Centrosymmetric crystal structures described as non-centrosymmetric: An analysis of reports in Inorganica Chimica Acta

Layered Perovskite (CH3NH3)2Pb(SCN)2I2 Single Crystals: Phase Transition and Moisture Stability

To study stability issues of three-dimensional perovskites, there is a strategy to introduce the thiocyanate ion (SCN^-) into CH₃NH₃PbI₃ (MAPbI₃) to solve these problems. Here, we report the bulk growth of layered perovskite MA₂Pb(SCN)₂I₂ single crystals by different growth methods in an ambient atmosphere. We also investigate the structural determination and refinements, band gap, and photoluminescence of MA₂Pb(SCN)₂I₂ single crystals. More importantly, the phase transition and stability of MA₂Pb(SCN)₂I₂ are systematically demonstrated. MA₂Pb(SCN)₂I₂ undergo the reversible single-crystal to single-crystal phase transition in the orthorhombic systems from the space group Pmmn (no. 59) to the space group Pmn2₁ (no. 31) at low temperature. Moreover, the temperature-dependent recovery behaviors of MA₂Pb(SCN)₂I₂ single crystals, powders, and thin films at high temperature are studied in detail. Besides, the moisture stability of MA₂Pb(SCN)₂I₂ is described when exposed to moisture condition by the experiment and theoretical calculations. It would be interesting to not only conduct a comprehensive study on the crystal structures and the phase transition processes of layered perovskites but also provide guidance for further optoelectronic applications of these perovskite materials.

The Publications of Howard Flack (1943–2017)

Howard Flack was a driving force in the development of modern crystallography. Today “the Flack parameter” has entered into the common parlance of crystallography but his influence was far wider. This article provides an overview of his scientific output and a full bibliography.

Kryptoracemic compound hunting and frequency in the Cambridge Structural Database

Determination of kryptoracemic compound frequency in the Cambridge Structural Database using CCDC Python API script.

The use of X-ray crystallography to determine absolute configuration

Essential background on the determination of absolute configuration by way of single-crystal X-ray diffraction (XRD) is presented. The use and limitations of an internal chiral reference are described. The physical model underlying the Flack parameter is explained. Absolute structure and absolute configuration are defined and their similarities and differences are highlighted. The necessary conditions on the Flack parameter for satisfactory absolute-structure determination are detailed. The symmetry and purity conditions for absolute-configuration determination are discussed. The physical basis of resonant scattering is briefly presented and the insights obtained from a complete derivation of a Bijvoet intensity ratio by way of the mean-square Friedel difference are exposed. The requirements on least-squares refinement are emphasized. The topics of right-handed axes, XRD intensity measurement, software, crystal-structure evaluation, errors in crystal structures, and compatibility of data in their relation to absolute-configuration determination are described. Characterization of the compounds and crystals by the physicochemical measurement of optical rotation, CD spectra, and enantioselective chromatography are presented. Some simple and some complex examples of absolute-configuration determination using combined XRD and CD measurements, using XRD and enantioselective chromatography, and in multiply-twinned crystals clarify the technique. The review concludes with comments on absolute-configuration determination from light-atom structures.