The cation-dependent structural phase transition and dielectric response in a family of cyano-bridged perovskite-like coordination polymers

Abstract

A family of cyano-bridged perovskite-like coordination polymers (CPs), [(CH3)nNH(4-n)]2[KFe(CN)6] (n = 1 (1), 2 (2), 3 (3), and 4 (4)), were synthesized and characterized. The differential scanning calorimetry measurements and variable-temperature single-crystal X-ray structural analyses revealed that, owing to the deformation of the host framework as well as the dynamic transition of the guest cation between the static/ordered and dynamic/disordered states, the four CPs undergo structural phase transitions (at 429, 226, 316, and 350 K, respectively) with the symmetry breakings dependent on the symmetries of the encapsulated guest cations. The modulated differential scanning calorimetry measurement suggested that the phase transitions of 1 and 3 have more striking kinetic processes related to their drastic deformation of the host framework as well as a very significant alteration of the host-guest interaction during the phase transition. Moreover, accompanying the transitions between low- and high-temperature phases, the step-like transitions between low and high dielectric states were observed in 1-3, and the corresponding change in amplitude of the dielectric constant is dependent on the total dipole moment of each cage in the high-temperature phase. The investigation on these host-guest CPs deepens the understanding of the relationship between the dipole moment of guest cations and the dielectric behaviour of materials, shedding light on the search for new switchable molecular dielectrics.

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

“…For temperatures below 370 K our data were well described in terms of an I4/m structural model based on that reported for (NMe 4 ) 2 K[Fe(CN) 6 ] [38]; at higher temperatures our data were consistent with the aristotypic F m3m structure as reported for the hexacyanoferrate in Ref. 38. Full details of our refinements are given as SI, but we show representative fits and structural models for data collected at 300 and 400 K in Fig.…”

“…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 .…”

“…We turn now to the related issue of ferrooctupolar order in the compositional family [NMe 4 ] 2 B[Co(CN) 6 ] (B = K, Rb, Cs). All but the K-containing member of this specific family have been characterised previously [51][52][53], and the corresponding hexacyanoferrates are known for all three B-site cations [38,54]. The system (NMe 4 ) 2 K[Fe(CN) 6 ] undergoes an order-disorder phase transition at ∼350 K from a low-temperature I4/m phase-in which the NMe 4 + cations are collectively rotated around a common 100 axis-to a high-temperature F m3m phase in which these rotations are disordered throughout the structure [38].…”