2O5·xAl2O3 series were synthesized, and the structures investigated using synchrotron X-ray powder diffraction and neutron powder diffraction data for the first time. Structural models were developed and refined using the Rietveld method and a [3 + 1] dimensional incommensurately modulated composite structure approach with a composition dependent modulation vector q, and superspace group Xmmm(0β0)s00. Displacive atomic modulation functions across the (1 – x)Ta2O5·xAl2O3 series were found to be very similar, and strongly resemble those for the Ta2O5–WO3 system, in line with the notion that there are structure types in higher dimensional space just as there are in 3D space. Bond valence sum calculations and bond distance plots showed that the introduction of the modulation to the structural model generally led to more favourable bond valence sum values and bond distances. Fourier difference plots were examined, and the occupational modulation of aluminium refined to determine that the aluminium atoms preferentially occupy the octahedral sites.