X-ray diffraction studies of room and intermediate tempertaure phases of phosphate based solid acids

In this work, the MH2PO4-type solid acids (where M= Cs, K, Rb) structure and properties are studied and their contribution towards fuel cell industry is understood. Some research groups have shown that, CsH2PO4 (CDP) and RbH2PO4 (RDP) shows sudden, several order of magnitude jumps in their proton conductivity at certain high temperatures.(31, 32) This so called superprotonic behavior of these compounds makes them a fuel cell electrolyte at elevated temperatures. (33) Most of the research groups working on two possible causes to get the reliable information for the high proton conductivity: polymorphic phase transition (Botez et al.,) and thermal dehydration (Ortiz et al.,) (1, 34). We have used synchrotron x-ray diffraction to clear the doubts on the structure and chemical changes undergone by polycrystalline KH2PO4 (KDP) upon heating within 30°C-250°C. The data shows a polymorphic phase transition at 190°C from the room temperature tetragonal phase to intermediate temperature monoclinic phase. The newly formed monoclinic KDP phase is isomorphic to monoclinic phases of CDP (35) and RDP (36) at room temperature and intermediate temperature respectively. But, on further heating under high pressure the later ones are showing a phase transition to a stable cubic phase whereas KDP is not showing any major structural changes even under ambient and high pressure conditions. The purpose of this work is to support the idea that the smaller ion size of K plays a major role in the absence of superprotonic behavior of KDP