Structure-property relationships in ferroelectric tungsten bronze oxides

Abstract

Tungsten bronze-type compounds have been extensively studied as ferroelectric materials. Due to their large degree of compositional flexibility, focus has historically been on the tetragonal (TTB) structure. The hexagonal (HTB) structure has been studied to a lesser extent but has been found to exhibit interesting electrical properties such as ferroelectricity and superconductivity. The work detailed in this thesis focuses on the structural and electrical characterisation of various tungsten bronze oxides.

Detailed structural analyses, utilising Rietveld refinements and symmetry mode analysis, shows that the HTB CsNbW₂O₉ is a new class of improper ferroelectric where combination of Γ₂⁻ (displacive) and K₃ (cell-tripling) modes result in a net polarisation. Three phase transitions occur in the polar phase, with Cmc2₁ adopted at ambient temperature and TC ~ 1100 K. Piezoresponse force microscopy (PFM) indicates the same topologically-protected 6-fold domain configuration as the improper h-RMnO₃ materials. However, unlike the manganites the improper mechanism in this HTB is due to electronic rather than geometric effects.

The room temperature structures of RbNbW₂O₉ and KNbW₂O₉ were found to be orthorhombic Cmc2₁ using neutron diffraction (PND). This is different to the published structures due to the sensitivity of PND to oxygen positions. Both compositions undergo successive phase transitions upon cooling from high-temperature centrosymmetric P6/mmm via second-order transitions driven by the octahedral tilting. Unlike the analogous Cs composition, both systems are shown to be proper ferroelectrics driven by a conventional Γ₂⁻ mode.

The solid solution Cs₁₋ₓRbₓNbW₂O₉ shows that the improper ferroelectric structure adopted at x = 0 is maintained at values of x ≤ 0.75; at higher x the proper ferroelectric structure of RbNbW₂O₉ is adopted. With decreasing A-site occupancy, peaks attributable to the improper ferroelectric structure appear broadened indicating the tripled cell is not uniformly adopted.

Total neutron scattering (PDF) studies of disorder in the ‘empty’ TTB Ba₄La₀.₆₇☐₁.₃₃Nb₁₀O₃₀ suggest this is attributable to oxygen atom displacements. This is supported by comparison of both Rietveld and PDF structures and observed variation of atomic displacements.

Date of Award	26 Jun 2019
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Finlay Morrison (Supervisor)