Transition metal borates have attracted considerable interest due to their rich crystal chemistry and diverse functional properties (magnetic, electrochemical, optical and others). These materials exhibit rare phenomena such as high magnetic ordering temperatures, cascades of magnetic transitions, and spin-liquid, glass and ice states. Owing to its unique combination of properties, FeBO₃ has been developed as a commercial Mössbauer source for synchrotron facilities [1]. In FeBO₃, the magnetic ordering temperature is high (T_N ≈ 348 K), whereas in Fe_1–xCr_xBO₃, it decreases with increasing Cr content [2].

In this work, FeBO₃ and CrBO₃ were investigated by in situ high-temperature powder X-ray diffraction (Rigaku Ultima IV, Cu Kα, 295–1173 K). Near T_N, FeBO₃ exhibits anomalous unit-cell behavior, whereas CrBO₃ shows no such anomalies. Both borates display positive uniaxial thermal expansion with low average coefficients: α = 8 (FeBO₃) and 6 × 10⁻⁶K⁻¹ (CrBO₃). The slight reduction in the expansion upon the Cr → Fe substitution arises from the smaller ionic radius of ^VICr³⁺ and shorter Cr–O bonds. FeBO₃ remains stable up to ~900 K, where a recrystallization to α-Fe₂O₃appears, while CrBO₃ is stable up to 1173 K. Thermal expansion is maximum along the c axis, i.e., perpendicularly to planes of the isolated BO₃ triangles, and minimum in the ab plane. These findings suggest that Fe_1–xCr_xBO₃ borates are promising candidates for synchrotron, optical and magnetic applications requiring tunable low thermal expansion.

The work is funded by the Russian Science Foundation [25-73-00080].

1. Potapkin, A.I. Chumakov, G.V Smirnov, J.-P. Celse, R. Rüffer, C. McCammon & L. Dubrovinsky, The ⁵⁷Fe Synchrotron Mössbauer Source at the ESRF, J. Synchrotron Radiation. 19 (2012) 559–569.
2. Muller, M.P. O'Horo, J.F. O'Neill, FeBO₃ solid solutions: Synthesis, crystal chemistry, and magnetic properties, J. Solid State Chemistry. 23 (1978) 115–128.