Properties of interfaces in uniaxial ferroelectrics

Abstract

Interfaces in ferroelectrics commonly display distinct emergent phenomena driven by the material's spontaneous electric polarisation. In a uniaxial system, two general categories of interfaces can be considered: external crystal boundaries and internal domain walls.

External crystal boundaries are interfaces between the crystal and the surrounding medium. In the first part of this thesis, surface potentials associated with the pyroelectric response of uniaxial ferroelectrics were investigated with High-Voltage Kelvin Probe Force Microscopy (HV-KPFM). A new method for measuring the pyroelectric coefficients of monodomain uniaxial ferroelectrics was developed, with the calculated pyroelectric coefficients for lithium niobate and lithium tantalate agreeing with literature values. HV-KPFM was also used to explore the spatial variation of surface potentials across differently polarised domain states in polydomain systems, as well as the temperature dependence of the pyroelectric coefficient in triglycine sulphate.

Internal domain walls are interfaces within ferroelectrics that separate regions with distinct polarisation directions. These two-dimensional functional materials can exhibit emergent conductivity when supporting polar discontinuities; however, novel functional properties associated with the introduction of a lattice twist are forbidden. In the second part of this thesis, an established technique was used to thermocompressively bond two lithium niobate single crystals by their polar faces. This allowed the formation of a strong interfacial polar discontinuity and the introduction of a lattice twist across the "artificial" domain wall. The strong polar discontinuities at the atomically sharp interfaces were found to be associated with the emergence of interfacial conductivity. However, at some specific twist angles associated with short-range aperiodicity, a dramatic change in microstructure occurred, driven by a twist-induced collapse in the interfacial conductivity.

Finally, the interfacial polar textures in thermomechanically bonded lithium niobate were explored with DPC-STEM. Cross-sectional and plan-view imaging of interfaces was conducted, revealing a dramatic reorientation of the polarisation associated with a polar skyrmion lattice.

Thesis is embargoed until 31 July 2027.

Date of Award	Jul 2026
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Marty Gregg (Supervisor) & Amit Kumar (Supervisor)