Giant Dielectric Anomaly of a Metal–Organic Perovskite with Four‐Membered Ring Ammonium Cations

Abstract

Ferroelectric materials with dielectric constants larger than 10 4 over a broad temperature range near room temperature have great potential for technological applications. Such highly polarizable materials have been found only in the perovskite-related metal oxides called relaxors. [1][2][3] Recently, the development of molecule-based highly polarizable materials has attracted increasing interest. For example, antiferroelectric and ferroelectric porous coordination polymer crystals with guest water molecules, a ferroelectric crystal consisting of single-component molecules with very large spontaneous polarization, and an exotic ferroelectric crystal based on supramolecular rotators have been reported. [4,5] However, there has been no report of molecule-based solids with broad dielectric peaks higher than 10 4 . High polarizability in a material is usually related to its structural freedom, such as the orientational motion of the permanent electric dipole and the freezing of the optical lattice vibration. Since molecules generally possess large structural freedom, the realization of molecule-based materials exhibiting giant polarizability will be a promising challenge. One of the structural freedom characteristics of the molecules is the ring-puckering motion of four-memberedring molecules. It has been known for over half a century that the four-membered-ring molecules, such as trimethylene sulfide, trimethylene oxide, and cyclobutane as well as fivemembered-ring molecules with one double bond, such as 2,5-dihydrofurane, have double minimum potentials for the outof-plane bending deformation of the molecule and therefore exhibit characteristic ring-puckering molecular vibrations (Figure 1 a). [6][7][8] If the energy barrier between the double minima is sufficiently high, the molecule will adopt a nonplanar structure even at high temperatures. However, if the magnitude of the energy barrier is thermally accessible, it may be possible that the molecule adopts a nonplanar structure at low temperatures and a planar structure at high temperatures. In such a case, it would be possible for the crystal to exhibit a large dielectric susceptibility coupled with a cooperative change of the molecular conformation. However, to our knowledge, studies to develop molecule-based dielectrics utilizing the conformational freedom characteristics of the four-membered-ring molecule have not been reported. Herein we report a metal-organic perovskite containing four-membered-ring ammonium cations, [(CH 2 ) 3 [9] which has been regarded as a metal-organic perovskite ABX 3 (A = ammonium cation, B = M 2+ (M = Zn, Mn), and X = HCOO À ) with three-dimensional CaTiO 3 -like structure. [10][11][12] Similar to well-known inorganic ferroelectric and antiferroelectric perovskites such as BaTiO 3 and PbZrO 3 , these metal-organic perovskites have been reported to exhibit dielectric transitions. [10, 11] With the aim of utilizing the ring-puckering molecular motion in inducing the dielectric transition of metal-organic perovskite, we have prepare...

Phase transition and cationic motion in the perovskite formate framework [(CH3)2NH2][Mg(HCOO)3]

“…The compounds with the azetidinium (four-membered ring ammonium) ion, [(CH 2 ) 3 3 ] (M = Mn, Cu, Zn), have been reported to have extraordinarily large dielectric constants over a wide temperature range [4,5].…”

Raman and IR Studies of Pressure- and Temperature-Induced Phase Transitions in [(CH2)3NH2][Zn(HCOO)3]

“…7−18 Azetidinium metal formates ([CH 2 ) 3 NH 2 [M(HCOO) 3 ], AzM) with M = Mn, Cu, and Zn, crystallize in the orthorhombic structure with space group Pnma and undergo phase transitions into the monoclinic structures. 3,19,20 For AzZn, the transition occurs at 255 K to the P2 1 /c structure. 20 These compounds were also reported to exhibit giant dielectric anomalies.…”

“…20 These compounds were also reported to exhibit giant dielectric anomalies. 19,20 Here it is important to note that AzZn is unique in the family of azetidinium metal formates because it shows the presence of an intermediate polar orthorhombic phase with space group Pna2 1 , stable between T c2 = 255 and T c1 = 300 K. 20 Although there is no confirmation that the Pna2 1 phase is ferroelectric, it was suggested that the dielectric properties of this material might be related to the formation of small ferroelectric domains near T c1 . 20 AzM compounds with M = Cu, Zn, and Mn have also attracted attention because of the high flexibility of their frameworks, which is advantageous for creating novel ferroelastics exhibiting large shear strains.…”

“…21,22 Temperature-induced phase transitions were studied in AzM compounds by means of X-ray diffraction, NMR, differential scanning calorimtry and dielectric data. [19][20][21]23 These studies showed that, in the high-temperature Pnma phases, the azetidinium cations (Az + ) are planar, whereas they adopt a ring-puckered conformation in the low-temperature phases. [19][20][21]23 Pressure-induced phase transitions in this family of MOFs were reported only for [NH 4 3 ], possessing chiral framework of 4 9 ·6 6 topology and perovskite-like [C(NH 2 ) 3 ][Mn(HCOO) 3 ] and AzMn.…”

“…[19][20][21]23 These studies showed that, in the high-temperature Pnma phases, the azetidinium cations (Az + ) are planar, whereas they adopt a ring-puckered conformation in the low-temperature phases. [19][20][21]23 Pressure-induced phase transitions in this family of MOFs were reported only for [NH 4 3 ], possessing chiral framework of 4 9 ·6 6 topology and perovskite-like [C(NH 2 ) 3 ][Mn(HCOO) 3 ] and AzMn. 22,24 The phase transition pressure was found to be only 0.41−0.66 GPa for AzMn, and it was shown that the high-pressure phase of AzMn at room temperature has the same monoclinic structure (space group P2 1 /n) as the ambient pressure low-temperature phase stable below 273 K. 22 Although temperature-induced phase transitions in AzM compounds have already been studied, the detailed mechanism of the phase transitions in AzZn is still not fully understood.…”