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

Abstract 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) ⇋ M n H 2 R n O 3 n + 1 (s) + ( n − 1) H 2 O (v), n ≥ 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 KH 2 PO 4 and RbH 2 PO 4 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.

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