Hybrid organic–inorganic perovskites providing integrated functionalities for multimodal switching applications are widely sought-after materials for optoelectronics. Here, we embark on a study of a novel pyrrolidinium-based cyanide perovskite of formula (C 4 H 10 N) 2 KCr(CN) 6 , which displays thermally driven bimodal switching characteristics associated with an order–disorder phase transition. Dielectric switching combines two features important from an application standpoint: high permittivity contrast (Δε′ = 38.5) and very low dielectric losses. Third-order nonlinear optical switching takes advantage of third-harmonic generation (THG) bistability, thus far unprecedented for perovskites and coordination polymers. Structurally, (C 4 H 10 N) 2 KCr(CN) 6 stands out as the first example of a three-dimensional stable perovskite among formate-, azide-, and cyanide-based metal–organic frameworks comprising large pyrrolidinium cations. Its stability, reflected also in robust switching characteristics, has been tracked down to the Cr 3+ component, the ionic radius of which provides a large enough metal–cyanide cage for the pyrrolidinium cargo. While the presence of polar pyrrolidinium cations leads to excellent switchable dielectric properties, the presence of Cr 3+ is also responsible for efficient phosphorescence, which is remarkably shifted to the near-infrared region (770 to 880 nm). The presence of Cr 3+ was also found indispensable to the THG switching functionality. It is also found that a closely related cobalt-based analogue doped with Cr 3+ ions displays distinct near-infrared phosphorescence as well. Thus, doping with Cr 3+ ions is an effective strategy to introduce phosphorescence as an additional functional property into the family of cobalt-cyanide thermally switchable dielectrics.

We report the synthesis, crystal structure, and dielectric, vibrational and emission spectra of two novel heterometallic perovskite-type metal-organic frameworks (MOFs) of the following formula: [(CH)NH][KCr(HCOO)] (DMAKCr) and [CHNH][KCr(HCOO)] (EtAKCr). DMAKCr crystallizes in a trigonal structure (R3[combining macron] space group) and undergoes an order-disorder phase transition to the monoclinic system (P1[combining macron] space group) at about 190 K. The dielectric studies confirm the presence of first-order relaxor-like structural transformation. In the high-temperature phase, the dimethylammonium cations are dynamically disordered over three equal positions and upon cooling the dynamical disorder evolves into a two-fold one. This partial ordering is accompanied by a small distortion of the metal-formate framework. EtAKCr crystallizes in a monoclinic structure (P2/n space group) with ordered EtA cations and does not experience any phase transition. The differences in the thermal behavior caused by the substitution of Na ions by larger K ions in the [cat]MM (cat = DMA, EtA, M = Na, K and M = Cr and Fe) heterometallic MOF family are discussed taking into account the impact of the hydrogen bond (HB) pattern and other factors affecting the stability of metal-formate frameworks. The optical studies show that DMANaCr and EtAKCr exhibit Cr-based emission characteristics for intermediate ligand field strength.

Two novel three-dimensional metal–organic compounds of formula FA2KM(CN)6, where M = Co, Fe and FA = formamidinium (CH(NH2)2+), have been found to crystallize in a perovskite-like architecture.

Inorganic–organic hybrid QMnCl (Q = quinuclidinium) crystals were synthesized and characterized. The X-ray and variable-temperature IR/Raman analysis demonstrate that the crystals undergo a reversible structural phase transition, which originates from an order–disorder process and is related to the dynamics of the organic Q cation. Dielectric function measurements disclose a switchability between low (“OFF”) and high (“ON”) dielectric states centered at around 285 K. Owing to a remarkable temperature-dependent dielectric function, this type of molecular compound can represent an interesting tunable and switchable dielectric material for a diverse range of applications.