Electrical conductivity of geikielite (MgTiO3) at lunar mantle conditions: the role of metastable defect states and thermal history

The electrical conductivity of geikielite (MgTiO 3 ), the Mg endmember of the ilmenite group, was investigated at mantle pressures of 2 and 4.3 GPa and temperatures up to 1850 K using a Kawai-type multi-anvil apparatus. Electrical conductivity increases by more than seven orders of magnitude between 800 and 1700 K and exhibits two distinct conduction regimes separated by a transition at ~ 1500–1700 K. The high-temperature regime is characterized by large activation enthalpies (Δ H ≈ 1.8–2.3 eV), whereas the low-temperature regime shows much lower values (Δ H ≈ 0.19–0.31 eV). Stepwise annealing experiments reveal a pronounced thermal-history dependence: repeated heating to progressively higher maximum temperatures ( T max ) produces metastable conductivity states, enhancing low-temperature conductivity by up to six orders of magnitude and systematically reducing activation enthalpy. This behavior indicates activation and freezing-in of defect-related charge carriers. Negative activation volumes further support a hopping-type conduction mechanism. Although Ti³⁺ was not directly detected, the combination of reducing experimental conditions, Al³⁺ impurities (~ 0.35 wt% Al₂O₃), low activation energies, and strong thermal memory is most consistent with small-polaron hopping involving Ti³⁺–Ti⁴⁺ pairs. At lunar core–mantle boundary temperatures, geikielite reaches conductivities of 10¹–10² S/m, exceeding those of olivine and overlapping estimates for the lunar low-velocity zone. Our results demonstrate that solid-state Ti-rich oxides can produce high electrical conductivity without partial melting, providing new constraints on the thermochemical evolution and electromagnetic structure of the lunar interior.