Structural stability, cation ordering, and local relaxation along the AlNbO₄-Al_0.5Cr_0.5NbO₄ join

Abstract

(Al_1-xCr_x)³⁺Nb⁵⁺O₄ (with 0 ≤ x ≤ 0.5) compounds have been investigated through the combination of X-ray powder diffraction (XRPD) and electronic absorption spectroscopy (EAS). In spite of the natural occurrence of AlTaO₄, the lack of a mineral with composition AlNbO₄ contrasts with the strong geochemical affinity between Nb and Ta elements. Rietveld refinements of XRPD data showed that the effective coordination numbers of the two non-equivalent octahedral sites (M1 and M2) in the AlNbO₄ structure are much lower than expected, especially the one mainly occupied by Nb. This is in agreement with the very low crystal field strength values (10Dq) found by EAS for Cr³⁺ replacing Al at site M2. These findings imply that an unfavorable bonding situation occurs for Nb, Al, and Cr ions in the AlNbO₄ structure, which can be regarded as substantially strained compared to AlTaO₄, thus explaining the lack of a natural AlNbO₄ isomorph. The observed long local Cr-O distances (low 10Dq) reveal that the AlNbO₄ lattice is not relaxed as a consequence of the Cr-Al substitution (the relaxation coefficient ε is close to zero) and the AlNbO₄ structure appears to follow the Vegard’s law. This is due to the fact that the Cr³⁺ for Al³⁺ substitution, for the limited range of solid solution (up to 0.2 apfu at site M2), does not induce any additional octahedral strain in a lattice already significantly strained.