A Metal-Free Molecular Antiferroelectric Material Showing High Phase Transition Temperatures and Large Electrocaloric Effects

Abstract

Antiferroelectric (AFE) materials, featuring an antiparallel alignment of electric dipoles in the adjacent sublattices, are keeping a great promise toward solid-state refrigeration applications on account of their electrocaloric (EC) effects. Although extensive studies have been performed on inorganic oxide counterparts (e.g., PbZrO 3 and AgNbO 3 ), metal-free molecular AFE alternatives with the above-room-temperature EC activities are quite scarce but urgently demanded in terms of environmental issues. Herein, we present a new metal-free molecular AFE, cyclohexylmethylammonium bromide (CMB), which exhibits the unusual antiferroelectric−ferroelectric−paraelectric phase transitions around 364 and 368 K upon heating. The phase transition temperatures are much higher than the majority of known molecular AFE materials. The practical utilization level of electric polarization (∼6 μC/cm 2 ) is clearly evidenced by the typical double polarization−electric field hysteresis loops. Strikingly, large positive and negative EC responses with the temperature changes (ΔT) of 4.2 and −3 K are achieved under an electric field of 20 kV/cm. The origin of its antiferroelectricity and EC properties is elucidated by the antipolar reorientation of cations along with displacement of bromine anions, being distinct from the known mechanism of inorganic oxides. Such intriguing AFE behaviors, including large polarization and EC effects, reveal great potentials of CMB for the solid-state refrigeration. This study sheds light on further exploration of new AFE candidates toward environmentally friendly solid-state cooling devices.

Record high-Tc and large practical utilization level of electric polarization in metal-free molecular antiferroelectric solid solutions

“…Structure comparison reveals that the order–disordering accounts for their phase transitions and the vanishing of bulk polarization. The halogen anions of these solid solutions exhibit relative atomic displacements in FEP, thus coinciding with the variation of crystal symmetry 24 . Hence, the origin of AFE-FE-PE phase transitions can ascribe to the collaboration between organic cationic ordering and anionic displacement.…”

“…Notably, this binary solid-solution family has a delicate compositional dependence on intermolecular H-bonds, as shown by Hirshfeld surface, which results in the enhanced T c . Taking CMB as a prototype 24 , the height of energy barriers can be successfully regulated by I/Br/Cl halogen modification (Fig. 1b, c ), which leads to a wide tunable rang of T c from 324 K of CMI to 453 K of CMC.…”

Thermally-driven unusual dual SHG switching with wide SHG-active steps triggered by inverse symmetry breaking

“…Fig. 5c shows the switching SHG behaviors of [C 4 H 10 N][CdCl 3 ], 32 cyclohexylmethylammonium bromide, 33 [Co II (en) 3 ](SO 4 ) 34 and [azetidinium]CdCl 3 . 38 Here, this feature has been further significantly improved in the current work, and it also has good chemically stable SHG properties (Fig.…”

“…However, rare cases can exhibit twoor more step SHG-switching behaviors, including normal "high-low-off" SHG switching, [27][28][29][30] "low-high-off" SHG switching-MHyPbBr 3−x Cl x (x = 1.33, 1.95, 2.25, and 2.55) 31 and an unusual "off-on-off" SHG switching. [32][33][34] In terms of material construction, NLO materials with multi-step SHG switching can be prepared by designing multi-step phase transitions. However, the centrosymmetric crystal structures make SHG signals inactive in most multi-step phase transition compounds.…”

“…35–37 In particular, realizing the unusual “off–on–off” SHG switching material is a considerable challenge. In the limited relevant reports, the examples of [C 4 H 10 N][CdCl 3 ], 32 [CoII(en) 3 ](SO 4 ), 33 cyclohexylmethylammonium bromide, 34 and [azetidinium]CdCl 3 38 show the “off–on–off” two-step SHG switching behaviors, which promote the development of novel two-step NLO switches. Regrettably, most of their applications and research categories are limited by the low phase transition temperature or narrow SHG-active temperature range.…”