Screening Ferroelastic Transitions in Switchable Cyano-Bridged Perovskites: [CH₃C(NH₂)₂]₂[KM(CN)₆], M = Cr³⁺, Fe³⁺, Co³⁺. Crystal Structure Characterization, Dielectric Properties, ¹H NMR, and Quasielastic Neutron Scattering Studies

Abstract

Here we report the dynamics of organic cations as guest molecules in the novel perovskite host framework. The novel switchable crystals from the coordination family [CH3C­(NH2)2]2­[KB″(CN)6] were constructed by employing acetamidinium cations (ACE = [CH3C­(NH2)2]) as A-site and B″: Cr3+, Fe3+, Co3+ in the inorganic cage. The molecular motion of ACE+ as well as the effect of the trivalent metallic cation type on two-step transitions were studied using a combination of experimental methods: (i) X-ray diffraction experiments, (ii) thermal analysis, (iii) dielectric and electric studies, (iv) optical observations, (v) 1H NMR spectroscopy, and (vi) quasielastic neutron scattering (QENS). On the basis of the X-ray analysis, transitions occur from a frozen-order phase (III, monoclinic, C2/m) to an in-plane rotational disorder state (II, trigonal, R3̅m) and to a melt-like disorder one (I, cubic, Fm3̅m). A markedly temperature-dependent dielectric constant indicates the tunable and switchable properties of all complexes. The QENS, as well as 1H NMR spectroscopy, were applied to measure the dynamics of the organic cations residing in the inorganic framework. The ferroelastic nature of the phases III and II were proven based on the polarized light microscopy photographs. Also, the modified equation of the tolerance factor t was used to estimate the theoretical phase transition temperatures for a crystal with Mn3+.

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

“…Generally, such behavior in the frameworks based on cyanide ligands is related to structural changes starting from the phase where organic cations are frozen to the orientationally disordered phase, which consequently leads to a significant change in the dielectric constant. ,− Thus, the difference in the observed frequency dispersions of 1Co and 2Fe indicates the disparate range of resonant frequencies attributed to the dynamic motion of the organic cation. Taking the above findings into consideration, we can conclude that for 2Fe the interaction between the Pyr + and the framework is stronger in comparison to that in the 1Co crystal, which is consistent with the crystallographic data.…”

Ferroelasticity in Organic–Inorganic Hybrid Perovskites

“…And Zamponi groups reported a series of switchable cyano-bridged 3D perovskites, [CH 3 C(NH 2 ) 2 ] 2 [KM(CN) 6 ] (M = Cr 3 + , Fe 3 + , and Co 3 + ), which display two consecutive ferroelastic phase transitions from C2/m to R � 3m to Fm � 3m space group. [42] According to Aizu's notation, the ferroelastic phase transitions belong to the m � 3mF � 3m and � 3mF2/m, respectively. The two-step phase transitions in the three crystals are attributed to the changes in the organic cationic dynamics from the orientational disorder state in high temperature to the in-plane rotational disorder in intermediate temperature and finally to the orientational order state in low temperature.…”

“…In addition, the metal B 3+ could also be replaced by Cr 3+ and Co 3+ to construct new A 2 B I B III X 6 ‐type ferroelastics. And Zamponi groups reported a series of switchable cyano‐bridged 3D perovskites, [CH 3 C(NH 2 ) 2 ] 2 [KM(CN) 6 ] (M=Cr 3+ , Fe 3+ , and Co 3+ ), which display two consecutive ferroelastic phase transitions from C 2/ m to R $${\bar{3}}$$ m to Fm $${\bar{3}}$$ m space group [42] . According to Aizu's notation, the ferroelastic phase transitions belong to the m$${\bar{3}}$$ mF$${\bar{3}}$$ m and $${\bar{3}}$$ m F2/ m , respectively.…”

Switchable dielectric constant, structural and vibrational studies of double perovskite organic–inorganic hybrids: (azetidinium)2[KCr(CN)6] and (azetidinium)2[KFe(CN)6]

“…With this combination, the inorganic part is responsible for stability and periodicity, while the organic cation is responsible for molecular dynamics in the crystal, thus generating states with different energies induced thermally. These states are formed by phase transitions (PTs) and differ in many parameters such as the symmetry of the crystal structure, [9][10][11] the value of electric permittivity, [12][13][14] generation of spontaneous polarization, 15,16 NLO (nonlinear optical coefficient), 12,16 and DC and AC conductivity, 13,17 to name a few. The organic cation dynamics change, and thus the dielectric response in the vicinity of PT is primarily affected by the dipole moment orientation.…”