Crystal-structure refinements for orthorhombic boracite, Mg3ClB7O13, and a trigonal, iron-rich analogue*

Abstract

Abstract The crystal structures of orthorhombic boracite, Mg3ClB7O13, and of a trigonal iron-rich analogue, Fe2.4Mg0.6ClB7O13, have been refined using 1509 reflections for boracite (Pca21 a = b = c/√2 = 8.5496 Å, c = 12.0910 ± 0.0009 Å, Z = 4) and 650 reflections for the analogue (R3c, a = 8.612 ± 0.002 Å, c = 21.065 ± 0.004 Å, Z = 6). The residuals R are, respectively, 0.038 and 0.065. The two structures are quite similar, differing only in the arrangement of symmetry-equivalent units. They consist of unbroken boron-oxygen frameworks with large interstices in which the metal cations and chloride anions reside. The basic unit of the borate framework is made up of three rings of three boron-oxygen tetrahedra sharing corners and joined at a common oxygen atom, O(1); the ring systems are cross-linked to one another, as well as through a single boron-oxygen triangle. The O(1) oxygen atom is not common to four “BO3O pyramids”, as previously described. Instead one of these four is a normal triangle and the other three are normal tetrahedra. Magnesium and iron cations are five coordinated in an arrangement best described as transitional between square pyramidal and trigonal bipyramidal. The nature of the cavities in the borate framework probably accounts for the unusual coordination and some anomalous cation-chlorine distances. For most compounds with boracite-like structures, in which various cations substitute for magnesium and various halogens for chlorine, the ferroelectric transition temperature is found to increase with the ionic radius ratio, Me2+/halogen.