A complementary study of perovskites : combining diffraction, solid-state NMR and first principles DFT calculations
Abstract
Perovskites, ABX₃, and their associated solid-solutions are a
particularly important and attractive area of research within materials
chemistry. Owing to their structural and compositional flexibility and
potential physical properties they are one of the largest classes of materials
currently under investigation. This thesis is concerned with the synthesis
and structural characterisation of several perovskite-based materials using
a combined approach of high-resolution synchrotron X-ray and neutron
powder diffraction (NPD), solid-state Nuclear Magnetic Resonance (NMR)
and first-principles Density Functional Theory (DFT) calculations.
Initial investigations concentrated on room temperature NaNbO₃, a
perovskite widely debated in the literatue. Published crystallographic
data indicate NaNbO₃ possesses two crystallographically distinct Na sites
in space group Pbcm. Whilst some of our materials appear in agreement
with this (notably a commercially purchased sample) many of our
laboratory-synthesised samples of NaNbO₃ routinely comprise of two
phases, which we show to be the antiferroelectric Pbcm and polar P2₁ma
polymorphs. Several different synthetic methods were utilised during this
investigation and the quantity of each phase present was found to vary as
a function of preparative method. ²³Na,
⁹³Nb and ¹⁷O DFT calculations
were used in conjunction with experiment to aid in spectral analysis,
assignment and interpretation. In addition,
ab initio
random structure
searching (AIRSS) was utilised in an attempt to predict the most stable
phases of NaNbO₃. This proved to be both successful and highly
informative.
A series of NaNbO₃-related solid-solutions, namely K[subscript(x)]Na[subscript(1-x)]NbO₃
(KNN), Li[subscript(x)]Na[subscript(1-x)]NbO₃ (LNN) and Na[subscript(1-x)]Sr[subscript(x/2)]□[subscript(x/2)]NbO₃
(SNN) have also been
synthesised and characterised. The substitution of K⁺ , Li⁺ and Sr²⁺ cations
onto the A site appears to produce the same polar P2₁ma phase initially
identified in the room temperature NaNbO₃ investigation. The abrupt
change in cation size in the KNN and LNN series, and the introduction of
vacancies in the SNN series, is thought to result in a structural distortion
which, in turn, causes the formation of the P2₁ma phase.
A low temperature synchrotron X-ray powder diffraction study (12
< T < 295 K) was completed for a sample of NaNbO₁ composed of the
P2₁ma polymorph (~90%) and a small quantity of the Pbcm phase (~10%).
A region of phase coexistence was identified between the P2₁ma, R3c and
Pbcm phases over a relatively large temperature range. Full conversion of
the P2₁ma phase to the low temperature R3c phase was not possible and,
consistently, the P2₁ma phase was the most abundant phase present.
Factors such as structural, strain, crystallite size and morphology are
thought to be crucial in determining the exact phases of NaNbO₃
produced, both at low and room temperature.
The solid-solution La[subscript(1-x)]Y[subscript(x)]ScO₃ was also investigated. Compositions
x = 0, 0.2, 0.4, 0.6, 0.8 and 1 were successfully synthesised and
characterised. Refined high-resolution NPD data indicates that an
orthorhombic structure, in space group Pbnm, was retained throughout
the solid-solution. Using ⁴⁵Sc and ⁸⁹Y MAS NMR each sample was found
to exhibit disorder, believed to result from both a distribution of
quadrupole and chemical shifts. NMR parameters were calculated for
several model Sc and Y compounds using DFT methods to determine the
feasibility and accuracy of ⁴⁵Sc and ⁸⁹Y DFT calculations. These proved
successful and subsequent calculations were completed for the end
members LaScO₃ and YScO₃. DFT calculations were also utilised to gain
insight into the disorder exhibited in the La[subscript(1-x)]Y[subscript(x)]ScO₃ solid-solution.
Type
Thesis, PhD Doctor of Philosophy
Collections
Items in the St Andrews Research Repository are protected by copyright, with all rights reserved, unless otherwise indicated.