Ferroelastic domain wall orientations in the ( NH 4 ) 3 H ( SeO 4 ) 2 crystal

Experimental and theoretical studies of the ferroelastic domain structure of the (NH 4) 3 H(SeO 4) 2 crystal in phases III and IV are performed. Using the refined structural data the orientations of Wand W 0-type domain walls, as well as the temperature evolution of the domain structure are investigated. It is shown that in spite of the fact that the room temperature phase III has a triclinic symmetry with small deviations from monoclinic, the domain structure in this phase differs strongly from the structure in the case of a monoclinic symmetry which has been previously accepted in this temperature range. In the monoclinic phase IV all the W 0-type domain walls lie almost in parallel to the (001)-plane which explains the invisibility of W 0 walls after III-IV phase transition during the observation of the domain structure in the (001)-plane.