Inorganic Metal Thiocyanates

Metal thiocyanates were some of the first pseudohalide compounds to be discovered and adopt a diverse range of structures. This review describes the structures, properties, and syntheses of the known binary and ternary metal thiocyanates. It provides a categorization of their diverse structures and connects them to the structures of atomic inorganic materials. In addition to this description of characterized binary and ternary thiocyanates, this review summarizes the state of knowledge for all other binary metal thiocyanates. It concludes by highlighting opportunities for future materials development.

Three-Dimensional Bimetallic Ammonium K-Eu Nitrate with a Rare (6,6)-Connected Ion Topology Exhibiting Structural Phase Transition and Photoluminescence Properties

A K-Eu bimetallic ammonium metal-nitrate three-dimensional (3D) framework incorporating R-N-methyl-3-hydroxyquinuclidine, (RM3HQ)2KEu(NO3)6 (RM3HQ = R-N-methyl-3-hydroxyquinuclidine, 1), was characterized and reported. Distinguishing from the former hybrid rare-earth double perovskites, 1 adopts a mixed corner- and face-sharing K+/Eu3+-centered polyhedral connectivity to form a 3D inorganic framework, showing a rare (6, 6)-connected ion topology with a 66 framework. Notably, 1 exhibits clear phase transition, and the switchable thermodynamic behavior is confirmed by variable-temperature dielectric measurements and second-harmonic generation response. Moreover, 1 also shows photoluminescence properties. The activator Eu3+ plays a crucial role in this process, leading to a significant narrow emission at 592 nm with a photoluminescence quantum yield (PLQY) of 20.76%. The fluorescence lifetime (FLT) of 1 is 4.32 ms. This finding enriches the bimetallic hybrid system for potential electronic and/or luminescence applications.

Dimension-Dependent Phase Transitions, Ferroelasticity, and Photoluminescence in Hybrid Organic-Inorganic Materials

[(CH3)3P(CH2)2OH]2Cd3(SCN)8 (1) and [(CH3)3P(CH2)2OH]Cd(SCN)3 (2) were obtained with completely different structures and properties under the same synthesis conditions. Compound 1, showing green fluorescence, has a rare three-dimensional 4,6-connected fsh topology having (43.63)2(46.66.83) Schläfli notation, while compound 2 with blue-violet phosphorescence displays a one-dimensional perovskite structure with an infinite {[Cd(SCN)3]-}∞ chain and exhibits both ferroelastic and dielectric switching characteristics.

Above-Room-Temperature Ferroelastic Phase Transitions in Two Tetrafluoroborate-Based Hexagonal Molecular Perovskites

ABX₃-type molecular perovskites provide an important platform to tune phase transitions, via judiciously choosing A-, B-, and X-site components, to approach advanced functional materials for applications. Although tetrafluoroborate can act as X-site component to assemble ten instances of ABX₃ molecular perovskites, only two of them possess hexagonal perovskite structures. Herein, we report two tetrafluoroborate-based hexagonal molecular perovskites, A[Na(BF₄)₃], by judiciously choosing two different A-site cations: 1-methyl-1,4-diazabicyclo[2.2.2]octane-1,4-diium (Hmdabco²⁺) for 1 and 1-methylpiperazine-1,4-diium (H₂mpz²⁺) for 2. They have high-temperature phases in the same space group (P6₃/mmc) revealing highly disordered A-site cations. Upon cooling, 1 undergoes two-step P6₃/mmc ↔ P3̅c1 ↔ P2₁/n transitions at 344 and 338 K, respectively, including a ferroelastic one (3̅mF2/m) accompanied by a spontaneous strain of 0.013. In contrast, the smaller H₂mpz²⁺ cation with more adoptable conformations induces a one-step sharp P6₃/mmc ↔ P2₁/c ferroelastic transition (6/mmmF2/m(s)) at 418 K in 2, leading to more significant symmetry breaking and a considerable spontaneous strain of 0.129. This study provides important clues to modulate structural phase transitions by tuning diverse components for the multicomponent dense hybrid crystals.

Dual-channel control of ferroelastic domains in a host–guest inclusion compound

By replacing [PF6]− with the larger [TFSA]−, the phase transition temperature is increased from 305 K to 342 K in a host–guest inclusion ferroelastic crystal, [(3,4-DFA)(18-crown-6)][TFSA], which can realize dual-channel (thermal and stress) control of ferroelastic domains.

Room-temperature dielectric switching in a host–guest crown ether inclusion complex

Using the “momentum matching” theory, we have designed a new host–guest crown ether inclusion complex, which exhibits prominent room temperature bistable dielectric switching.