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

MARTÍN E. SALETA; DANIELA VALDÉS; LILIANA MOGNI; DINA TOBIA; SANTIAGO J. A. FIGUEROA; JUNIOR C. MAURICIO; ENIO LIMA JR.; GUILLERMO ZAMPIERI; RODOLFO SÁNCHEZ

Conferencia; International Conference on X-Ray Absorption Fine Structure (XAFS21); 2021

Metal-oxide semiconductors have been studied for theirpotential technological applications due to their interesting physical and chemicalproperties. In particular the solid solution of Cr oxide substituted with Ti(Cr2−xTixO3+z) has been studied due to itstechnological application as a gas or volatile organic compound detector. Inparticular the electrical properties of the extremes of the solid solution arevery different. The Ti2O3 has been widely studied due tothe metal-to-insulator transition that this sesquioxide exhibits, without achange in its crystalline symmetry. Meanwhile, Cr2O3 is a semiconductor. Theoreticaland experimental results suggest that the conduction mechanism of Cr2O3can be explained by the small polaron model. The solid solution Cr2−xTixO3+zpresents p- type conductivity predominantly determined by the Cr defects and itwas reported that its conduction is not affected significantly by theincorporation of Ti ions, except for a gradual decrease of the conductivitywith respect to that of pure Cr2O3.In this work, we present magnetic and electricalconduction characterizations of Cr1.8Ti0.2O3+z(CTO) synthesized by a sol-gel method followed by two thermal treatments at1273 K. Magnetic susceptibility studies showed a para- to antiferromagnetic transitionat around 305-310 K, which is confirmed by electron spin resonancespectroscopy. X-ray photoemission spectra (XPS) and X-ray absorptionspectroscopy 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 Cr2O3structure, a fraction of the Ti ions is in the +4 oxidation state and a minorpart in the +3 state. By XPS, we confirmed that the Ti ions located on thesurface are all practically in the +4 oxidation state, meaning that the core ofthe 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 thenonadiabatic 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, inthe temperature range of 750-790 K, thecharacteristic parameters of the NA-SPH model (exponential hopping energy andpre-exponential conductivity factor) change. Both parameters are lower at hightemperatures than their respective values at low temperatures. It is importantto mention that no changes, in this temperature range, are observed in thethermal evolution of the cell parameters (or volume) studied by X-raydiffraction, which discards any possible structural rearrangement ortransition. However, the thermal evolution of the intensity of some selectedpeaks of the Ti K-edge of the XANES spectra presents a break at the sametemperature range. The changes observed in both experiments strongly suggestthat these are associated with a variation in the electron-phonon interaction in the CTO system. This is a noveleffect, and this result opens new perspectives for understanding the physicalphenomena of the electron-phononinteraction, which could be complemented with further theoretical calculations.M.E. Saleta, et al. J. Phys. Chem. C. (2021) DOI:10.1021/acs.jpcc.1c00366