Mark T. Weller

Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide

The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural behaviour, with successive transitions between orthorhombic, tetragonal and cubic polymorphs at ca. 165 K and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The equilibrium structures compare well to solutions of temperature-dependent powder neutron diffraction. By following the normal modes we calculate infrared and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degradation of the sample. Despite a clear separation in energy between low frequency modes associated with the inorganic PbI3 network and high-frequency modes of the organic CH3NH3+ cation, significant coupling between them is found, which emphasises the interplay between molecular orientation and the corner-sharing octahedral networks in the structural transformations. Soft modes are found at the boundary of the Brillouin zone of the cubic phase, consistent with displacive instabilities and anharmonicity involving tilting of the PbI6 octahedra around room temperature.

Cubic Perovskite Structure of Black Formamidinium Lead Iodide, α-[HC(NH2)2]PbI3, at 298 K

The structure of black formamidinium lead halide, α-[HC(NH2)2]PbI3, at 298 K has been refined from high resolution neutron powder diffraction data and found to adopt a cubic perovskite unit cell, a = 6.3620(8) Å. The trigonal planar [HC(NH2)2]+ cations lie in the central mirror plane of the unit cell with the formamidinium cations disordered over 12 possible sites arranged so that the C–H bond is directed into a cube face, whereas the −NH2 groups hydrogen bond (NH···I = 2.75–3.00 Å) with the iodide atoms of the [PbI3]− framework. High atomic displacement parameters for the formamidinium cation are consistent with rapid molecular rotations at room temperature as evidenced in ab initio molecular dynamic simulations.

Structure and oxygen stoichiometry in Sr3Fe2O7−y, 0 ≤ y ≤ 1.0

Abstract Powder neutron diffraction has been used to determine the structure of four compounds from the series Sr 3 Fe 2 O 7− y , y = 0, 0.25, 0.42, and 1.0. All these compounds crystallize with structures based on that of Sr 3 Ti 2 O 7 with oxygen systematically removed from a site linking the FeO 6 octahedra together. Complete removal of oxygen from this site produces an unusual, square-pyramidal coordination around each iron(III) ion and a stoichiometry of Sr 3 Fe 2 O 6 ; this compound is isostructural with La 2 SrCu 2 O 6 . A bond valence parameter, R 0 , for Fe 4+ has been derived as 1.788(2) A.

Structure and oxidation state relationships in ternary copper oxides

Powder neutron diffraction has been used to determine accurate oxygen positions in five simple ternary copper(II) oxides. SrCuO{sub 2}, Sr{sub 2}CuO{sub 3}, Bi{sub 2}CuO{sub 4}, Ca{sub 2}CuO{sub 3}, and Li{sub 2}CuO{sub 2}. These materials have structures based on corner- or edge-sharing CuO{sub 4} square planes with Cu-O distances between 1.907 and 2.04 {angstrom}. In KCuO{sub 2} the CVuO{sub 4} square planes are distorted with two short copper oxygen bonds of 1.81 {angstrom}. The structures of the more complex materials BaCuO{sub 2+x} (x = 0, 0.07) have also been studied using powder neutron diffraction techniques and the oxygen distribution in these materials was examined. Variations in copper(II) and copper(III) geometries are interpreted in terms of bond valence calculations which have been carried out for all the materials studied.

Temperature- and Pressure-induced Proton Transfer in the 1:1 Adduct Formed between Squaric Acid and 4,4 '-Bipyridine

We have applied a combination of spectroscopic and diffraction methods to study the adduct formed between squaric acid and bypridine, which has been postulated to exhibit proton transfer associated with a single-crystal to single-crystal phase transition at ca. 450 K. A combination of X-ray single-crystal and very-high flux powder neutron diffraction data confirmed that a proton does transfer from the acid to the base in the high-temperature form. Powder X-ray diffraction measurements demonstrated that the transition was reversible but that a significant kinetic energy barrier must be overcome to revert to the original structure. Computational modeling is consistent with these results. Modeling also revealed that, while the proton transfer event would be strongly discouraged in the gas phase, it occurs in the solid state due to the increase in charge state of the molecular ions and their arrangement inside the lattice. The color change is attributed to a narrowing of the squaric acid to bipyridine charge-transfer energy gap. Finally, evidence for the possible existence of two further phases at high pressure is also presented.

Experimental and theoretical optical properties of methylammonium lead halide perovskites

The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3](+) cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS).

Structure and magnetic properties of Sr2FeO4 and Sr3Fe2O7 studied by powder neutron diffraction and Mössbauer spectroscopy

Sr2FeO4 and Sr3Fe2O7 have been studied using low-temperature powder neutron diffraction, magnetometry and 57Fe Mossbauer spectroscopy. Sr2FeO4 orders antiferromagnetically at 60 K whilst the Neel temperature of Sr3Fe2O7 is 110 K. At room temperature the Mossbauer spectrum of Sr3Fe2O7 is interpreted in terms of a disproportionation, Fe4+↔ Fe3++ Fe5+, though neutron diffraction measurements show a single, crystallographic iron site. Neutron diffraction data collected below the Neel temperatures from Sr2FeO4(4 K) and Sr3Fe2O7(6 and 90 K) only show broad weak magnetic reflections indicative of antiferromagnetic ordering in two dimensions. Mossbauer data collected below TN from Sr2FeO4 indicate the presence of at least four magnetically different Fe4+ sites.

Hydrothermal synthesis of zeolites

Recent developments have included the synthesis of new, large pore zeolite structures, zeotypes and metal substituted zeolites, along with the in situ investigation of zeolite synthesis and control of morphologies. In terms of new zeolite structures there is increasing realisation of the aim to specifically design templates to control zeolite structures, and the use of the fluoride ion as a mediating agent has been influential. In terms of zeotypes considerable effort has been directed at transition metal containing systems, particularly those of cobalt, with the potential to generate redox active catalytic centres. Growth of zeolite films on substrates has also been developed and progress is being made in studying directly, using diffraction and NMR techniques, the crystallisation of zeolites.

Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites

We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations. Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm-1. Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamic effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity. We show that optical rather than acoustic phonon scattering is likely to prevail at room temperature in these materials.

Superconductivity and localisation in the Bi2Sr2Ca1−xRExCu2O8+d(RE = Nd, Sm, Gd and Dy) system

Abstract A systematic study has been made of the effect on the structure, electrical conductivity and hole concentration of Bi 2 Sr 2 CaCu 2 O 8+ d resulting from the substitution of rare earths for Ca. The area of the basal plane increased and the c -axis decreased with an increase of rare earth concentration. T c remained at 73 K for x x ⪕0.6. For x ⪖0.6 the resistivity appears to correspond to variable range hopping. The effective Cu-O charge has been determined chemically and the resulting variation of T c with the number of holes per sub cell is in agreement with the behaviour of other systems in which holes occur in Cu-O planes. The smooth transition from superconducting to localised nature is believed to follow from a decrease of the in-plane Cu-O hybridization.