X-ray diffraction and Mössbauer spectroscopy study of oxoborate azoproite (Mg,Fe2+)2(Fe3+,Ti,Mg,Al)O2(BO3): an in situ temperature-dependent investigation (5 ≤ T ≤ 1650 K)

This work is devoted to an investigation of elemental composition, crystal structure and thermal expansion of natural oxoborate azoproite from the Tazheran massif (Siberia, Russia) in the temperature range 5–1650 K. Elemental composition was determined by energy-dispersive X-ray spectroscopy (EDX). Its empirical formula based on five oxygen atoms is (Mg1.81Fe2+ 0.19)∑2.00(Fe3+ 0.36Ti0.26Mg0.26Al0.12)∑1.00O2(BO3). Local environment, oxidation states and ratio of Fe atoms are determined using Mössbauer spectroscopy and compared with EDX and single-crystal X-ray diffraction (SCXRD) data. A refinement of the crystal structure from SCXRD data collected at 293 K was provided for the first time. The structure could be described both in terms of cation- and anion-centered polyhedra. It is composed of vertex- and edge-sharing metal–oxygen [MO6] n − octahedra that form extended zigzag chains along the a axis building up a framework with the [BO3]3− triangles located in its distorted trigonal channels. From the other point of view, there are double chains consisting of oxocentred [OM 4] n + tetrahedra and [OM 5] n + tetragonal pyramids forming six-membered rings with the triangles in its cavities. Four non-equivalent Mn + sites are occupied by cations as follows: M(1) (2a) and M(2) (2d) – Mg, M(3) (4g) – Mg and Fe2+, M(4) (4h) – Fe3+, Ti4+, Mg and Al3+. According to differential scanning calorimetry, low- and high-temperature powder X-ray diffraction (LT- and HT-XRD) data, Mössbauer spectroscopy and magnetometry data (5 ≤ T ≤ 1650 K), there are no phase transitions obtained in the temperature range investigated. However, some anomalies in temperature dependencies of unit-cell parameters caused by a partial Fe2+ → Fe3+ oxidation are found in the range 873–1173 K. Azoproite melts at a temperature higher than 1600 K. Eigenvalues of the thermal expansion tensor are calculated for the oxoborate and thermal expansion is described in comparison with its crystal structure.