The influence of temperature, pressure and Ag doping on the physical properties of TiO2 nanoceramics

Abstract

Undoped and Ag-doped TiO₂ ceramics have been prepared at temperatures between 500–1000 °C and under pressures up to 8 GPa. Their crystal structures and physical properties were investigated by means of EDX, SEM, TEM, X-ray powder diffraction, and magnetization M, specific heat C_p and electrical resistance ρ measurements. It is found that the anatase-structured As-cast powder transforms into rutile and columbite-type at 500 °C and 5.5 GPa. The stabilization of the latter phase is fulfilled under a pressure of 8 GPa and at temperatures above 800 °C. On the basis of experimental results, we conclude that the physical properties of TiO₂ can be tailored along with its crystal structure. In particular, magnetic properties change from paramagnetic in anatase and rutile to magnetic correlations and in all likelihood magnetic-field-induced antiferromagnetic short-range order in columbite-structured TiO₂. Contrasting behaviour in the temperature dependences of specific heat between anatase/rutile and columbite-type TiO₂ is obvious. Differently from anatase/rutile, the C_p of columbite-type TiO₂ exhibits a low-temperature excess, being interpreted as due to magnetic correlations, or else the prevalence of soft modes. An analysis of ρ(T) for columbite-type TiO₂ in the temperature range of 280–400 K reveals the presence of a new trapping state at an energy level of ∼28 meV within the originally forbidden gap. Furthermore, thermal fluctuation-induced tunnelling and hopping conductivities are suggested to govern in a lower temperature range. We recognize that the Ag-doped contents do not alter the crystal structure but considerably enhance magnetic correlations, compared to undoped samples.