Hidden nature of the high-temperature phase transitions in crystals of KH2PO4-type: Is it a physical change?

On the assumption that dehydration occurs on heating above T-p and hydrolysis takes place under the appropriate water vapor pressure, i.e. nMH(2)RO(4) (s)reversible arrow M(n)H(2)R(n)O(3n+1)(s) + (n-1) H2O (v), n greater than or equal to 1 we could successfully interpret the physicochemical properties related to the high-temperature behaviors in crystals of MX(2)RO(4)-type ( M = K, Rb, Cs,; X = H, D; R = P, As). From the phase interrelationships based on chemical analyses, the so-called high-temperature phase (HTP) above T-p is regarded as a mixture consisting of monomeric reactant and polymeric product. The transformation from the metastable HTP to tetragonal structure in KH2PO4 and RbH2PO4 under the free atmosphere can be accounted for as a hydrolytic degradation or depolymerization. From this interpretation, it is suggested that the nomenclature high-temperature phase transition (HTPT) around T-p should be replaced by that onset of partial polymerization at reaction sites distributed on the surface of solids.