441 research outputs found
Improper Ferroelectricity: An Electronically Driven Improper Ferroelectric: Tungsten Bronzes as Microstructural Analogs for the Hexagonal Manganites (Adv. Mater. 40/2019)
No full textThe rapidly emerging field of domain wall nanoelectronics exploits the different functionality of ferroelectric domain walls relative to the bulk. In article number 1903620, Finlay D. Morrison and co-workers present a new improper ferroelectric with the hexagonal tungsten bronze structure. This material displays the same complex labyrinthine domain structure, including six-fold “cloverleaf” domain vertices, as the widely studied hexagonal manganites. These domain structures have generated much interest due to their rich functionality associated with a high density of charged domain walls
Developmental and environmental effects on the assembly of glutenin polymers and the impact on grain quality of wheat
No full textWheat kernel development can be divided into three phases i.e. cell division, cell enlargement and dehydration. Accumulation of gluten proteins continues till the end of the cell enlargement phase. During the dehydration phase, post-translational polymerization of the glutenin subunits occurs to form very large glutenin polymers. Assembly of the glutenin polymers has been monitored by increase in the unextractable polymeric protein. Lines possessing HMW-GS related to dough strength (e.g. 5+10) started accumulating large polymers several days earlier than lines with HMW-GS related to dough weakness (e.g. 2+12) and maintained their higher amounts till maturity. This may be explained by faster polymerization resulting from a higher concentration of cysteine residues in the x-type HMW-GS.
Sulphur deficiency leads to an increase in the ratio of HMW- to LMW-GS, causing a shift in the MWD to higher MWs, resulting in bucky dough properties. High temperature during grain development appears to shift the MWD to lower MWs with corresponding lowering of dough strength but the presence of strength-associated HMW-GS appears to confer greater tolerance to heat stress. Since sulphur deficiency and higher global temperatures may be expected to increase in the future, some suggestions how breeders may use strategies to counter these effects are put forward
Learning disabilities : barriers to choice in residential services.
No full textA recent study has revealed the extent of the obstacles to choice and control in residential settings for people with learning disabilities. In the first part of this article the first two authors highlight the key messages for practice. In the second part of the article the Social Care Institute for Excellence provides an overview of other research and resources on residential services and learning disabilities
Vacancy ordering and polytypism in B-site deficient halide perovskites
No full textB-site deficient halide perovskites show promising optoelectronic properties as potential candidates for the replacement for conventional lead-based halide perovskites. To further understand the relationship between crystal structure and properties, it would be of great interest to explore B-site deficient halide perovskites with various vacancy distributions. This work focuses on the preparation and characterisation of novel B-site deficient halide perovskites with unusual vacancy ordering.
The successful preparation of Cs₄CuSb₂Cl₁₂ in 2017 provided insight into the study of cubic closed packed B-site deficient halide perovskites. Theoretical screening and experimental work were carried out in this study to explore other potential A₄B’²⁺B³⁺₂X₁₂ halide perovskites. One novel compound Cs₄MnBi₂Cl₁₂ was synthesised and analysed, with typical d-d transition induced photoluminescence (PL) at 602 nm activated by octahedral coordinated 3d⁵ Mn²⁺.
By manipulating the fraction of B-site vacancies, another novel B-site deficient halide perovskite Cs₁₀MnSb₆Cl₃₀ with unusual one-dimensional 10H (hcccc)₂ structure was synthesised and studied. A similar d-d transition induced PL was observed at 620 nm. However, an enhancement of the quantum yield in comparison with previously reported Cs₄MnSb₂Cl₁₂ suggested that this is due to the different vacancy ordering and reduced dimensionality of octahedral connectivity. Preliminary DFT calculations of the electronic structure of Cs₁₀MnSb₆Cl₃₀ suggested an indirect band gap of 2.69 eV for this compound.
By using azetidinium (Az⁺) as the A-site cation, one novel B-site deficient perovskite polytype with a 6H (hcc)₂ layered structure was observed in Az₃Sb₂Cl₉ and Az₃Sb₂Br₉. The unique vacancy distribution formed an intrinsically polar structure, which was further confirmed by the converse piezoelectric effect. Variable temperature structural analysis and property measurements revealed several phase transitions from the hexagonal prototypical structure to an orthorhombic superstructure between 300 K and 20 K for both compounds. A tuneable band gap and preferential site occupation of chlorine and bromine anions were also observed in the solid-solution Az₃Sb₂Cl₉₋ₓBrₓ.
