FERROELASTIC DOMAIN-STRUCTURE OF (NH4)(3)H(SEO4)(2) CRYSTAL

Theoretical and experimental studies of the ferroelastic domain struct ure of (NH4)(3)H(SeO4)(2) crystal in phases III and IV are discussed. Using the refined structural data the orientations of W- and W'-type d omain walls as well as temperature evolution of the domain structure a re investigated. It is shown that despite the fact that the room-tempe rature phase III is triclinic with small deviations from monoclinic, t he domain structure in this phase differs strongly from the structure in the case of monoclinic symmetry. In the monoclinic phase IV all W'- type domain walls lie almost parallel to the (001)-plane, which explai ns the invisibility of W' walls after the III-TV phase transition duri ng observation of domain structure in the (001)-plane.