On the high-temperature phase transitions of some KDP-family compounds: a structural phase transition? A transition to a bulk-high proton conducting phase?

Thermal analysis, X-ray diffraction, and impedance analysis were used to st udy the high-temperature nature of the KDP-family compounds: KH2PO4 (KDP), RbH2PO4 (RDP) and CsH2PO4 (CDP). Our results show that the structural phase transition from tetragonal to monoclinic modification in RDP and RDP earli er reported at around T-p approximate to 180 degrees C and T-p approximate to 86 degrees C, respectively, does not exist. For CDP we found that the st ructural phase transition from a monoclinic to a cubic symmetry superproton ic phase above 230 degrees C does not take place. Instead, in close agreeme nt with Lee's suggestion, we found evidence that at all these temperatures a partial polymerization takes place at reaction sites distributed on the s urface of the samples. (C) 1999 Published by Elsevier Science B.V. All righ ts reserved.