Metal–organic frameworks (MOFs) are crystalline compounds consisting of metal atoms and organic molecules and which often feature pores capable of hosting different types of molecular ‘guests’. This intrinsic host–guest chemistry has attracted considerable attention in numerous areas including gas storage, sensing and catalysis. Weng Zhang and co-workers at Southeast University in Nanjing, China, and Nagoya University in Japan1 have now demonstrated that a specific type of MOF also shows exceptional dielectric properties.

The possibility of using MOFs to make dielectric materials — electric insulators that can be polarized under an external electric field — has been considered before. The approach relies on the introduction of a polar molecule within the MOF’s pores. The dielectric permittivity of a molecule is related to its rotational freedom. In most cases, unfortunately, the polar guests are ‘frozen’ in a fixed position, giving rise to only a very small dielectric response.

Zhang and his fellow researchers circumvented this problem by investigating a switchable dielectric. “These materials can undergo transitions between high and low dielectric states under external stimuli, with potential applications in data processing and sensing,” says Zhang. The team studied an MOF in which the orientation of the guest is frozen at low temperature and unrestricted at high temperature.

Fig. 1: Low-temperature ordered (left) and high-temperature disordered (right) structures of the switchable dielectric (HIm)2[KFe(CN)6].

The perovskite-type cage compound studied has an anionic Fe–CN–K host framework, in which imidazolium cations are present as guest molecules. Thermal and X-ray diffraction analyses revealed that the compound undergoes a phase transition from a lower-temperature ordered phases in which the guests are frozen, to a higher-temperature disordered phase in which the guest can move freely (Fig. 1).

Measurements of the variations of the dielectric permittivity as a function of temperature confirmed that the compound is a switchable dielectric. The team also observed a very high anisotropy in permittivity, reflecting the specific arrangement of the cations within the pores.

The findings could have an important impact on future research. “A deep understanding of the collective motion of dipole moments provides insights that are valuable in the search for new electric ordering materials, such as ferroelectrics,” says Zhang.