Tunable negative and low thermal expansion in magnetic calcite-type Fe1−xCrxBO3 (x = 0.0, 0.14, 1.0) borates: a comprehensive in situ study

Abstract

Polycrystalline and single-crystal samples of calcite-type Fe_1−xCr_xBO₃ borates (x = 0.0, 0.14, and 1.0) were synthesised via multi-step solid-state reaction and flux methods, respectively. Crystal structures were refined using in situ high-temperature single-crystal X-ray diffraction data collected over 295–473 K. Increasing the x(Cr³⁺) reduces the unit cell parameters and volume in accordance with Vegard's law. The magnetic transition temperatures determined by magnetometry and Mössbauer spectroscopy are 350, 314–318 and 11 K for x(Cr³⁺) = 0.0, 0.14, and 1.0, respectively. Both in situ single-crystal and powder X-ray diffraction reveal an unusual behavior of the unit cell parameters and volume near the magnetic transitions for FeBO₃ and Fe_0.86Cr_0.14BO₃. The end-members, FeBO₃ and CrBO₃, exhibit positive thermal expansion with low average linear coefficients over 295–903 K: 〈α〉 = 8 (FeBO₃) and 6 × 10⁻⁶ K⁻¹ (CrBO₃), respectively. In contrast, Fe_0.86Cr_0.14BO₃ displays rare negative linear and volumetric thermal expansion up to 305 K (〈α〉 = −8 × 10⁻⁶ K⁻¹), which is attributed to magnetostriction followed by positive expansion at higher temperatures (〈α〉 = 7 × 10⁻⁶ K⁻¹). Thermal stability improves with Cr³⁺ substitution: FeBO₃ and Fe_0.86Cr_0.14BO₃ undergo a partial decomposition to α-Fe₂O₃ at ∼903 and 993 K, respectively, whereas CrBO₃ remains stable up to 1173 K. The Cr³⁺ substitution shifts the magnetic transition towards room temperature, enables tunable negative-to-low thermal expansion and improves thermal stability (decomposition temperature), which allows consideration of the Fe_1−xCr_xBO₃ series as promising candidates for high-precision optical, spintronic and electronic applications.