Crystal structures and phase transition of the cyanospinel K2Hg(CN)4

Abstract

The crystal structures of the high temperature (paraelastic) and low temperature (ferroelastic) phases of the cyanospinel K2Hg(CN)4 have been determined by neutron powder diffraction. At 298 K the crystal has the spinel structure and the cyanide coordination is ordered in the sequence N–C–Hg–C–N. Below the phase transition (111 K) the structure is trigonal with space group R3̄c. At the transition Hg(CN)4 tetrahedra adjacent along [111] rotate in antiphase as rigid units by ≊7° and the K ion shifts by ≊0.1Å. The transition has an ordering wave vector q0≡{1/2 1/2 1/2}. Comparison is made with the structural characteristics of other cyanospinels and it is suggested that the transitions cannot be understood in terms of steric interactions but must be a dynamic effect.