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 nu(3), and out-of-plane bending mode 2 nu(2) is generally more prominent than the in-plane bending mode nu(4) and symmetric stretching mode nu(1) except for siderite FeCO3. Combining thermal expansion coefficients alpha(V) with the relative changes in frequency delta nu i/delta T of Raman spectra, the average thermodynamic Gruneisen parameters (gamma(th)) 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 gamma(th) illustrates that the concentration of Fe2+ causes a negative effect while the concentration of Mn2+ causes a positive effect on gamma(th). 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.