A-Site Compensated Donor-Doped SrTiO3

The initial motivations of this thesis were to dope paraelectric SrTiO3 with niobium alongside an A-site vacancy compensation scheme so as to study the resulting chemical defects and potentially manipulate them in order to create a macroscopic dipole to induce a piezoelectric effect. However, it is made clear that this thesis is not an attempt at creating a new, nor modifying an existing, ferroelectric system.
This thesis will cover an introduction to piezoelectric and ferroelectric materials, due to the author’s inclusion within a piezoelectric research group, from which stemmed the idea for this research. A literature review into the defect chemistry of SrTiO3 was carried out, including the effects of non-stoichiometric SrTiO3, acceptor and donor doping, followed by a review of existing defect dipole research. Samples of Sr1-x/2Ti1-xNbxO3, Sr0.9Ti1-xNbxO3, SrTi1-xNbxO3, SrTi1-xMnxO3 and SrTi1-xCoxO3 were produced via the mixed oxide solid state reaction technique. Samples were characterised using X-ray diffraction and scanning electron microscopy. Following this electrical characterisation was carried out and impedance, electric modulus and permittivity data were examined in order to better understand the defect mechanisms taking place. Complex impedance analysis was used as an initial comparison of conductivity before deeper analysis of the frequency dependence of impedance and modulus data was conducted.
Detailed analysis of the impedance and modulus data showed that electronic conduction dominated the electrical behaviour of all Nb-doped SrTiO3 samples. The conductivity was much more electron-dominated in the B-site Nb-doped samples compared to the A-site vacancy samples. This resulted in a blue colouration of sample pellets, indicating the reduction of Ti4+ to Ti3+ and higher conductivities. The influence of Sr vacancies is also apparent from the lack of increase in conductivity in the Sr1-x/2 samples despite ten times the increase in Nb content. It was shown that the addition of large concentrations of A-site vacancies to the perovskite system enabled it structurally to accommodate large amounts of Nb, but it also significantly reduced the increase in conductivity seen by Nb doping. Equally, the large amount of Nb doping enabled the SrTiO3 system to accept large amounts of Sr deficiencies (up to 15%) by charge compensation. The vacancy/donor compensation scheme is shown to be key to maintaining the SrTiO3 perovskite structure by charge compensation, proven by the Sr0.9 sample set which began to eject TiO2 as a secondary phase as Nb dopant decreased.