Nonadiabatic small polarons produced by ti ions in granular and paramagnetic cr1.8ti0.2o3+z particles

Abstract

Cr1.8Ti0.2O3+z (CTO) particles were grown by a sol-gel route followed by two thermal treatments at 1273 K. Magnetic susceptibility studies showed a para-to antiferromagnetic transition at around 305-310 K, which is confirmed by electron spin resonance spectroscopy. X-ray absorption spectroscopy studies reveal two different oxidation states for Ti ions: +3 and +4. Even more, X-ray photoemission spectra showed that Ti ions located on the surface of CTO are in the +4 state. At the same time, both spectroscopy studies can confirm that the chromium ions of CTO are only in the +3 state. The electrical transport measurements at high temperatures (between 500 and 950 K) reveal that the nonadiabatic small polaron model describes the conduction of CTO, which is associated with electron hopping between Ti3+ and Ti4+ ions. In the range of 750-790 K, the electrical conductivity curve presents changes, which are manifested in the thermal activation energy gap value and also in the pre-exponential factor. In this temperature range, no magnetic or structural changes are observed. However, the intensity of selected peaks of the Ti K-edge X-ray absorption near-edge structure (XANES) spectra as a function of temperature exhibits a break in this range. Based on both experiments, we associated these changes with variations in the electron-phonon interaction.