Ferroelastic domain structure of (NH4)(3)H(SeO4)(2) crystal

Theoretical and experimental studies of the ferroelastic domain structure 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 domain walls as well as temperature evolution of the domain structure are investigated. It is shown that despite the fact that the room-temperature phase III is triclinic with small deviations from monoclinic, the 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 explains the invisibility of W' walls after the III-TV phase transition during observation of domain structure in the (001)-plane.