Nonadiabatic small polarons produced by Ti ions in Cr1.8Ti0.2O3+z particles: A study by XANES

M E SALETA; DANIELA P. VALDÉS; LILIANA MOGNI; DINA TOBIA; SANTIAGO J. A. FIGUEROA; JÚNIOR C. MAURICIO; ENIO LIMA, JR.; GUILLERMO ZAMPIERI ; RODOLFO D. SÁNCHEZ

Sydney

Conferencia; International Conference X-Ray Absorption and Fine Structure, Virtual (XAFS2021); 2021

University of Sydney

Metal-oxide semiconductors have been studied for their potential technological applications due to their interesting physical and chemical properties. In particular the solid solution of Cr oxide substituted with Ti (Cr2−xTixO3+z) has been studied due to its technological application as a gas or volatile organic compound detector. In particular the electrical properties of the extremes of the solid solution are very different. The Ti2O3 has been widely studied due to the metal-to-insulator transition that this sesquioxide exhibits, without a change in its crystalline symmetry. Meanwhile, Cr2O3 is a semiconductor. Theoretical and experimental results suggest that the conduction mechanism of Cr2O3 can be explained by the small polaron model. The solid solution Cr2−xTixO3+z presents p- type conductivity predominantly determined by the Cr defects and it was reported that its conduction is not affected significantly by the incorporation of Ti ions, except for a gradual decrease of the conductivity with respect to that of pure Cr2O3.In this work, we present magnetic and electrical conduction characterizations of Cr1.8Ti0.2O3+z (CTO) synthesized by a sol-gel method 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 photoemission spectra (XPS) and X-ray absorption spectroscopy studies confirm that the Cr ions are only present in the +3 state. Meanwhile, in the Ti K-edge, it is observed that inside the Cr2O3 structure, a fraction of the Ti ions is in the +4 oxidation state and a minor part in the +3 state. By XPS, we confirmed that the Ti ions located on the surface are all practically in the +4 oxidation state, meaning that the core of the CTO particles is constituted by ions in both oxidation states, +3 and +4, and those on the surface are in the +4 state. The electric conduction of CTO is described by the nonadiabatic small polaron hopping (NA-SPH) model. The mixed valence in the Ti3+/Ti4+ system improves notably the mobility of the electrons. On the other hand, in the temperature range of 750-790 K, the characteristic parameters of the NA-SPH model (exponential hopping energy and pre-exponential conductivity factor) change. Both parameters are lower at high temperatures than their respective values at low temperatures. It is important to mention that no changes, in this temperature range, are observed in the thermal evolution of the cell parameters (or volume) studied by X-ray diffraction, which discards any possible structural rearrangement or transition. However, the thermal evolution of the intensity of some selected peaks of the Ti K-edge of the XANES spectra presents a break at the same temperature range. The changes observed in both experiments strongly suggest that these are associated with a variation in the electron-phonon interaction in the CTO system. This is a novel effect, and this result opens new perspectives for understanding the physical phenomena of the electron-phonon interaction, which could be complemented with further theoretical calculations.M.E. Saleta, et al. J. Phys. Chem. C. (2021) DOI: 10.1021/acs.jpcc.1c00366.