TDPAC characterization of Ni2HfF8·12H2O and its decomposition products

The hyperfine interaction in Ni₂HfF₈·12H₂O has been determined between 77 K and 1100 K by means of the time-differential perturbed angular correlation technique. From 200 K on, the one-site phase existing at lower temperatures undergoes a gradual phase transition until, at room temperature, the populations of both phases attain a 2:1 ratio. While the quadrupole frequencies characterizing them exhibit aT ^3/2 thermal dependence, their population ratio seems to obey a Boltzmann distribution. At 350 K, when the η-value of the high temperature phase electric field gradient approaches its maximum value, the starting compound decomposes to NiHfF₆·6H₂O. A kinetics study of the Ni₂HfF₈·12H₂O recovery at room temperature seems to indicate that a tri-dimensional diffusion mechanism is responsible for the corresponding reaction process. The first decomposition product of NiHfF₆·6H₂O left to atmospheric pressure is found to be NiHfF₆·4H₂O at 368 K and, between 414 K and 590 K, the high temperature cubic phase of NiHfF₆ and Hf₂OF₆ can be simultaneously observed. Finally, monoclinic HfO₂ appears from 1020 K on, having been preceded by an interaction which can be though of as depicting a preliminary stage in hafnia formation.