(Al1−xCrx)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 ɛ 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.

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