Ga2.52V2·48O7·33(OH)0.67, a synthetic member of the nolanite/akdalaite-type family of oxyhydroxides containing trivalent vanadium

Abstract

The oxyhydroxide Ga2.52V2·48O7·33(OH)0.67 is prepared by reaction between Ga metal and Na3VO4 in a 1:1 monoethanolamine:water mixture at 240 ​°C. Powder neutron diffraction shows the material to be isostructural with the minerals nolanite and akdalaite, with cations occupying tetrahedral and octahedral interstitial sites in a hexagonal close-packed array of oxide/hydroxide (P63mc, a ​= ​5.7906(2) Å, c ​= ​9.2550(5) Å). Rietveld refinement against the data shows that Ga preferentially occupies tetrahedral sites, as well as some octahedral sites, and hence all V is octahedrally coordinated. The oxidation state of vanadium is confirmed as close to V3+ using V K-edge X-ray absorption near-edge structure spectroscopy, consistent with the refined chemical composition. The material is metastable, dehydrating around 300 ​°C and then decomposing above 500 ​°C, as shown by thermogravimetric analysis and thermodiffraction. The oxide Ga2.52V2.48O8 produced after dehydration at 300 ​°C is shown to contain a larger proportion of V4+ than the parent oxyhydroxide, to ensure charge balance, but the essential hexagonal structure is maintained. Variable temperature magnetisation measurements show that although both materials appear to obey the Curie-Weiss law at high temperatures, at low temperatures the inverse susceptibility curves are non-linear. There is, however, no evidence for strong magnetic exchange and the extracted effective moments are consistent with the presence of more V3+ in the oxyhydroxide compared to the oxide.