Stabilisation of disordered superprotonic phases in crystals of the M 5H 3(AO 4) 4· xH 2O family

Abstract

In order to clarify the nature of the suppression of the order–disorder phase transitions and the stabilisation of the disordered highly-conducting state, conductivity, thermal (TGA and DSC) and infrared spectroscopic measurements have been carried out on single crystals of Cs 5H 3(SO 4) 4· xH 2O, Cs 5H 3(SeO 4) 4·H 2O and K 5H 3(SO 4) 4·H 2O. The proton dynamics and structural superprotonic phase transitions are briefly reviewed for these compounds. Protonic conductivity is demonstrated and is shown to be due mainly to the acid protons. An opposite influence of the water content on the stability of poorly conducting and highly conducting phases was found, where a stoichiometric water content tends to stabilise, at low temperature, ordered phases with a low conductivity; whilst a loss of water through an increase in temperature leads to stabilisation of superprotonic phases characterised by disordered acid hydrogen bond networks and in which the phase transition from the superionic phase is suppressed.