On the succesive phase transitions in RbHSeO4 and NH4HSeO4 crystals

Effect of external electric field and external mechanical stress influence on domain structure of RbHSeO4(RHSe) and NH4HSeO4 (AHSe) crystals was studi ed. Taking into account microscopic observations of domain structure in par ticular phases we consider the successive changes of symmetry in this famil y as 222 --> 2 --> 1. It is in accordance with predictions of Aizu and Sapr iel and the 222 is a symmetry of nonexisting prototype phase. The monoclini c phases are potentially ferroelectric and fully ferroelastic and the tricl inic phases are fully ferroelastic and fully ferroelectric.