Structural stability, cation ordering, and local relaxation along the AlNbO4-Al0.5Cr0.5NbO4 join

Abstract

(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 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

Effect of Impregnation on the Structure of Niobium Oxide/Alumina Catalysts Studied by Multinuclear Solid-State NMR, FTIR, and Quantum Chemical Calculations

“…In a related recent study, M. Ardit investigated a series of (Al 1– x Cr x ) 3+ Nb 5+ O 4 (with 0 ≤ x ≤ 0.5) compounds by the means of X-ray powder diffraction and electronic absorption spectroscopy . The author confirmed that AlNbO 4 was indeed highly disordered and was able to deduce the occupation equation.…”

“…According to Ardit, in AlNbO 4 , the proportion of aluminum satisfies the relation provided that Al M1 = Nb M2 .…”

On the Microstructure and Isothermal Oxidation at 800 and 1200 °C of the Nb-24Ti-18Si-5Al-5Cr-5Ge-5Sn (at.%) Silicide-Based Alloy

“…In the alloy ZF6, both AlNbO 4 and CrNbO 4 were formed, while only the former was formed with Cr in solution in OHS1, which means that more Cr was present in the scale of ZF6 because there is incomplete solid solubility between AlNbO 4 and CrNbO 4 [68]. Indeed, up to some critical Cr content, the (Al,Cr)NbO 4 is one phase with the structure of AlNbO 4 (monoclinic, C2/m space group) compared with two phases AlNbO 4 plus CrNbO 4 (tetragonal (rutile type structure), P42/mnm) when the critical Cr concentration is exceeded, in which there is no structural relaxation [68].…”

Precise Characterization of the Rich Structural Landscape Induced by Pressure in Multifunctional FeVO4

“…Figure 3 shows the enthalpy difference between all of the structures considered in the calculations. They include not only all of the previously known polymorphs but also the new FeVO 4 -I′ polymorph, a monoclinic structure described with the same space group as that of FeVO 4 -II′ but with different unit-cell parameters, 21 and a triclinic structure that is a distortion of FeVO 4 -II′. We used for the last two structures the names FeVO 4 -II″ and FeVO 4 -I″, respectively.…”

“…To obtain this polymorph in addition to pressure, a temperature of 800 °C has to be applied . This indicates that there is a large kinetic barrier blocking the transition from FeVO 4 -I to FeVO 4 -II.…”