The other azetidinium based halides were preliminarily investigated. Az₃Sb₂l₉, Az₃Bi₂Br₉ and Az₃Bi₂I₉ showed typical zero-dimensional 6H (hcc)₂ perovskite structure with isolated octahedral dimers; whereas Az₂BiCl₅ and Az₂BiBr₅ were found to adopt non-perovskite cis-connected chain structures
Synthesis and structure-property relationships in selected metal fluorides
No full textThere has been an increase in the interest in fluoride materials over the
last decade. This interest has focused on multiferroic materials and kagome
lattices, to name but a few areas. This thesis focuses on the synthesis and
crystallographic characterisation of selected transition metal fluorides and
oxyfluorides. Work is presented on the tetragonal tungsten bronze solid
solutions of K[subscript(x)]FeF₃, where x = 0.58 and x ≈ 0.5, and the copper analogue,
K₃Cu₃Fe₂F₁₅; the kagome structure of Cs₂ZrCu₃F₁₂; and hydrothermal reactions
using vanadium, manganese, or molybdenum as the transition metals in the
formation of new fluorides and oxyfluorides.
The tetragonal tungsten bronze compounds K[subscript(x)]FeF₃ (x = 0.58 and x ≈ 0.5)
are both tetragonal at 500 K. In the variant with the lower K-content, there is a
clear phase separation into two tetragonal phases even at this temperature. The
K₀.₅₈FeF₃ sample separates into two distinct phases below 340 K to possess one
tetragonal and one orthorhombic phase. Then at roughly 300 K, both samples
undergo a phase transition where the tetragonal phase in the P4/mbm space
group in K₀.₅₈FeF₃ changes to an orthorhombic phase with a larger unit cell; and
the tetragonal phase in P4₂bc for the K₀.₅FeF₃ sample changes to the same
orthorhombic model, whilst the P4/mbm model remains unchanged. The
evolution of the lattice parameters and phase fractions is studied in detail using
synchrotron powder X-ray diffraction (sPXRD).
The kagome structure investigated, Cs₂ZrCu₃F₁₂, possesses the “ideal”
kagome lattice at room temperature, but previous work has suggested that
there is a phase transition at 225 K. The two structures are determined by
single crystal X-ray diffraction at 300 K and 125 K. Variable temperature
sPXRD studies are performed between these two temperature ranges to
determine the phase evolution as a function of temperature. The structure
changes from a rhombohedral to a monoclinic phase at low temperature. This
is the result of the buckling of the kagome layers at the phase transition. The
Zr⁴⁺ ion changes from 6 to 7 coordinate and this is seen as the main driving
force for the distortion of the kagome layer from its “ideal” planar arrangement.
ii
The phase transition is first-order as seen from the electrical impedance
measurements.
The hydrothermal reactions presented reveal seven new materials and
their crystal structures. Sr₂V₂F₁₀·H₂O is new and found to be isostructural to
Sr₂Fe₂F₁₀·H₂O. BaVO₂F₃ is a cubic material that is potentially piezoelectric. Two
hybrid organic inorganic manganese compounds are reported. The ladder
structure (C₃N₂H₅)[Mn₂F₆(H₂O)₂] crystallises in a polar space group and shows
promise as a candidate for multiferroic studies. The second hybrid material,
(C₇NH₁₆)₂[MnF₅(H₂O)]·2H₂O, crystallises in a centrosymmetric space group.
The Mo hybrid materials are all centrosymmetric and possess isolated
molybdenum-centred monomeric or dimeric octahedral units
Synthesis and structure-property relationships in rare earth doped bismuth ferrite
No full textThere has been significant interest in BiFeO₃ over the past decade. This interest has focused on the magnetic and electrical properties, which in the long term may prove useful in device applications.
This thesis focuses on the synthesis, electrical characterisation, and structural origin of the electrical properties of rare earth doped bismuth ferrite. Two systems have been studied: BiFeO₃ doped with lanthanum and neodymium (Bi₁₋ₓREₓFeO₃ RE= La, Nd). Specific examples have been highlighted focusing on a detailed structural analysis of a lanthanum doped bismuth ferrite, Bi₀.₅La₀.₅FeO₃, and a neodymium analogue, Bi₀.₇Nd₀.₃FeO₃. Both adopt an orthorhombic GdFeO₃-type structure (space group: Pnma) with G-type antiferromagnetism.
