High-temperature behavior of FeTiO3 and MgTiO3: in-situ synchrotron X-ray diffraction study

Abstract

The high-temperature volumetric and axial thermal expansion behaviors of ilmenite (FeTiO 3 ) and geikielite (MgTiO 3 ) were examined up to 773 K using in situ synchrotron X-ray diffraction. No phase transitions were observed within the experimental temperature range. The temperature–volume data were fitted using the Berman (1988)’s equation, yielding V 0 = 316.50(3) Å 3 and α V 0 = 3.73(4) × 10 –5 K −1 for FeTiO 3 , and V 0 = 308.20(1) Å 3 and α V 0 = 2.95(9) × 10 –5 K −1 for MgTiO 3 . In addition, axial thermal expansion analysis showed that FeTiO 3 has a 0 = 5.0865(1) Å, α a 0 = 1.41(1) × 10 –5 K −1 , c 0 = 14.0858(4) Å, and α c 0 = 0.92(2) × 10 –5 K −1 , while MgTiO 3 has a 0 = 5.0585(1) Å, α a 0 = 0.85(3) × 10 –5 K −1 , c 0 = 13.9084(4) Å, and α c 0 = 1.23(3) × 10 –5 K −1 . The results demonstrate that FeTiO 3 exhibits more stable volumetric expansion with increasing temperature than MgTiO 3. However, FeTiO 3 and MgTiO 3 display opposite trends in axial expansion: the a -axis expands more than the c -axis in FeTiO 3 ( α a 0 / α c 0 = 1.53), whereas in MgTiO 3 , the c -axis expands more ( α a 0 / α c 0 = 0.69). These differences mainly reflect the distinct nature of the A-site cations: the 3d electrons of Fe 2+ affect the rigidity and thermal response of Fe–O and Ti–O bonds, while Fe-related magnetic exchange interactions and associated magnetoelastic coupling may further constrain Fe–O bond expansion and reduce the thermal expansion of FeTiO 3 .