The crystal structure of synthetic ludwigite-type oxoborate Cu 2 FeO 2 (BO 3 ) was first determined using in situ high-temperature single-crystal X-ray diffraction data collected at 300, 800 and 950 K. The structure is described as a heteropolyhedral framework composed of edge- and vertex-sharing M O 6 n − ( M = Cu 2+ , Fe 3+ ) octahedra that form extended zigzag chains in the bc plane with isolated trigonal planar BO 3 3− located in triangular cavities of the cationic framework. Oxo-centered O M 4 n + tetrahedra and O M 5 n + tetragonal pyramids are observed and described for the first time. The structure is disordered: the Cu1 and Cu3 sites are fully occupied by the Cu 2+ ions, while the M 2 and M 4 sites are occupied by the Cu 2+ and Fe 3+ ions in ratios of 0.59:0.41 and 0.20:0.80, respectively. The O4 site is split into two sites, O4 A and O4 B , which leads to the formation of distorted M 2O 6 n − , Cu3O 6 ] n − and M 4O 6 n − polyhedra. Thermal behavior of Cu 2 FeO 2 (BO 3 ) was investigated using in situ high-temperature single-crystal and powder X-ray diffraction, simultaneous thermal analysis and high-temperature heat capacity measurements. Unusual behavior of the unit-cell parameters and specific heat is observed at ∼690 K. The borate is stable up to ∼1040 K, when an incomplete solid-phase decomposition begins with the formation of CuO and (Cu,Fe) 3 O 4 phases. The anisotropy of thermal expansion is weak and it is explained by (i) the preferable orientation of the most chemically rigid BO 3 3− units, (ii) the theory of shear deformations of the monoclinic ac plane and (iii) an arrangement of the oxo-centered O 2 M 7 n + and O 2 M 8 n + double chains. Volume expansion increases with an increase in temperature from 24.6 × 10 −6 K −1 (at 300 K) to 35.4 × 10 −6 K −1 (at 1000 K). The degree of anisotropy of the expansion of Cu 2 FeO 2 (BO 3 ) is similar to that of orthorhombic ludwigite-type oxoborates.
Biryukov et al. (Fri,) studied this question.