CATION NONSTOICHIOMETRY IN TIN-MONOXIDE-PHASE SN1-DELTA O WITH 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 Sn12dO. This nonstoichiometry is an intrinsic feature of this material and is accommodated through the formation of static transverse displacive modulations along the ^hh0& directions, giving a tweed micro- structure without the introduction of complex arrangements of vacancy interstitials ~as in wustite!. Our obser- vation 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 a-PbO (B-10) 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 ther- mal 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. @S0163-1829~97!07233-0# I. INTRODUCTION