THE EFFECT OF IONIC GROUP ROTATIONS ON PROTON ORDERINGS IN M3H(XO4)(2) SUPER-IONIC CRYSTALS

The various proton orderings occurring in M3H(XO4)(2) crystals are inv estigated in the framework of a lattice-gas-type model with account ta ken of the interactions of the proton subsystem with the orientations of the XO4 ionic-groups. In particular, it is shown that the orientati onal ordering of SeO4 ions is strongly involved in the I-II-III-IV pha se transitions which take place in (NH4)(3)H(SeO4)(2) and appears to b e the basic mechanism determining the observed phase sequence. The pha se diagram obtained together with the microscopic parameter values cal culated for (NH4)(3)H(SeO4)(2) agree with the experimental data for th e phase orderings.