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 Fe1-xCrxBO3 borates (x = 0.0, 0.14, 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. Magnetic transition temperatures determined by magnetometry and Mössbauer spectroscopy are 350, 314-318 and11 K for x(Cr3+) = 0.0, 0.14, and1.0, 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 Fe0.86Cr0.14BO3. The end-members FeBO3 and CrBO3 exhibit positive thermal expansion with low average linear coefficients over 295-903 K: = 8 (FeBO3) and 6 × 10 -6 K -1 (CrBO3), respectively. In contrast, Fe0.86Cr0.14BO3 displays rare negative linear and volumetric thermal expansion up to 305 K ( = -8 × 10 -6 K -1 ), which is attributed to magnetostriction, followed by positive expansion at higher temperatures (= 7 × 10 -6 K -1 ). Thermal stability improves with the Cr3+ substitution: FeBO3 and Fe0.86Cr0.14BO3 undergo a partial decomposition to α-Fe2O3 at ~903 and 993 K, respectively, whereas CrBO3 remains stable 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 Fe1-xCrxBO3 series as promising candidate for high-precision optical, spintronic and electronic applications.