Polymorphs and solid solutions: materials with new luminescent properties obtained through mechanochemical transformation of dicyanoaurate(I) salts

We report the use of mechano- and thermochemical methods to create new solid-state luminescent materials from well-known inorganic salts, potassium dicyanoaurate(I) KAu(CN)₂, and potassium dicyanocuprate(I) KCu(CN)₂. In particular, manual grinding or ball milling of commercial samples of KAu(CN)₂ led to the formation of a novel polymorph of the salt, herein termed m-KAu(CN)₂, evident by a significant change in color of the fluorescence emission of the solid material from orange to violet. The formation of m-KAu(CN)₂ is reversible upon addition of small amounts of solvents, and powder X-ray diffraction analysis indicates that the structure of m-KAu(CN)₂ might be related to that of pristine KAu(CN)₂ through a change in ordering of Au(CN)₂^- ions in a layered structure. Thermal treatment of KAu(CN)₂ led to the discovery of another polymorph of this well-known salt, herein termed t-KAu(CN)₂, making KAu(CN)₂ a rare example of a system in which thermochemical and mechanochemical treatments lead to the formation of different, in each case previously not reported, polymorphic forms. The thermally-induced transformation from KAu(CN)₂ to t-KAu(CN)₂ takes place around 250 °C and proceeds in a crystal-to-crystal fashion, which enabled the preliminary structural characterisation through single crystal X-ray diffraction, revealing the retention of the layered structure and a change in ordering of Au(CN)₂^- ions. Milling of the simple salt KAu(CN)₂ in the presence of equimolar amounts or less of its copper(I)-based analogue coordination polymer KCu(CN)₂ leads to the formation of a series of solid solution materials, isostructural to m-KAu(CN)₂ and with visible fluorescence emission distinct from KCu(CN)₂ or any herein investigated forms of KAu(CN)₂.

Hybrid Lead Iodide Perovskites with Mixed Cations of Thiourea and Methylamine, From One Dimension to Three Dimensions

Hybrid halide perovskites featuring as new materials of high-performance solar cells have attracted great research interest. The temperature-dependent dimensional transition of halide perovskites is a crucial handle in the preparation of perovskite films. Only the small cations of methylammonium (MA) or formamidinium (FA) have been involved for most of the dimensional transition materials. In this work, thiourea (tu) is introduced into hybrid halide materials. A new series of 1D ribbonlike hybrid lead iodides with tu and MA cations are reported that were crystallographically characterized as MA_n(Htu)_n+1Pb_nI_4n+1 (n = 1-4 denoted as 1-4, respectively; in 1, MA is replaced by tu). The width of the perovskite ribbon increases from one PbI₆ octahedron to four corner-fused octahedra. Compounds 2 and 3 can be turned into a black 3D perovskite after annealing. This is an unusual mixed MA-tu hybrid halide perovskite system, in which the tu molecule plays an important role in manipulating the dimensions and their photoconductive properties. Scanning electron microscopy of the blackened sample shows that there are a lot of regular vent holes on the smooth crystal surface with sizes of hundreds of nanometers. The tunable structures and porous crystals might be advantageous in the sense of material modulation.

Three-Dimensional Metal-Free Molecular Perovskite with a Thermally Induced Switchable Dielectric Response

Temperature-responsive materials with switching physical properties have been widely researched. Among them, the switchable dielectric perovskite materials show potential applications in the electrical and electronic industries and even the intelligence industries. However, perovskite oxides and hybrid organic-inorganic perovskites, as the most representative switchable dielectric materials, are limited by bad biocompatibility. Herein, we report temperature-dielectric-responsive metal-free perovskite (H₂dabco)(NH₄)[BF₄]₃ constructed by the strategy of substituting the B site in the general formula ABX₃ (doubly protonated 1,4-diazabicyclo[2.2.2]octane = H₂dabco). Meanwhile, structurally similar hybrid material (H₂dabco)Rb[BF₄]₃ was designed as a control. They exhibit similar phase-transition characteristics and dielectric response behaviors around 333 K. More interestingly, the ordered-disordered transformation of their organic "spherical" cations (H₂dabco) was deemed to produce their phase transitions and dielectric response switching. Given its ability to generate a dielectric response, (H₂dabco)(NH₄)[BF₄]₃ will show the potential application of metal-free perovskite in a future thermal sensing device.

Dielectric transitions and relaxations in Ca(ii)Co(iii)-based cyanometallate frameworks with a rare (6,6)-connected nia topology

A series of cyanometallate frameworks A[CaCo(CN)₆] (A = protonated guanidine, urea or thiourea guest) exhibit a rare (6,6)-connected nia topology with the Schläfli symbol of (4¹²·6³)(4⁹·6⁶) and thermally driven dielectric transitions and relaxations without evident structural phase transitions.

Distinct room-temperature dielectric transition in a perchlorate-based organic–inorganic hybrid perovskite

Distinct room-temperature dielectric transition is realized in a perchlorate-based organic–inorganic hybrid perovskite with the introduction of a polar cationic guest.