Electrical properties of natural and synthetic nano-crystalline MgTiO3 geikielite at mantle pressure and temperature conditions

Abstract

Electrical resistivity of nanocrystalline (30–40 nm crystallite size) and a crystalline sample of geikielite MgTiO 3 has been measured at simultaneous high pressure and high temperature up to 6 GPa and 800 K, respectively. The temperature dependence of the electrical resistivity of both the and sample obeys the Arrehenius behavior in the temperature range between 3 and 800 K and pressure range up to 6.0 GPa. The activation volume of the electrical conduction coarse crystalline sample of geikielite is almost twice that of the nanocrystalline geikielite indicating the increase of activation volume with the crystallite size. The activation energy the electronic conduction decreases from eV at room pressure to 0.25 eV at 6.0 GPa geikielite, and to 0.225 eV in the same pressure range geikielite. The pressure dependence of the activation energy of geikielite sample is found to obey the following expressions \[{\Delta}\mathit{E} (eV) = 0.39 {-} 0.026(1) \mathit{P} + 0.0036 \mathit{P}^{2} for natural sample, and\] \[{\Delta}\mathit{E} (eV) = 0.68 {-} 0.080 (2) \mathit{P} + 0.0007 \mathit{P}^{2} for synthetic sample,\] where P is pressure in GPa. We observe a crossover from extended state type conduction to hopping conduction at 4.0 GPa and 350 K nano-crystalline geikielite. However, there is no such change of conduction mechanism observed the geikielite at high pressures and high temperatures. The present study reveals the phase stability of nano-crystalline geikielite and geikielite up to mantle pressure and temperature conditions, viz. 6 GPa and 800 K, and no phase transition or decomposition is observed in the sample.