Fe-bearing vanadium dioxide–paramontroseite: structural details and high-temperature transformation

Abstract

A natural sample of Fe-bearing paramontroseite (V0.84Fe0.19Al0.03O2; a = 4.8960(14), b = 9.395(3), c = 2.9163(5) Å, V = 134.14(6) Å3; space group Pbnm) from Prachovice mine (Czech Republic) was investigated to shed light on cation partitioning and behavior upon heating. XRD experiments showed that V and Fe are not vicariant of one another, as V occupies the octahedral site at (0.09, 0.14, 0.25), whereas Fe enters a tetrahedral site at (0.41, 0.06, 0.25), the latter expected to be empty in the ideal structure. Thermal expansion is anisotropic, leading to the following β coefficients: -2.0×10-5, 3.0×10-5, 0.8×10-5 and 1.8×10-5 ∘C−1 for a, b, c and V, respectively. At T higher than 350 ∘C, V undergoes oxidation, from [4+] to [5+], and paramontroseite decomposes into Fe-tetrapolyvanadate (Fe2V4O13) and V-pentoxide (V2O5). µ-Raman spectroscopy analyses confirmed that paramontroseite is sensitive to heating: the crystal surface invested by the laser beam degrades very quickly, leading to the phases revealed by diffraction measurements. There is no evidence for the formation at high T of a rutile-type phase, as we observed for iso-structural ramsdellite MnO2.