The isomorphous transition between Co(NH 3 ) 6 (PF 6 ) 2 and Ni(NH 3 ) 6 (PF 6 ) 2

The unusual phase transition at 20 K in Ni(NH3)6(PF6)2 in which there is no change of symmetry is investigated via the quantum-rotational spectrum of a series of mixed NixCo(1-x)(NH3)6(PF6)2 crystals. Below x=0.65 the spectra reveal the lattice of the high-temperature phase of Ni(NH3)6(PF6)2, whilst above this composition the low-temperature phase is present. This observation is in contrast to the smooth transition as a function of composition that was expected, in which the dynamics is characterized by many closely spaced shallow potential-energy minima.