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Superionic materials have intrigued researchers because they exhibit structural transitions from the paraelectric to the superionic phase which are accompanied by an increase of the electrical conductivity as much as five orders of magnitude. Notwithstanding the number of studies and variety of experiments that have been carried out on superprotonic crystals of the KH 2 PO 4 type, no generally agreed-upon model describing the structural and chemical features that induce superionic transitions exists. Discrepancies between the high-temperature results reported by different groups of scientists have been well summarized in recent papers. 1,2 Lee 1 has discussed the similarities of the phase transitions in the KH 2 PO 4 -type compounds and concluded that the high-temperature phenomena of these compounds are not related to physical changes like structural phase transitions, but related to chemical ones and has suggested that the term ''high-temperature phase transition'' should be replaced by ''onset of partial polymerization at reaction sites distributed on the surface of solids. '' In order to check if the reported high-temperature phase transitions of CsH 2 PO 4 ͑CDP͒ are only related to thermal dehydration, power x-ray measurements were performed by Ortiz et al.2 A fresh powder sample was subsequently heat treated 1 min at 130, 165, 200, 238, and 250°C, and their respective x-ray diffraction patterns taken under dry conditions after cooling at room temperature ͑25°C͒ were plotted ͑see Fig. 3 of Ref. 2͒. That strategy of the experiment is very good for presenting the thermal decomposition process of CDP as a function of annealing temperature; however, it is insufficient to conclude that the reported transition 3-12 associated with endothermic anomaly at ca. 231°C does not take place and, moreover, that the high conductivity above 231°C is only a consequence of the dehydration of the crystal surface.Under normal air conditions the high-temperature transition occurs very close to the region where CDP decomposes by dehydration and these two simultaneous effects considerably complicate the interpretation of the phase relation.Ten years ago the Bond method of precise lattice parameter determination was applied as a sensitive monitor of the structural changes occurring in the CDP crystal.9 Lattice parameters of CDP were calculated on the basis of the measurements of the Bragg angles of the reflections with high angles using the least-square method. The accuracy of the measurements of 2, which was better than 10Љ of arc, allowed the determination of the lattice parameters to an accuracy better than 3ϫ10Ϫ5 . Investigations of the lattice parameters of CDP above room temperature have revealed that up to ca. 137°C all parameters are almost linear. In the vicinity of 149°C a discrepancy of the linearity of the lattice parameters has been noticed, but the space group ( P2 1 /m) of the crystal above room temperature up to ca. 231°C does not change. The investigation of the single-crystal samples of CDP with the Bond diff...

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