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

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 <i>D</i>_3<i>h</i> nonpolar guanidinium results in an <i>R</i>3̅<i>c</i> ↔ <i>R</i>3̅<i>m</i> ↔ <i>Fm</i>3̅<i>m</i> transition, whereas the <i>C</i>_2<i>v</i> polar acetamidinium results in a <i>C</i>2/<i>m</i> ↔ <i>R</i>3̅<i>m</i> ↔ <i>Fm</i>3̅<i>m</i> 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

Zeolite CAN and AFI-Type Zeolitic Imidazolate Frameworks with Large 12-Membered Ring Pore Openings Synthesized Using Bulky Amides as Structure-Directing Agents

Using bulky amides as the structure-directing agents (SDAs) is an alternative synthetic strategy for the exploration of crystalline large pore (≥12-membered ring) zeolitic imidazolate frameworks (ZIFs). Specifically, by using the bulky amides, dibutylformamide (DBF) and dipropylformamide (DPF) as solvent and imidazole (Im) as a ligand, two ZIFs mimicking the CAN and AlPO-5 (AFI) zeotypes with 12-membered ring (MR) pore openings were synthesized, and denoted as CAN-[Zn­(Im)₂] and AFI-[Zn­(Im)₂], respectively. These two materials are the first known examples of Zn­(Im)₂ polymorphs with 12-MR pores and AFI-[Zn­(Im)₂] has the largest pore apertures reported to date for ZIF materials. The concept that the bulky amides used were not simply acting as the solvent, but were in fact acting as SDAs or templates during the synthesis of the large pore ZIFs, was suggested by the closeness of the geometrical fit between the guest DBF and the <i>can</i> cages (composite building units) of the CAN-[Zn­(Im)₂]

Plastic Crystals with Polar Halochromate Anion: Thermosensitive Dielectrics Based upon Plastic Transition and Dipole Rotation

Plastic crystals functioning with rotatable components offer new opportunities in areas such as modern optoelectronic materials. Here, by taking advantage of controllable rotation of the polar component within the ion-pair plastic-crystal system, we present two such crystals, namely, (Et₄N)­(CrO₃X) (X = Cl or Br), which are unusual examples exhibiting two-staged thermosensitive dielectric responses above room temperature. The frequency-dependent response in the first stage is due to the structural phase transitions, whereas that in the second stage is induced by dynamic rotation of the polar halochromate anions in their NaCl-type plastic-crystal phases. The intrinsic mechanisms were also explicated by molecular dynamics simulations, providing a direct insight into the dynamic characteristics of these two compounds. These studies show that ionic plastic crystals functioning with polar groups are an attractive candidate as sensitive thermoresponsive dielectric materials

Plastic Crystals with Polar Halochromate Anion: Thermosensitive Dielectrics Based upon Plastic Transition and Dipole Rotation

Plastic crystals functioning with rotatable components offer new opportunities in areas such as modern optoelectronic materials. Here, by taking advantage of controllable rotation of the polar component within the ion-pair plastic-crystal system, we present two such crystals, namely, (Et₄N)­(CrO₃X) (X = Cl or Br), which are unusual examples exhibiting two-staged thermosensitive dielectric responses above room temperature. The frequency-dependent response in the first stage is due to the structural phase transitions, whereas that in the second stage is induced by dynamic rotation of the polar halochromate anions in their NaCl-type plastic-crystal phases. The intrinsic mechanisms were also explicated by molecular dynamics simulations, providing a direct insight into the dynamic characteristics of these two compounds. These studies show that ionic plastic crystals functioning with polar groups are an attractive candidate as sensitive thermoresponsive dielectric materials