Isostructural phase transition, quasielastic neutron scattering and magnetic resonance studies of a bistable dielectric ion-pair crystal [(CH3)2NH2]2KCr(CN)6

Abstract

We have synthesised and characterised a novel organic–inorganic hybrid crystal, [(CH₃)₂NH₂]₂KCr(CN)₆. The thermal DSC, TMA, DTG and DTA analyses indicate two solid-to-solid structural phase transitions (PTs). According to the X-ray diffraction experiments, the first PT at 220 K is isostructural, since it does not involve a change of the space group. This transition occurs between the states, where the (CH₃)₂NH₂⁺ cations are orientationally disordered and ordered (frozen). The other reversible PT at 481 K leads to a melt-like phase similar to the one observed in plastic crystals or polar liquids. Dielectric spectroscopy has been used to characterise the switching properties of the dipole moments in the vicinity of the PTs. Continuous-wave electron paramagnetic resonance spectroscopy was employed to investigate the effect of ordering on the local environment of the Cr³⁺ ions. We have also applied the quasielastic neutron scattering (QENS) technique as well as ¹H NMR spectroscopy to measure the dynamics of the (CH₃)₂NH₂⁺ cations residing in the inorganic framework.