ANOMALOUS THERMAL-BEHAVIOR AND HYDROGEN-BOND IN VOPO4.2H2O

An anomalous thermal behaviour of the vanadyl phosphate dihydrate has been found by means of X-ray powder diffraction. In the tetragonal layered structure of VOPO4.2H2O the monotonic decrease of the c-parameter with increasing temperature in the 3-42-degrees-C range has been observed, whereas the a-parameter remains constant at a = 6.2098(21) angstrom. The upper and lower limits of the c-parameter for the tetragonal phase of VOPO4.2H2O are c = 7.4593(25) angstrom at 3-degrees-C and c = 7.3907(25) angstrom at 42-degrees-C. Based on the temperature dependence of the lattice parameters and diffraction line profiles, the structural changes with variation of the temperature have been derived and the mechanism of the topotactic dehydration proposed. It is evident that the interlayer hydrogen bonds play the key role in the observed effects. This implies that the present study confirms certain common structural features of ices and hydrates and illustrates the influence of the crystalline environment on the behaviour of the water-water dimer.