Low temperature structural phase transition in hafnium and zirconium tetrafluoride trihydrates

From time-differential perturbed angular correlation (TDPAC) measurements, the monoclinic and triclinic crystal structures in hafnium and zirconium tetrafluoride trihydrates are found to be present simultaneously in both the compounds. From previous TDPAC and XRD investigations, a monoclinic crystal structure for HfF4 center dot 3H(2)O but, for its analogues zirconium compound, a triclinic structure was reported. Contrary to earlier reports, the triclinic fraction in HfF4 center dot 3H(2)O is found to be maximum (80%) at room temperature. In fact, the triclinic crystal structure of HfF4 center dot 3H(2)O is reported here which was not known prior to this report. In ZrF4 center dot 3H(2)O, a strong signal (80-90%) for the triclinic structure is found at room temperature while the monoclinic fraction appears as a weak signal (10-15%). Structural phase transitions in these trihydrate compounds have been observed in the temperature range 298-333 K. (C) 2015 Elsevier Ltd. All rights reserved.