Molecular motion of imidazolium cations and phase transitions in (ImH)2KCo(CN)6 with double perovskite structure

[Display omitted] •Phase transitions of (ImH)2KCo(CN)6 are related to ImH+ in-plane reorientation.•Low- and high-temperature phase transitions are of first and second order, respectively.•Reorientation of the cations is frozen in low-temperature phase.•ImH+ motion takes place with a distributed correlation time at C3 symmetry site. Temperature dependence of 1H nuclear magnetic resonance spin–lattice relaxation time T1 of (ImH)2KCo(CN)6, ImH+: imidazolium cation, is reported. The asymmetric T1 minimum observed in intermediate-temperature phase between 198 and 112 K is analyzed assuming the Cole-Davidson distribution of correlation time. Temperature dependences of 14N nuclear quadrupole resonance frequencies and spin–lattice relaxation times are also reported. The phase transitions at 198 and 112 K are shown to be of second and first order, respectively. The 14N relaxation is shown to be governed by the electric field gradient fluctuation due to the in-plane reorientational motion of the imidazolium cations.