Thermoanalytical study, phase transitions, and dimensional changes of alpha-Zr(HPO4)(2)center dot H2O large crystals

Crystals of millimetric dimensions of layered alpha-Zr(HPO4)(2) . H2O start to lose hydration water at temperatures higher than 270 degrees C when heated at rates of 2-50 degrees C/min. This allowed the observation, at about 143 and 257 degrees C, of two new reversible phase transitions of monohydrate zirconium phosphate. The connected enthalpic effects were 3.8 +/- 0.2 and 2.7 +/- 0.1 kJ/mol, respectively. Measurements of the thickness of the crystal, along the perpendicular to the layers, were also performed as a function of the temperature. At temperatures very close to those of the two-phase transitions, the thickness of the crystal decreased by 0.69% and by a further 0.44% of its initial value. A thermal behavior very similar to that found for large crystals was observed when microcrystalline alpha-Zr(HPO4)(2) . H2O (average crystal size 1-10 mu m) was analyzed at very high heating rates (i.e., 50 degrees C/min). X-ray powder diffraction analysis revealed that the phase transitions involve a discontinuous decrease of the interlayer distance from 7.56 to 7.50 Angstrom at 134 degrees C, and to 7.43 Angstrom at 257 degrees C, in agreement with dilatometric data. Finally, some structural aspects involving the new phases and the possible relations with those already reported are discussed. (C) 1997 Academic Press.