Soft X-ray absorption at the O K-edge in rutile-type natural oxides

Abstract

An X-ray absorption study on O 1s edge was carried out on natural oxides with rutile-type crystal structure – the minerals rutile (TiO 2 ), pyrolusite (MnO 2 ), cassiterite (SnO 2 ) and plattnerite (b-PbO 2 ). Despite the chemical bonding being essentially ionic, the differences in the outer electron shell of cations clearly influence post-edge features of XAS spectra. The implications of site symmetry on coordination environment are emphasized for [MO 6 ] octahedra (with quite regular geometry in cassiterite) and for the isosceles triangle [OM 3 ] that displays either two long plus one shorter (pyrolusite and plattnerite) or two short plus one longer (rutile and cassiterite) M–O distances, with M–O–M bond angles always close to 100 plus twice 130 . Theoretical modelling of XAS spectra was carried out. Details of O 1s absorption spectra like the presence of a single post-edge feature for plattnerite and cassiterite may be interpreted by the separation or mixing between O 2p and M nd plus mixing with M ns and np states. Spectra features are commented in relation to crystal structure details but the energy shift assigned for the O K-edge in cassiterite still lacks a plausible explanation.