Pamela S. Whitfield

Lattice dynamics and the nature of structural transitions in organolead halide perovskites

Organolead halide perovskites are a family of hybrid organic-inorganic compounds whose remark- able optoelectronic properties have been under intensive scrutiny in recent years. Here we use inelastic X-ray scattering to study low-energy lattice excitations in single crystals of methylammonium lead iodide and bromide perovskites. Our ndings conrm the displacive nature of the cubic-to- tetragonal phase transition, which is further shown, using neutron and x-ray diraction, to be close to a tricritical point. The experimental sound speed, around 100-200 m/s, suggests that electron- phonon scattering is likely a limiting factor for further improvements in carrier mobility. Lastly, we detect quasistatic symmetry-breaking nanodomains persisting well into the high-temperature cubic phase, possibly stabilized by local defects. These ndings reveal key structural properties of these materials, but also bear important implications for carrier dynamics across an extended temperature range relevant for photovoltaic applications.

Structural Competition and Reactivity of Rare-Earth Oxide Phases in YxPr2–xO3 (0.05 ≤ x ≤ 0.80)

We report, for the first time, members of the Y xPr2- xO3 system with non-bixbyite or defect fluorite structures. The synthesis, structure, phase transitions, and high temperature reactivity of the trigonal A-type and monoclinic B-type structures are reported along with those of the cubic C-type phase (bixbyite). Combined powder X-ray and neutron diffraction Rietveld refinements are used to report structural details of all three reported phases. Phase transitions are investigated, showing a clear dependence on average cation size. Using neutron diffraction, phase transitions are followed in situ, revealing that all high temperature phases are quenchable. In-situ powder X-ray diffraction experiments in flowing oxygen allow insights into mechanistic details of redox processes in the reported phases. In contrast to the C-type cubic bixbyite, the trigonal A-type and monoclinic B-type structures do not allow for topotactic oxygen uptake, displaying instead a phase transition to either the bixbyite C-type capable of accommodating additional oxide anions or the direct oxidation to the cubic defect fluorite structure. The findings reported here agree with the accepted lanthanide sesquioxide phase diagrams and provide exceptional control of phases. The work is important for the prediction of structures, and the synthetic control needed for rational design of functional materials.

Structure Evolution and Reactivity of the Sc(2–x)VxO3+δ (0 ≤ x ≤ 2.0) System

Solid oxide fuel cells (SOFCs) are solid-state electrochemical devices that directly convert chemical energy of fuels into electricity with high efficiency. Because of their fuel flexibility, low emissions, high conversion efficiency, no moving parts, and quiet operation, they are considered as a promising energy conversion technology for low carbon future needs. Solid-state oxide and proton conducting electrolytes play a crucial role in improving the performance and market acceptability of SOFCs. Defect fluorite phases are some of the most promising fast oxide ion conductors for use as electrolytes in SOFCs. We report the synthesis, structure, phase diagram, and high-temperature reactivity of the Sc(2- x)V xO3+δ (0 ≤ x ≤ 2.00) oxide defect model system. For all Sc(2- x)V xO3.0 phases with x ≤ 1.08 phase-pure bixbyite-type structures are found, whereas for x ≥ 1.68 phase-pure corundum structures are reported, with a miscibility gap found for 1.08 < x < 1.68. Structural details obtained from the simultaneous Rietveld refinements using powder neutron and X-ray diffraction data are reported for the bixbyite phases, demonstrating a slight V3+ preference toward the 8b site. In situ X-ray diffraction experiments were used to explore the oxidation of the Sc(2- x)V xO3.0 phases. In all cases ScVO4 was found as a final product, accompanied by Sc2O3 for x < 1.0 and V2O5 when x > 1.0; however, the oxidative pathway varied greatly throughout the series. Comments are made on different synthesis strategies, including the effect on crystallinity, reaction times, rate-limiting steps, and reaction pathways. This work provides insight into the mechanisms of solid-state reactions and strategic guidelines for targeted materials synthesis.

Quantitative phase analysis of challenging samples using neutron powder diffraction. Sample #4 from the CPD QPA round robin revisited

Quantitative phase analysis (QPA) using neutron powder diffraction more often than not involves non-ambient studies where no sample preparation is possible. The larger samples and penetration of neutrons versus X-rays makes neutron diffraction less susceptible to inhomogeneity and large grain sizes, but most well-characterized QPA standard samples do not have these characteristics. Sample #4 from the International Union of Crystallography Commission on Powder Diffraction QPA round robin was one such sample. Data were collected using the POWGEN time-of-flight (TOF) neutron powder diffractometer and analysed together with historical data from the C2 diffractometer at Chalk River. The presence of magnetic reflections from Fe 3 O 4 (magnetite) in the sample was an additional consideration, and given the frequency at which iron-containing and other magnetic compounds are present during in-operando studies their possible impact on the accuracy of QPA is of interest. Additionally, scattering from thermal diffuse scattering in the high- Q region ( d -spacing reflections by magnetic contributions, but the constant-wavelength data were adversely impacted if the magnetic structure was not included. A robust refinement weighting was found to be effective in reducing quantification errors using the constant-wavelength neutron data both where intensities from magnetic reflections were ignored and included. Results from the TOF data were very sensitive to inadequate modelling of the high- Q (low d -spacing) background using simple polynomials.

Asymmetric band flipping for time-of-flight neutron diffraction data

Charge flipping with powder diffraction data is known to produce a result more reliably with high-resolution data,i.e.visible reflections at smalldspacings. This data are readily accessible with the neutron time-of-flight technique but the assumption that negative scattering density is nonphysical is no longer valid where elements with negative scattering lengths are present. The concept of band flipping was introduced in the literature, where a negative threshold is used in addition to a positive threshold during the flipping. But, it was not tested with experimental data at the time. Finallly, band flipping has been implemented inTOPAStogether with the band modification of low-density elimination and tested with experimental powder and Laue single-crystal neutron data.

Synthesis of a Ferrolite: A Zeolitic All-Iron Framework

Crystals of the first sodalite-type zeolite containing an all-iron framework, a ferrolite, Ba8 (Fe12 O24 )Nay (OH)6 ⋅x H2 O, were synthesized using the hydroflux method in nearly quantitative yield. Ba8 (Fe12 O24 )Nay (OH)6 ⋅x H2 O crystallizes in the cubic space group Pm3‾m with a=10.0476(1) Å. Slightly distorted FeO4 tetrahedra are linked to form Fe4 O4 and Fe6 O6 rings, which in turn yield channels and internal cavities that are characteristic of the sodalite structure. Barium, sodium, and hydroxide ions and water molecules are found in the channels and provide charge balance. Magnetic measurements indicate that the ferrolite exhibits magnetic order up to at least 700 K, with the field-cooled and zero-field-cooled curves diverging. Analysis of the 57 Fe Mössbauer spectra revealed two spectral components that have equal spectral areas, indicating the presence of two subsets of iron centers in the structure. Dehydrated versions of the ferrolite were also prepared by heating the sample.

Diffraction studies from minerals to organics: lessons learned from materials analyses

In many regards the study of materials and minerals by powder diffraction techniques are complimentary, with techniques honed in one field equally applicable to the other. As a long-time materials researcher many of the examples are of techniques developed for materials analysis applied to minerals. However in a couple of cases the study of new minerals was the initiation into techniques later used in materials-based studies. Hopefully they will show that the study of new minerals structures can provide opportunities to add new methodologies and approaches to future problems. In keeping with the AXAA many of the examples have an Australian connection, the materials ranging from organics to battery materials.