Structural variations were investigated by Rietveld refinement of temperature dependent powder neutron diffraction using a combination of both conventional “bond angle/bond length” and symmetry-mode analysis. The latter was particularly useful as it allowed the effects of A-site displacements and octahedral tilts/distortions to be considered separately. This in-depth structural analysis was complemented with ac-immittance spectroscopy using the multi-formulism approach of combined impedance and modulus data to correlate structural changes with the bulk electrical properties. This approach was essential due to the complex nature of the electrical response with contributions from different electroactive regions.
The structural variations occur due to a changing balance between magnetic properties and other bonding contributions in the respective systems. This results in changes in the magnitude of the octahedral tilts, and A-site displacements giving rise to phenomena such as negative thermal expansion and invariant lattice parameters i.e., the invar effect.
More specifically, analysis of Bi₀.₅La₀.₅FeO₃ highlights a structural link between changes in the relative dielectric permittivity and changes in the FeO₆ octahedral tilt magnitudes, accompanied by a structural distortion of the octahedra with corresponding A-site displacement along the c-axis; this behaviour is unusual due to an increasing in-phase tilt mode with increasing temperature. The anomalous orthorhombic distortion is driven by magnetostriction at the onset of antiferromagnetic ordering resulting in an Invar effect along the magnetic c-axis and anisotropic displacement of the A-site Bi³⁺ and La³⁺ along the a-axis. This contrasts with the neodymium analogue Bi₀.₇Nd₀.₃FeO₃ in which a combination of increasing A-site displacements in the ac-plane and decrease in both in-phase and anti-phase tilts combine with superexchange giving rise to negative thermal expansion at low temperature. The A-site displacements correlate with the orthorhombic strain.
By carefully changing the synthesis conditions, a significant change in bulk conductivity was observed for a number for Bi₁₋ₓLaₓFeO₃ compositions. A series of Bi₀.₆La0.₄FeO₃ samples are discussed, where changes in the second step of the synthesis result in significantly different bulk conductivities. This behaviour is also observed in other compositions e.g. Bi₀.₇₅La₀.₂₅FeO₃. Changes in the electrical behaviour as a function of temperature are discussed in terms of phase composition and concentration gradients of defects. Activation energies associated with the conduction process(es) in Bi₁₋ₓLaₓFeO₃ samples, regardless of composition, fall within one of two broad regimes, circa. 0.5 eV or 1.0 eV, associated with polaron hopping or migration of charge via oxygen vacancies, respectively.
The use of symmetry-mode analysis, in combination with conventional crystallographic analysis and electrical analysis using multi-formulism approach, presents a new paradigm for investigation of structure-property relationships in rare earth doped BiFeO₃
Vogel-Fulcher analysis of relaxor dielectrics with the tetragonal tungsten bronze structure : Ba6MNb9O30 (M = Ga, Sc, In)
No full textIn-depth analysis of the relaxor behaviour of Ba6MNb9O30 (M= Ga, Sc, In) tetragonal tungsten bronze (TTB) ceramics was carried out. Powder x-ray diffraction and scanning electron microscopy were performed in order to confirm the formation of desired phases and to determine the microstructure. Low-temperature dielectric spectroscopy was used in order to characterise the dielectric properties of these materials; the degree of relaxor behaviour were investigated in relation with the increase of ionic radius of the M-cation on the B-site of the TTB structure. The dynamics of dielectric relaxation of dipoles was studied by fitting the dielectric permittivity data to the Vogel-Fulcher (VF) model in order to monitor the reproducibility and validity of the physical results. Restrictions to the VF fit were attempted besides the regular “free-fit” by constraining some of the fundamental relaxation parameters to physically sensible values. We show that Vogel-Fulcher fits are very sensitive to the fitting range resulting in a large range of fundamental parameters for the dielectric relaxation processes, and that the restriction of the frequency domain due to experimental noise or to instrumentation limits has a dramatic influence on the values obtained.Peer reviewe
Exploring azetidinium as the A-site in organic-inorganic hybrid perovskites
No full textOrganic inorganic hybrid perovskites (OIHPs) have appealing optoelectronic properties.
As sample preparation is relatively straightforward, there are opportunities to investigate
novel compositions and structures. This thesis concerns the synthesis, structure and
optoelectrical properties of OIHPs with azetidinium as an A-site cation.
Azetidinium [(CH₂)₃NH₂⁺, Az] is a four-member ring ammonium, and its size is
calculated to be promising as an A-cation for OIHPs. A stable 6H hexagonal perovskite
AzPbBr₃ was synthesised and analysed, and its bandgap was determined to be 2.81 eV.
