Phase transitions and thermal expansion of BaCO₃ and SrCO₃ up to 1413 K

The phase evolution and thermal expansivity of synthetic orthorhombic BaCO₃ (α-BaCO₃) and SrCO₃ (α-SrCO₃) were studied up to 1413 K using high-temperature powder X-ray diffraction. The α-BaCO₃ phase transformed into a trigonal β phase at 1073–1093 K with an increase in molar volume by 2.7%, and further to a cubic γ phase at 1233–1253 K with 0.2% increase in molar volume. The α-SrCO₃ phase transformed into a trigonal β phase at ~1173 K. The fitted parameters for volume thermal expansion coefficients (α_V(T) = a₀ (10⁻⁵ K⁻¹) + a₁(10⁻⁸ K⁻²)T) are a₀ = 4.0(4), a₁ = 4.0(6) for α-SrCO₃; a₀ = 4.4(5), a₁ = 3.9(7) for α-BaCO₃; a₀ = 6.5(2) and a₀ = 6.8(1) for the β and γ phase of BaCO₃, respectively. The β-BaCO₃ phase exhibits negative expansion along the a-axis, which may help to design new materials with tunable thermal behavior when it is mixed with materials with positive thermal expansion. The aragonite-group carbonates (CaCO₃, SrCO₃, and BaCO₃) have similar bulk thermal expansivity, but their anisotropy and linear expansivities vary with the size of the cation.