Evolution structurale de nouvelles pérovskites lacunaires Y x(Ta 3 xW 1-3 x)O 3

New non-stoichiometric perovskites Y _x(Ta _{3 x}W _{1-3 x})O ₃ were isolated and their homogeneity ranges determined. Structural evolution as a function of composition and the atomic positions were studied. The perovskite Y _0.33TaO ₃ was particularly studied; two forms were isolated: a quadratic one (high temperature) and an orthorhombic one (low temperature). The orthohombic form observed by electron microscopy and diffraction shows nearly periodic twin bands; an hypothesis is made to explain the occurrence of these twins.