Non-adiabatic small polaron hopping transport above metal-like to insulator transition in the vacant 3d-orbital Tb2Ti2O7 pyrochlore oxide

We report the validity of Mott variable range hopping (VRH) conduction mechanism and small polaron hopping in Tb₂Ti₂O₇ in the temperature range 603–803 K. The temperature-dependent resistivity data are in good agreement with Mott 3d VRH. The other parameters estimated such as hopping range, hopping energy and density of states near Fermi level are in good agreement with those of many semiconductor oxides. The Holstein’s condition for non-adiabatic conduction mechanism is also satisfied. The ac conductivity is governed by Jonscher’s power law as \( {\sigma}_{a.c}^{\prime}\left(\upsilon, T\right)={\sigma}_{d.c}(T)+\alpha (T){\upsilon}^n \) and the exponent n values in the range of 1.02–1.23 suggest that conduction takes place due to the small polaron hopping mechanism. The possible polaron formation and their mechanism are discussed systematically.