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

The equation of state of ilmenite (FeTiO 3 ) and geikielite (MgTiO 3 ) 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 FeTiO 3 , the zero-pressure unit-cell volume V 0 = 312.2(1) Å 3 , the zero-pressure bulk modulus K 0 = 165(4) GPa, and its pressure derivative K ′ 0 = 6.6(6); for MgTiO 3 , V 0 = 304.17(8) Å 3 , K 0 = 157(3) GPa, and K ′ 0 = 7.3(4). Additionally, the axial compressional behavior of FeTiO 3 and MgTiO 3 were also fitted with a linearized third-order Birch-Murnaghan EoS. The axial compressibility coefficients for FeTiO 3 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 MgTiO 3 . Both FeTiO 3 and MgTiO 3 exhibit the axial compression anisotropy, of which FeTiO 3 shows stronger axial compressibility anisotropy than MgTiO 3 . 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 FeTiO 3 and MgTiO 3 were further discussed.