Structural Family of the CaMg[B2O5] Kurchatovite: Structure Features, Polytypical Complexity, DFT Analysis, and Comparative Crystal Chemistry of Divalent Cation Pyroborates

Abstract

Natural and synthetic borates occupy a special place in the extensive class of polytypism-exhibiting layered inorganic compounds with modular structures. The present work presents a repeated modular analysis of kurchatovite and clinokurchatovite CaMg[B₂O₅] crystal structures using the OD formalism as well as a topological analysis of the geometry and complexity of their polytypes crystal structures which were optimized by quantum chemical methods. The polytypic nature of kurchatovite, clinokurchatovite, and SrCd[B₂O₅] structures is confirmed by the data obtained for these compounds. Theoretical IR spectra obtained for clinokurchatovite completely agree with the experimental ones. For polytypic structures, the IR spectrum can be used as a kind of empirical measure of structural complexity: the number of normal modes increases with decreasing symmetry and increasing number of vibrational degrees of freedom per unit cell. Comparative crystal chemical analysis reveals a symmetrical relationship between a large number of divalent \(M_{2}^{2+}[{{\text{B}}_{2}}{{\text{O}}_{5}}]\) pyroborates (M = Ca, Sr, Pb, Mg, Mn–Ni, Zn, Cd) which can be also combined into a single structural shimazakiite (Cd₂[B₂O₅]) OD family consisting of three types of OD layers.