Pyrrolidinium-Based Cyanides: Unusual Architecture and Dielectric Switchability Triggered by Order–Disorder Process

Abstract

Two three-dimensional metal–organic compounds of the formula Pyr 2 KM(CN) 6 , where M = Co, Fe and Pyr = pyrrolidinium ((CH 2 ) 4 NH 2 + ), have been found to crystallize at room temperature in a monoclinic structure, space group P 2 1 / c . They are cyano-bridged compounds with an unprecedented type of architecture containing pyrrolidinium cations in the voids. The materials have been investigated by X-ray diffraction, dielectric, and spectroscopic methods as a function of temperature in order to determine their properties and the mechanism of the reversible phase transitions occurring at ca. 345–370 K. The phase transitions in both crystals are first order and are associated with a symmetry increase to a rhombohedral structure (space group R 3 ®m ) as well as a significant disorder of organic cations above T c . On the basis of Raman scattering and IR spectroscopy it has been assumed that the phase transition in both crystals is triggered by thermally induced pseudorotation of the organic cation and large out-of-plane motions of its atoms followed by a “click-in” of the cyanide bridges. The materials have been proposed as possible switchable dielectrics due to their respective high differences in dielectric permittivities across the phase transition.

Near-Infrared Phosphorescent Hybrid Organic–Inorganic Perovskite with High-Contrast Dielectric and Third-Order Nonlinear Optical Switching Functionalities

“…It is worth noting that the Δ S value of Pyr 2 KCr(CN) 6 perovskite is about three times larger than for nonperovskite Pyr 2 KM(CN) 6 (M = Co or Fe) analogues that exhibit phase transition from the ordered LT P 2 1 / c phase to the HT disordered R 3̅ m phase. 48 Thus, DSC data indicate that the disorder of Pyr + cations is significantly larger in the HT phase of perovskite Pyr 2 KCr(CN) 6 compared to nonperovskite Pyr 2 KM(CN) 6 analogues.…”

“…71 , 72 The assignment of internal modes of Pyr + cations, proposed in Table S3 , is based on Raman and IR data reported for nonperovskite Pyr 2 KM(CN) 6 analogues (M = Co or Fe). 48 …”

“… 39 , 47 It is worth noting that we have discovered switchable properties controlled by temperature also for a nonperovskite cobalt cyanide network comprising pyrrolidinium (Pyr + ) cations. 48 Furthermore, we demonstrated for the first time that the change between on and off states in this compound may also be achieved by applying external pressure. 49 Regarding chromium perovskite analogues, step-like dielectric anomalies were reported for two analogues only, i.e., DMA 2 KCr(CN) 6 and MA 2 KCr(CN) 6 .…”

Metal ion modulation triggers dielectric double switching and green fluorescence in A2MX4-type compounds

“…[1][2][3][4][5][6][7][8][9][10][11] Based on the above characteristics, these types of materials have potential applications in the fields of sensors, optoelectronic devices, information storage, signal processing, and switches. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] Organic-inorganic hybrid (OIH) materials possess good structural tunability and overcome the disadvantages of conventional inorganic materials such as low flexibility and high cost. Therefore, they are considered as ideal materials for designing multifunctional responsive materials.…”

Organic‐Inorganic Hybrid Crystal [1‐methylpiperidinium]2[ZnCl4] with High Tc Phase Transition and Dielectric Switches

“…In the process of phase transition, the latent heat of phase transition is absorbed or released. Among them, dielectric phase transition materials can switch the dielectric constant between low dielectric state and high dielectric state under the stimulation of external temperature, and have potential applications in the fields of information storage, sensors, signal processing and storage equipment [23–35] . In the past few years, they have received great attention and became a hotspot of multifunctional materials research.…”