Defect engineering in controlling the dielectric behavior of rutile tio2

Abstract

Defects have gained increasing attention to tailor/explore novel dielectric properties in metal oxide due to the correlation of defect complexes and the interaction between defects and crystal environment. For example, colossal permittivity (CP, >1000) was successfully achieved by electron-pinned defect-dipoles (EPDDs) with a concrete example In+Nb co-doped rutile titanium dioxide. This dissertation, therefore, aims to use defect engineering to control the dielectric behaviour of rutile titanium dioxide. Continuing works were first carried out on acceptor/donor ion co-doped rutile titanium dioxide. Ga, with much smaller ionic size than In, was used as acceptor. The 0.5% Ga+Nb co-doped case also shows similar CP (>10000) with optimized loss in the range of 0.05-0.1. Systematic dielectric analysis suggest the CP is not dominated by EPDDs, which is significantly different compared with the EPDD-dominated CP in In+Nb co-doped rutile titanium dioxide and no-EPDD-dominated CP in Al+Nb co-doped case. In addition to the acceptor, another donor Ta(V) with similar ionic size but one more electron shell structure was used to replace the Nb(V). M+Ta co-doped rutile titanium dioxide (M = In, Mg) were synthesized. Both samples show similarly low dielectric loss below 0.01 but different intrinsic CP level (i.e. {u0303}10000 in the former, {u0303}7000 in the later) at appropriate co-doping level. Theoretical calculations reveals different EPDDs formed in each material. These results suggest the important role of acceptor and donor in controlling the CP behaviours in these materials. A part from the system with slight acceptor/donor ion co-doping, rutile titanium dioxide with heavily co-substitutional acceptor/donor ions were analyzed to have a more comprehensive understanding of the defect chemistry, correlation behavior of defect complexes, and their related dielectric properties. Upon the success in synthesizing Al+Ta co-substituted rutile titanium dioxide with co-substitution level (x) up to 50%. Structural analysis suggests that the materials maintain rutile phase even for x up to 50% with signature of disorder. Defect analysis suggests the significantly low ratio of Ti(III)/Ta(V) as well as lower magnitude of permittivity (800{u0303}3000) in the system compared with that previously reported. The permittivity increases when x < 20% but decreases after over 20% followed by exponential decreases of relaxation frequency as well as increase of the thermal activation energy E (e.g. E increases from 33.4 meV to 131.4 meV when x increases from 5% to 50%). The system presents a typical dipole glass character when x > 20%. Finally, this dissertation reports for the first time, by using defect engineering, the linear polarisation behavior of binary oxide, e.g. rutile titanium dioxide, can be changed to a nonlinear one at room temperature. With slightly doping acceptor Mg into the host matrix, a combination of electrical measurements and theoretical calculations suggest that Mg-oxygen vacancy defect dipole can facilitate certain neighboring Ti(IV) with large move-off-center ability, offering the doped titanium dioxide an unexpected role as a new family of room temperature nonlinear dielectrics. Moreover, a poling induced internal electric field abnormally varies with the direction of poling electric field can be observable, which is quite different from that observed in ferroelectrics and relaxor ferroelectrics.