Guanidinium-Formamidinium Lead Iodide: A Layered Perovskite-Related Compound with Red Luminescence at Room Temperature.

Two-dimensional hybrid organic-inorganic lead halides perovskite-type compounds have attracted immense scientific interest due to their remarkable optoelectronic properties and tailorable crystal structures. In this work, we present a new layered hybrid lead halide, namely [CH(NH2)2][C(NH2)3]PbI4, wherein puckered lead-iodide layers are separated by two small and stable organic cations: formamidinium, CH(NH2)2+, and guanidinium, C(NH2)3+. This perovskite is thermally stable up to 255 °C, exhibits room-temperature photoluminescence in the red region with a quantum yield of 3.5%, and is photoconductive. This study highlights a vast structural diversity that exists in the compositional space typically used in perovskite photovoltaics.

Crystal structure and electronic properties of Y3AlC3.

A novel ternary aluminum carbide, Y(3)AlC(3), has been synthesized under application of a lithium metal flux at high temperature (1523 K). Single-crystal structure determination of this compound revealed a new structure type with the Wyckoff sequence 2j3e and remarkable structural features at the border between Zintl and intermetallic phases. The puzzling bonding structure of Y(3)AlC(3) is analyzed with the aid of electronic structure calculations (energy bands and the electron localization function).

YAlC: A Bonding Chameleon with Heteropolyacetylene Features.

YAlC was prepared by a flux method. It crystallizes as a partially filled-up TlI structure, showing remarkable structural aspects at the border between Zintl phases and intermetallics. This novel ternary aluminide-carbide exhibits a unique one-dimensional multi-center bond and a polyacetylene-related aluminum carbide substructure. The different functionalities of aluminum and of yttrium are quite remarkable. While the latter behaves more like a trivalent ion, aluminum contributes considerably to covalent bonding with carbon. Still yttrium d states contribute, but hardly in a directed manner.

Guanidinium and Mixed Cesium-Guanidinium Tin(II) Bromides: Effects of Quantum Confinement and Out-of-Plane Octahedral Tilting.

Hybrid organic-inorganic main-group metal halide compounds are the subject of intense research owing to their unique optoelectronic characteristics. In this work, we report the synthesis, structure, and electronic and optical properties of a family of hybrid tin (II) bromide compounds comprising guanidinium [G, C(NH2)3 +] and mixed cesium-guanidinium cations: G2SnBr4, CsGSnBr4, and Cs2GSn2Br7. G2SnBr4 has a one-dimensional structure that consists of chains of corner-sharing [SnBr5]2- square pyramids and G cations situated in between the chains. Cs+ exhibits a pronounced structure-directing effect where a mixture of Cs+ and G cations forms mono- and bilayered two-dimensional perovskites: CsGSnBr4 and Cs2GSn2Br7. Furthermore, the flat shapes of the guanidinium cations induce anisotropic out-of-plane tilts of the [SnBr6]4- octahedra in the CsGSnBr4 and Cs2GSn2Br7 compounds. In G2SnBr4, the Sn lone pair is highly stereoactive and favors non-octahedral, that is, square pyramidal coordination of Sn(II). G2SnBr4 exhibits bright broad-band emission from self-trapped excitonic states, owing to its soft lattice and electronic localization. This emission in G2SnBr4 is characterized by a photoluminescence (PL) quantum yield of 2% at room temperature (RT; 75 ± 5% at 77 K) and a fast PL lifetime of 18 ns at room temperature.