Twin model for orthorhombic perovskites

We present a detailed single crystal x-ray diffraction study of twinned orthorhombic perovskites. The diffraction pattern can be indexed on a $2a_p$x$2a_p$x$2a_p$ lattice, but does not obey cubic symmetry relations. The data can be modelled in space group $Pnma$ with a twin based on a distribution of the $b$ axis over three perpendicular cubic axes. This model allows full structure determination in the presence of up to six twin fractions from single crystal x-ray diffraction data.