Controlling Two-Step Phase Transitions and Dielectric Responses by A-Site Cations in Two Perovskite-like Coordination Polymers

Abstract

Two new perovskite-like coordination polymers, A2[KFe­(CN)6], were constructed by employing guanidinium and acetamidinium as A-site cations, respectively. Their cation-controlled two-step phase transitions as well as the relevant dielectric responses were uncovered by the combined techniques of the variable-temperature single-crystal X-ray structural analyses and dielectric measurements. With a similar size and shape, the A-site cations reveal similar two-step thermal-induced transitions on their motional dynamics, i.e., from a frozen order state to an in-plane rotational disorder state, and to a melt-like disorder state. However, the variation of the A-site cations on their symmetries and dipole moments makes noticeable impacts on the symmetry breaking, the critical temperatures, and the dielectric responses for the two-step structural phase transitions, i.e., the D 3h nonpolar guanidinium results in an R3̅c ↔ R3̅m ↔ Fm3̅m transition, whereas the C 2v polar acetamidinium results in a C2/m ↔ R3̅m ↔ Fm3̅m transition. Investigations of these two coordination polymers demonstrate a fine modulation on the phase transition behaviors and dielectric responses by changing the symmetries and dipole moments of A-site cations.

Ferroic multipolar order and disorder in cyanoelpasolite molecular perovskites

“…On further heating of [C 3 N 2 H 5 ] 2 Rb[Co(CN) 6 ], its X-ray diffraction pattern reflected a firstorder phase transition at ∼480 K. The most obvious change in the diffraction pattern was the coalescence of peaks, consistent with an ascent in symmetry. The high temperature phase could be indexed in terms of the cubic spacegroup F m3m, which is consistent with the symmetry observed for other cubic double perovskites [30,37,38] and is related to the R3m ambient-temperature cell as shown in Fig. 2(c).…”

“…Alternatively, if a lower-symmetry A-site cation is statistically disordered over multiple equivalent orientations, then this disordered (presumably high-temperature) state may also have F m3m symmetry. This second case is realised at temperatures above 423 K in [CH 3 NH 3 ]K[Co(CN) 6 ], for example [30,38]. By contrast to these two scenarios, multipolar order involves long-range symmetry lowering at the A-site to a (proper) subgroup of T d .…”

Ferroic Multipolar Order and Disorder in Cyanoelpasolite Molecular Perovskites

“…Alternatively, if a lower-symmetry A-site cation is statistically disordered over multiple equivalent orientations, then this disordered (presumably high-temperature) state may also have F m3m symmetry. This second case is realised at temperatures above 423 K in [CH 3 NH 3 ]K[Co(CN) 6 ], for example [30,38]. By contrast to these two scenarios, multipolar order involves long-range symmetry lowering at the A-site to a (proper) subgroup of T d .…”

Dielectric Relaxation and Beyond Limiting Behavior of Alternating‐Current Conductivity in a Supermolecular Ferroelectric

“…On the one hand, the molecular structure, synthetic approach and functional properties of organic molecule are easily tailored.O nt he other hand, the inorganic network of materials has generally chemical, thermal, mechanical stabilities and aw ide range of electric characteristics as well. These advantages make them suitable for av ariety of low-cost optoelectronic devices and materials,s uch as lightemitting diodes, [29][30][31] switching materials, [32][33][34][35][36] proton-conducting materials [37,38] and solar cells. [39][40][41] Meanwhile, much progress has been achieved for organic-inorganic hybridc ompounds in ferroelectricity.I nt he wide field of hybrid ferroelectrics, the hybrid halometallates of M II (M = Pb, Mn, Cd) represent at echnologically important class, since they preserve the structural characteristics of perovskites.…”

“…On the other hand, the inorganic network of materials has generally chemical, thermal, mechanical stabilities and a wide range of electric characteristics as well. These advantages make them suitable for a variety of low‐cost optoelectronic devices and materials, such as light‐emitting diodes, switching materials, proton‐conducting materials and solar cells . Meanwhile, much progress has been achieved for organic‐inorganic hybrid compounds in ferroelectricity.…”