Equation of state of FeTiO3 and MgTiO3 by in-situ synchrotron X-ray diffraction at high pressures

Abstract

The equation of state of ilmenite (FeTiO₃) and geikielite (MgTiO₃) have been investigated using in situ synchrotron X-ray diffraction at high pressures up to ~ 21.8 GPa and ~ 23.0 GPa, respectively. No phase transitions were observed within the experimental pressure ranges for both samples. The pressure-volume data were fitted using the third-order Birch-Murnaghan equation of state (EoS), yielding the following results: for FeTiO₃, the zero-pressure unit-cell volume V₀ = 312.2(1) ų, the zero-pressure bulk modulus K₀ = 165(4) GPa, and its pressure derivative K′₀ = 6.6(6); for MgTiO₃, V₀ = 304.17(8) ų, K₀ = 157(3) GPa, and K′₀ = 7.3(4). Additionally, the axial compressional behavior of FeTiO₃ and MgTiO₃ were also fitted with a linearized third-order Birch-Murnaghan EoS. The axial compressibility coefficients for FeTiO₃ along the a- and c-axes are β_a = 1.43(4) × 10^− 3 GPa^-1 and β_c = 3.17(11) × 10^− 3 GPa^-1, respectively, with an anisotropic ratio of β_a: β_c = 0.45: 1.00, while β_a = 1.76 (7) × 10^− 3 GPa^-1 and β_c = 2.61(7) × 10^− 3 GPa^-1 with an anisotropic ratio of β_a: β_c = 0.67: 1.00 for MgTiO₃. Both FeTiO₃ and MgTiO₃ exhibit the axial compression anisotropy, of which FeTiO₃ shows stronger axial compressibility anisotropy than MgTiO₃. Moreover, the potential influencing factors (e.g., cation radius, crystal structure, and chemical bond strength) on the bulk moduli and the anisotropic linear compressibilities of FeTiO₃ and MgTiO₃ were further discussed.