Flexible crystals of perovskite-like coordination polymers with a tunable and switchable organic guest: (CH3NH3)2[KFe(CN)6] and (CH3NH3)2[KCo(CN)6]

Abstract

The crystal structure has been determined for (CH3NH3)2[KCo(CN)6] at temperatures 100 and 443 K. It crystallizes in the monoclinic space group C2/c (LT) and in the cubic one Fm3m (HT). The dielectric response has been investigated for single crystals of pure K3[A′(CN)6] and guest-hosts of (CH3NH3)2[KA′(CN)6], where A′ is a trivalent metal, Co or Fe. Their dielectric properties were measured in the frequency range between 135 Hz and 2 MHz in a wide temperature range between 200 and 460 K. The anisotropy of the electric permittivity for single crystals of (CH3NH3)2[KCo(CN)6] was analysed in terms of the crystal structure. The relaxation processes were observed in the case of the pure host K3Fe(CN)6 and the guest/host crystal (CH3NH3)2[KFe(CN)6]. The activation energies of the dielectric relaxation have been estimated to be equal to 44 and 40 kJ mol-1 for (CH3NH3)2[KFe(CN)6] and K3Fe(CN)6, respectively. The mechanism of the phase transitions found at 425 K and 421 K for (CH3NH3)2[KFe(CN)6] and (CH3NH3)2[KCo(CN)6], respectively, has been postulated. From the dielectric losses the value of the electric ac conductivity has been estimated and analyzed in terms of the activation process of the current carriers.