RbSbO₃: A Simple Structure with Complex Polytypism

Abstract

RbSbO3 crystallizes in different polytypes composed of trigonal [SbO3]− and [Rb]+ layers, which can appear in two orientations with respect to m_<100> and m_<120>, respectively. The order-disorder (OD) polytype family comprises four polytypes of a maximum degree of order (MDO), of which two are realized in actual crystals: MDO1 (R-3, c ≈ 19.8 Å, isotypic to ilmenite-like KSbO3) and MDO2 (R-3c, c ≈ 39.2 Å, isotypic to AgRuO3 ). Crystals grown from RbF:Sb2O5 in a 2:1 ratio are MDO1:MDO2 allotwins, whereas crystals grown from a 12:1 mixture are composed of MDO2 only. The stacking ambiguity leads to twinning by rotation about <100> or <120>. KSbO3 crystals grown from KF:Sb2O5 are composed of the MDO1 polytype only. The crystals are fourfold twins owing to stacking faults. A detailed description of the analysis of the diffraction data is given.