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

A natural sample of Fe-bearing paramontroseite (Vinline-formula_0.84Feinline-formula_0.19Alinline-formula_0.03Oinline-formula₂; inline-formulaa inline-formula= 4.8960(14), inline-formulab inline-formula= 9.395(3), inline-formulac inline-formula= 2.9163(5) Å, inline-formulaV inline-formula= 134.14(6) Åinline-formula³; 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 inline-formulaβ coefficients: inline-formula $M15inlinescrollmathml-\mathrm{normal\; 2.0}\times {\mathrm{normal\; 10}}^{-\mathrm{normal\; 5}}$ 59pt13ptsvg-formulamathimg4dfc02a6053a4294b6cbe98cdc222976 ejm-35-373-2023-ie00001.svg59pt13ptejm-35-373-2023-ie00001.png , inline-formula $M16inlinescrollmathml\mathrm{normal\; 3.0}\times {\mathrm{normal\; 10}}^{-\mathrm{normal\; 5}}$ 51pt13ptsvg-formulamathimg8cc417b3ec8f921814a72838e9c03d99 ejm-35-373-2023-ie00002.svg51pt13ptejm-35-373-2023-ie00002.png , inline-formula $M17inlinescrollmathml\mathrm{normal\; 0.8}\times {\mathrm{normal\; 10}}^{-\mathrm{normal\; 5}}$ 51pt13ptsvg-formulamathimg1e2705fa2131d85215d6eed4c0d0304c ejm-35-373-2023-ie00003.svg51pt13ptejm-35-373-2023-ie00003.png and inline-formula $M18inlinescrollmathml\mathrm{normal\; 1.8}\times {\mathrm{normal\; 10}}^{-\mathrm{normal\; 5}}$ 51pt13ptsvg-formulamathimg5851cc401e05a481a4c4a46ee741295c ejm-35-373-2023-ie00004.svg51pt13ptejm-35-373-2023-ie00004.png  inline-formula^∘Cinline-formula⁻¹ for inline-formulaa, inline-formulab, inline-formulac and inline-formulaV, respectively. At inline-formulaT higher than 350 inline-formula^∘C, V undergoes oxidation, from [4inline-formula+] to [5inline-formula+], and paramontroseite decomposes into Fe-tetrapolyvanadate (Feinline-formula₂Vinline-formula₄Oinline-formula₁₃) and V-pentoxide (Vinline-formula₂Oinline-formula₅). inline-formulaµ-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 inline-formulaT of a rutile-type phase, as we observed for iso-structural ramsdellite MnOinline-formula₂.