On cooling, AzPbBr₃ undergoes a symmetry lowering distortion which was identified by
variable temperature PXRD and dielectric spectroscopy. An anisotropic change in lattice
parameters on cooling marked a phase transition likely driven by the Pb⋯Pb repulsion in
the face sharing octahedra.
Compositional and structural analyses were performed on precipitation synthesised and
mechanosynthesised OIHPs Az₁₋ₓFAₓPbBr₃ and Az₁₋ₓMAₓPbBr₃ (0 ≤ x ≤ 1). For samples
obtained from precipitation synthesis, the actual FA% or MA% in the precipitate was
found to be less than the nominal composition in the reaction solution. No such mismatch
was found for mechanosynthesised samples. PXRD indicated partial solid solution
formation for Az-rich and MA- or FA-rich compositions, separated by an intermediate
two-phase region. The result suggests the extent of the solid solution of halide perovskites
is dependent only on the average A-cation size; the size mismatch is less of an influence.
This is in contrast to solid solution formation observed in oxide perovskites.
A tuneable bandgap was achieved ranging from 2.00 eV (AzPbI₃) to 3.41 eV (AzPbCl₃)
for the mixed halide perovskite AzPbBr₃₋ₓXₓ (X = Cl or I, 0 ≤ x ≤ 3). The structural
analyses revealed a complete 6H solid solution for AzPbBr₃₋ₓClₓ in comparison to the
structural progression from 6H, 4H to 9R polytypes, when varying the halide composition
from Br (x = 0) to I (x = 3) in AzPbBr₃₋ₓIₓ. A linear variation in unit cell volume as a
function of anion average radius was observed not only within the solid solution of each
polytype (following Vegard’s law) but continuously across all three polytypes.
Preliminary results on the synthesis and structural analysis indicate that Az₂PbBr₄ adopts
the = 1 Ruddlesden-Popper structure while azetidinium bismuth bromide has a 1D chain
structure. Detailed structural and optical analysis are planned in future projects."My PhD Studentship was supported by the Chinese Scholarship Council and the
University of St Andrews (CSC No. 201603780020)." -- Fundin
Synthesis and characterisation of the geometric ferroelectric LaTaO₄
No full textLaTaO₄ is one of very few reported n=2 Carpy-Galy oxides. It is isostructural to the BaMF₄ fluorides which were subject of much prior study due to their ferroelectric and multiferroic properties. Previous studies of LaTaO₄ have been consistent in their conclusion that it can adopt either a low temperature non-polar monoclinic, or high temperature polar orthorhombic phase, with the monoclinic – orthorhombic transition temperature (Tₘ₋ₒ) being dependent on the preparation method. Recent studies, however, have reported an unidentified dielectric anomaly in the orthorhombic phase which alludes to an additional subtle structural transition above Tₘ₋ₒ.
Powder neutron (PND) and electron diffraction studies have revealed that the previously unidentified dielectric anomaly denotes a transition between an incommensurately modulated and unmodulated orthorhombic (Cmc2₁) structure at ~503 K. The phase transition sequence is re-investigated in light of this new modulated phase, with resonant ultrasound (RUS) and Raman spectroscopy being utilised to elucidate the dynamics of transition and show the incommensurate – commensurate transition to be unusually 1ˢᵗ order, but nearly continuous (tri-critical).
Refinement of PND data in the Cmc2₁(α00)0s0 superspace group, indicates a modulation wavevector q = (0.456 0 0) at 483 K. The modulation arises from a variation in La³⁺ positions and octahedral tilts which propagate along the polar a-axis and is speculated to arise as a mechanism to improve overall bonding environment of La³⁺. Electron microscopy reveals that the modulation arises from partially ordered sub-units, which are 2.5× and 3× expansions of the original orthorhombic cell. A model has been constructed which combines the local structural origin with the observed macroscopic modulation, where sub-units partially order with an average periodicity of ~11a (~0.456 a*).
Previous studies report a relationship between the stability of each phase of LaTaO₄ and A-cation size. To explore the possibility that the modulation arises purely by an A-cation size effect, the series La₁₋ₓLnₓTaO₄ is synthesized, where Ln = Ce³⁺, Pr³⁺ and Nd³⁺ (0 ≤ x ≤ 0.3). Doping with these isovalent cations shows a general trend of stabilisation of the low temperature phases with decreasing average A-cation radii; the effect is amplified in Ce³⁺-doped samples and preliminary analysis suggests that this is due to a mix of oxidation states and the associated inter- and de-intercalation of interstitial oxygens during thermal cycling
- …
