The thermal behaviour of the quadrupole hyperfine interaction in (NH4)2HfF6

Abstract

A TDPAC investigation has been accomplished on (NH4)2 HfF6 between 14 and 620 K. A phase transition was observed below 390 K. The low temperature phase is characterized by two non-equivalent sites for hafnium atoms in a 1∶1 ratio. The high temperature phase, on the other hand, is depicted by a unique quadrupole interaction. Above 400 K, the compound decomposes successively to NH4 HfF5, NH4 Hf2F9 and HfF4. An enthalpy of 76±4 kJ/mol could be determined for the (NH4)2 HfF6→NH4 HfF5+NH4F chemical reaction. The hyperfine interaction and thermal evolution of (NH4)2 HfF6 was found to be quite similar to that of (NH4)2 ZrF6.