A TEM AND GROUP THEORETICAL-STUDY OF ALPHA-PBO AND ITS LOW-TEMPERATURE IMPROPER FERROELASTIC PHASE-TRANSITION

The results of a detailed TEM and group theoretical study of alpha-PbO and its low-temperature, improper ferroelastic phase transition are r eported. Ferroelastic twinning of the low-temperature, incommensuratel y modulated phase is observed and typically found to be on a scale rat her less than 1 mum. Some unusual features of the reciprocal space of this low temperature phase are reported including the coexistence of s harp incommensurate satellite reflections and a closely related diffus e intensity distribution as well as the presence of ''bands'' of diffu se intensity absence along {hkh} directions of reciprocal space. Sing le domain electron diffraction patterns of the low-temperature incomme nsurately modulated phase are found to be incompatible with a previous ly reported structure refinement and to require a 4-dimensional supers pace group symmetry of P : Cmma: s, -1, 1. The most general possible a tomic displacement pattern compatible with this symmetry is derived fr om group theoretical considerations. (C) 1994 Academic Press, Inc.