PHASE TRANSITIONS INTO SUPERPROTONIC STATE IN SOME HYDROGEN SULPHATES, SELENATES AND PHOSPHATES. L.A.Shuvalov, Inst. of Crystallography of RAS, Moscow, Russia
Not long ago there was found a new family of superprotonic MeHAO4(Me = Cs,NH4, Rb; A = S, Se). Soon we found other families of superprotonics: Me3H(AO4)2 (Me= Cs, NH4, Rb; A=S,Se) and CsH2AO4 (A=P,As). Later family Cs5H3(AO4)4*2H2O(A=S,Se) was found. All crystals have common features therefore we can unite them in new class of superprotonics.
1) They have phase transition (PT) at Tsp in high temperature superprotonic pha- se. 2) The PT is the first order improper feroelastic one.
3) Conductivity jumply increases at Tsp in several orders and with further growth of T reaches conductivity of melt.
4) The PT is accompanied by the anomalies of many physical properties.
5) In some crystals where melting point Tm<Tsp PT in superprotonic phase can be realized only at high hydrostatic pressure.
6) In low symmetry phase all possible sites for protons are occupied and all H- bonds are ordered.
7) Above Tsp quantaty of equivalent sites for protons increases in several times therefore protons on H-bonds system are dynamicaly disordered.
Such situation and high conductivity are realized even in ideal crystals. Crystals Cs5H3(AO4)4 have else unique properties. Dynamical disorder of pro- tons and H-bonds existing at high T frezes below room T and crystal turnes to glassy state with static disorder of structure. In the paper we consider structures of crystals ,mechanism of high conductivity and other results obtained by big informal group of recearchers from different institutions.