Single crystal growth, crystalline structure investigation and high-pressure behavior of impurity-free siderite (FeCO₃)

Abstract

Single crystals of impurity-free siderite were grown successfully using high-temperature–pressure annealing. The size of crystals ranged up to 100 µm, and they exhibited a rhomboid shape upon cleavage along the (101) plane. The composition of Fe_{0.9988±0.0011}CO₃ was quantified using electron probe analysis. Accurate crystalline structural data were investigated by means of single crystal X-ray diffraction (XRD) and the unit cell dimensions obtained in the rhombohedral symmetry of the $R\bar {3}c$ space group were a = 4.6861(3) and c = 15.362(2), and the final R = 0.0499. Using in situ synchrotron XRD, the high-pressure behavior of impurity-free siderite was investigated up to 20 GPa at ambient temperature. The pressure–volume (P–V) EoS was fitted by a third-order Birch–Murnaghan equation, and the isothermal bulk modulus was K₀ = 97.5(11) GPa for K₀′ = 4. High-pressure Raman spectroscopy was performed at up to 30 GPa at ambient temperature, and the Raman bands shifted as the increase of pressure ($\frac{{{\text{d}}{\nu _i}}}{{{\text{d}}P}}$) was determined. In combination with the high-pressure Raman results and the bulk modulus K₀, the mode Grüneisen parameters of each vibration were calculated. Meanwhile, high-temperature Raman spectroscopy was carried out at up to 300 °C and the Raman band shift ($\frac{{{\text{d}}{\nu _i}}}{{{\text{d}}t}}$) was also quantified.