In-Situ Single Crystal XRD and Raman Spectra Investigation of (Mg, Fe, Mn)CO3 at Various Temperatures

The in-situ X-ray diffraction (XRD) and Raman spectra of synthetic (Mg, Fe, Mn)CO3 single crystals for (Mg0.37Fe0.31Mn0.32)CO3, (Mg0.49Fe0.51)CO3, (Fe0.47Mn0.53)CO3, and (Mg0.50Mn0.50)CO3 were investigated up to 871 K. Lattice parameters and the volumetric thermal expansion coefficients were achieved, demonstrating an inherent anisotropy of c/a near to 3.0. The Raman spectra of our intermediate components all exhibit a negative temperature dependence similar to that of the end members. The temperature dependence of the external modes T, L, anti-symmetric stretching mode ν3, and out-of-plane bending mode 2ν2 is generally more prominent than the in-plane bending mode ν4 and symmetric stretching mode ν1 except for siderite FeCO3. Combining thermal expansion coefficients αV with the relative changes in frequency δνi/δT of Raman spectra, the average thermodynamic Grüneisen parameters (γth) for (Mg0.37Fe0.31Mn0.32)CO3, (Fe0.47Mn0.53)CO3, (Mg0.50Mn0.50)CO3, MgCO3, and MnCO3 were obtained as 1.18, 1.36, 1.34, 1.13, and 1.53, respectively. The relationship between Fe2+ and Mn2+ contents and γth illustrates that the concentration of Fe2+ causes a negative effect while the concentration of Mn2+ causes a positive effect on γth. These results could provide a reference for the contribution of Fe2+ and Mn2+ on the structural and thermodynamic properties of (Mg, Fe, Mn)CO3 carbonates.