THERMOANALYTICAL STUDY, PHASE-TRANSITIONS, AND DIMENSIONAL CHANGES OFALPHA-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 observa tion, at about 143 and 257 degrees C, of two new reversible phase tran sitions of monohydrate zirconium phosphate. The connected enthalpic ef fects were 3.8 +/- 0.2 and 2.7 +/- 0.1 kJ/mol, respectively. Measureme nts of the thickness of the crystal, along the perpendicular to the la yers, were also performed as a function of the temperature. At tempera tures 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 initia l value. A thermal behavior very similar to that found for large cryst als was observed when microcrystalline alpha-Zr(HPO4)(2) . H2O (averag e 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 interlay er distance from 7.56 to 7.50 Angstrom at 134 degrees C, and to 7.43 A ngstrom at 257 degrees C, in agreement with dilatometric data. Finally , some structural aspects involving the new phases and the possible re lations with those already reported are discussed. (C) 1997 Academic P ress.