Cation nonstoichiometry in tin-monoxide-phaseSn1−δOwith tweed microstructure

We report a chemical, thermogravimetric, and electron-diffraction/microscopy study of a tin-monoxide phase. A large deviation from the ideal stoichiometry is observed due to metal vacancies, resulting in the formula ${\mathrm{Sn}}_{1\mathrm{-}\mathrm{\delta }}\mathrm{O}.$ This nonstoichiometry is an intrinsic feature of this material and is accommodated through the formation of static transverse displacive modulations along the $〈\mathrm{hh}0\mathrm{}〉$ directions, giving a tweed microstructure without the introduction of complex arrangements of vacancy interstitials (as in wüstite). Our observation constitutes a different way of accommodating large deviations from ideal stoichiometries, especially in comparison with the well-known behavior of the transition-metal monoxides with the NaCl-type structure. The difference arises most likely from the layerlike nature of the α-PbO $\mathrm{}(B-10\mathrm{})\mathrm{}$ structural type with average tetragonal symmetry and $P4/nmm$ space group. Metal vacancies cause a strain coupling which stabilizes the highly disordered nonstoichiometric phase. Dynamical instabilities were not observed. An origin for the thermal instability of the material is suggested. A comparison with PbO, the only isostructural compound, is outlined. SnO is shown to be a beam-sensitive material.