On the high-temperature phase transitions of CsH2PO4: A polymorphic transition? A transition to a superprotonic conducting phase?

X-ray diffraction, thermogravimetric (TGA), differential scanning calorimet ric (DSC), and impedance analysis were used to study the reported high-temp erature phase transitions at 107, 149, 230, and 256 degrees C in crystals o f cesium dihydrogen phosphate, CsH2PO4 (CDP). Our results show strong evide nce that at all these temperatures, the observed DSC or differential therma l analysis (DTA) endothermic effects appear only as a consequence of a dehy dration process starting on the surface of the crystal. Our results thus sh ow that the reported transition at 230 degrees C is not a polymorphic trans ition. This means that the monoclinic symmetry, stable at room temperature, with space group P2(1/m)-C-2k(2), is maintained up to the final decomposit ion. Moreover, since we have not found any evidence for the existence of a superprotonic high-temperature phase above 230 degrees C, the high conducti vity above 230 degrees C is thus only a consequence of the dehydration of t he crystal surface. (C) 1999 American Institute of Physics. [S0021-9606(99) 51109-4].