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Above room‐temperature dielectric tunability is observed in the metal‐organic frameworks (MOFs) [NH2(CH3)2]n[FeIIIFeII(HCOO)6]n (1) and [NH2(CH3)2]n[FeIIIFeII(1−x)NiIIx(HCOO)6]n (x = 0.64–0.69) (2). The relative tunability (defined as [ϵ′(0) – ϵ′(E)]/ϵ′(0) × 100% = Δϵ′/ϵ′(0) × 100%) values for 1 are up to 35% (at 410 K) for E⊥c and 21% (at 380 K) for E||c, while these for 2 are 14% for E⊥c and 11.5% for E||c. Investigation on the mechanism of the dielectric tunability in 1 and 2 reveals that the activation energy for the electron hopping between two adjacent metal ions and the magnetic exchange interaction play a key role in the dielectric tunability of these materials.

Dielectric tunability has potential applications in electrically tunable microwave devices, such as voltage‐controlled oscillators, band pass filters, phase shifters. Although dielectric tunability has been reported in inorganic oxides, it has never been observed in metal‐organic frameworks (MOFs). The obstacle to MOFs acting as electric‐field dielectric tunability materials is attributed to that, (i) the dielectric tunability in MOFs often occurs at the temperature far below room‐temperature, due to the ferroelectric phase in the MOFs often occurring at low temperature (often less than 200 K); (ii) a high dielectric tunability in the MOFs‐based conventional ferroelectric materials often requires a large d.c. electric field (about 10 to 100 kV∙cm−1). As a result, the application of the MOFs in dielectric tunability is greatly limited. Jiang‐Bin Guo et al. (article no. http://doi.wiley.com/10.1002/pssr.201700425) have observed a large aboveroom‐ temperature dielectric tunability under a small d.c. bias electric field in the niccolite‐type formate frameworks for the first time, and the electron hopping between two adjacent metal ions and magnetic exchange interaction play a key role in their dielectric tunability. The present work not only provides a new route for the design of dielectric tunability materials, but also expands the function of MOFs.

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