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Crystal structures of monoclinic RbD 2 PO 4 were studied at 297 K in the ferrielectric phase (phase III) and at 350 K in the paraelectric phase (phase II) by means of the single crystal neutron diffraction. The oxygen and deuterium atoms associated with the hydrogen bonds along the b axis are ordered in phase III and are disordered in phase II. Other atoms are ordered in both phases II and III. There are four kinds of PO 4 tetrahedra in phase III. The ferrielectricity of phase III were confirmed in the present neutron structural analysis. The III-II phase transition is caused by an order-disorder of PO 4 tetrahedra accompanied by the motion of deuterium atoms within the hydrogen-bond chain. Fluctuations of the oxygen and deuterium atoms related hydrogen-bond chains along the b axis give rise to a quasi-onedimensionality.

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In this paper we first present recent experimental results related to superionic conduction in the zerodimensional hydrogen-bonded crystals of alkali-hydrosulfates or hydroselenates, M 3 HðXO 4 Þ 2 [M = K, Rb, Cs and X = S, Se]. Then a novel approach to the mechanism of proton conduction in the paraelastic phase is described. The key features of the conduction mechanism in the high temperature paraelastic phase which we call the superionic phase are: (1) Two kinds of ionic states, H 2 XO ionic states is that of the Bethe lattice; that is the appearance of the twin peak structure due to self-similarity. The calculated conductivity is very high such as the order of 10

Abstract.Tripotassium hydrogenbis(selenate),