Thermal expansivity of geikielite and ilmenite utilizing in-situ synchrotron X-ray diffraction at high temperature

The unit-cell parameters and volumes of geikielite (MgTiO 3 ) and ilmenite (FeTiO 3 ) were investigated at high temperatures up to 700 K and ambient pressure, using in-situ angle-dispersive synchrotron X-ray diffraction. No phase transition was detected over the experimental temperature range. Using (Berman in J Petrol 29:445–522, 1988. 10.1093/petrology/29.2.445) equations to fit the temperature-volume data, the volumetric thermal expansion coefficients at ambient conditions ( α V 0 ) of MgTiO 3 and FeTiO 3 were obtained as follows: 2.55 (6) × 10 –5  K −1 and 2.82 (10) × 10 –5  K −1 , respectively. We infer that the larger effective ionic radius of Fe 2+ (VI) (0.78 Å) than that of Mg 2+ (VI) (0.72 Å) renders FeTiO 3 has a larger volumetric thermal expansivity than MgTiO 3 . Simultaneously, the refined axial thermal expansion coefficients under ambient conditions are α a 0  = 0.74 (3) × 10 –5  K −1 and α c 0  = 1.08 (5) × 10 –5  K −1 for the a -axis and c -axis of MgTiO 3 , respectively, and α a 0  = 0.95 (5) × 10 –5  K −1 and α c 0  = 0.92 (12) × 10 –5  K −1 for the a -axis and c -axis of FeTiO 3 , respectively. The axial thermal expansivity of MgTiO 3 is anisotropic, but that of FeTiO 3 is nearly isotropic. We infer that the main reason for the different axial thermal expansivity between MgTiO 3 and FeTiO 3 is that the thermal expansion mode of the Mg-O bond in MgTiO 3 is different from that of the Fe–O bonds in FeTiO 3 .