Characterisation of competing orders in dielectric oxides

• Main Author: Giles-Donovan, Nathan James
• Format: Dissertation
• Language: English
• Published: University of Glasgow 2021
• DOI: 10.5525/gla.thesis.82685

Dielectric oxides exhibit many intriguing properties. For example, ferroic materials are central to developing transducers due to their ability to convert energy owing to the presence of coupled orders. In particular, lead-based perovskite relaxor-ferroelectric single crystal have attracted great attention in recent years, with exceptional piezoelectric and dielectric responses reported. Suitable for high-power industrial and underwater SONAR ultrasonic applications due to their high energy density, the performance of these materials has been linked to compositional disorder and short-range order but the mechanism is not yet well-understood. In general, coupling and competition of different orders can result in thought provoking physics and, in this work, this link was investigated by studying the fundamental behaviour of complex ferroelectrics and multiferroics. Polarised neutrons were used to characterise the magnetic ground state of Cu3Nb2O8, addressing an issue in the literature regarding the microscopic ordering mechanism. Furthermore, muon techniques were used to study the composition and magnetic structure of the relaxor-multiferroic Pb(Fe1/2Nb1/2)O3 which provided insight into the role of disorder and random fields. This work was then extended to study the Mn distribution and valence in doped Pb(In1/2Nb1/2)O3 - Pb(Mg1/3Nb2/3)O3 - PbTiO3. These materials are indicated to be amongst the highest performance piezoelectric but the microscopic mechanisms are not fully understood. This raised the question of best practice in material comparison, with unbiased comparison of transduction materials desired. To address this, a new method was developed to quantify the energy density of piezoelectric materials, which was verified in silico to be independent of a single use case or application. Overall, this work extends the understanding of three complex ferroelectric and multiferroic systems using fundamental characterisation methods with a foundation in applications.