A tem and group theoretical study of α-pbo and its low-temperature improper ferroelastic phase transition

Abstract

The results of a detailed TEM and group theoretical study of α-PbO and its low-temperature, improper ferroelastic phase transition are reported. Ferroelastic twinning of the low-temperature, incommensurately modulated phase is observed and typically found to be on a scale rather less than 1 μm. Some unusual features of the reciprocal space of this low temperature phase are reported including the coexistence of sharp incommensurate satellite reflections and a closely related diffuse intensity distribution as well as the presence of “bands” of diffuse intensity absence along {hkh}* directions of reciprocal space. Single domain electron diffraction patterns of the low-temperature incommensurately modulated phase are found to be incompatible with a previously reported structure refinement and to require a 4-dimensional superspace group symmetry of P : Cmma : s, -1, 1. The most general possible atomic displacement pattern compatible with this symmetry is derived from group theoretical considerations. Show more