ReO2 presents two crystalline variants, with monoclinic and orthorhombic structures. The former is metastable and irreversibly transforms to an orthorhombic structure above 460ºC. The structure of the latter was determined from studies on monocrystalline samples, whereas for the monoclinic variant there are no single crystals available so far. It was found only one monoclinic variant and the structure associated with this variant is based on studies on polycrystals. We analyzed a monoclinic oxide powder sample by X-ray diffraction and refined its pattern by means of the Rietveld Method. We obtained that the monoclinic variant belongs to space group P2(1)/c, with lattice parameters a = 5.615(3), b = 4.782(2), c = 5.574(2) Å, beta = 120.13(1)º.
