Investigation of thermal behavior of monoclinic ludwigite-type oxoborate Cu ₂ FeO ₂ (BO ₃ ) in the range 300–1273 K

The crystal structure of synthetic ludwigite-type oxoborate Cu₂FeO₂(BO₃) 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 [MO₆]^n- (M = Cu²⁺, Fe³⁺) octahedra that form extended zigzag chains in the bc plane with isolated trigonal planar [BO₃]^3- located in triangular cavities of the cationic framework. Oxo-centered [OM₄]ⁿ⁺ tetrahedra and [OM₅]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 Cu²⁺ ions, while the M2 and M4 sites are occupied by the Cu²⁺ and Fe³⁺ ions in ratios of 0.59:0.41 and 0.20:0.80, respectively. The O4 site is split into two sites, O4A and O4B, which leads to the formation of distorted [M2O₆]^n-, Cu3O₆]^n- and [M4O₆]^n- polyhedra. Thermal behavior of Cu₂FeO₂(BO₃) 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)₃O₄ phases. The anisotropy of thermal expansion is weak and it is explained by (i) the preferable orientation of the most chemically rigid [BO₃]^3- units, (ii) the theory of shear deformations of the monoclinic ac plane and (iii) an arrangement of the oxo-centered [O₂M₇]ⁿ⁺ and [O₂M₈]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 Cu₂FeO₂(BO₃) is similar to that of orthorhombic ludwigite-type oxoborates.