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

S.M.Aksenov^{a, b,} *, D.A.Banaru^{a, c}, A.M.Banaru^{a, d}, A.A.Antonov^e, N.A.Kabanova^f, A.N.Kuznetsov^d, E.L.Belokoneva^g, N.A.Yamnova^g, D.V.Deineko^{a, d}, N.A.Chervonnaya^h, N.V.Chukanov^g, and V.A.Blatov^f

^aLaboratory of Arctic Mineralogy and Materials Science, Nanomaterials Research Center, “Kola Science Center”, Russian Academy of Sciences Apatity, Russia

^bGeological Institute, “Kola Science Center” Russian Academy of Sciences, Apatity, Russia

^cVernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia

^dDepartment of Chemistry, Moscow State University, Moscow, Russia

^eLaboratory of Nature-Inspired Technologies and Environmental Safety of the Arctic, Nanomaterials Research Center, “Kola Science Center”, Russian Academy of Sciences, Apatity, Russia

^fSamara Center for Theoretical Materials Science, Samara State Technical University, Samara, Russia

^gFaculty of Geology, Moscow State University, Moscow, Russia

^hFederal Research Center of Problems of Chemical Physics and Medicinal Chemistry, Russian Academy of Sciences, Chernogolovka, Russia

email: *aks.crys@gmail.com

Received 15 May, 2024

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.

Keywords: kurchatovite, clinokurchatovite, pyroborates, OD structures, polytypism, IR spectroscopy, topology, complexity.

DOI: 10.1134/S0022476625020179