Michael K. Crawford

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.

Molecular spin-orbit excitations in theJeff=12frustrated spinel GeCo2O4

We describe powder and single-crystal inelastic neutron scattering experiments on a spinel-type antiferromagnet GeCo

A Raman Study of Fluorinated Ethanes Adsorbed on Zeolite NaX

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Compositions for microlithography

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Antireflective layer for use in microlithography

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Bases and surfactants and their use in photoresist compositions for microlithography

, represented by an effective total angular momentum J_eff = 1/2. Several types of non-dispersive short-range magnetic excitations were discovered. The scattering intensity maps in

Do Very Small POSS Nanoparticles Perturb s-PMMA Chain Conformations?

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Isotopic and isomorphic substitution of YBa_{2}Cu_{3}O_{6} and YBa_{2}Cu_{3}O_{7}: Far-infrared spectroscopy and assignment of phonons

space are well reproduced by dynamical structure factor analyses using molecular model Hamiltonians. The results of analyses strongly suggest that the molecular excitations below T_N arise from a hidden molecular-singlet ground state, in which ferromagnetic subunits are antiferromagnetically coupled. The quasielastic excitations above T_N are interpreted as its precursor. A combination of frustration and J_eff = 1/2 might induce these quantum phenomena.

Molecular Spin Excitations in the Orbitally Frustrated Spinel GeCo2Od4

Due to the destructive effect of chlorofluorocarbons (CFCs) upon the ozone layer, the major global producers of CFCs have agreed to cease production by 1996. As a result of this agreement there is a need to find replacements for CFCs in a number of applications. Hydrofluorocarbons (HFCs) offer reasonably effective alternatives in uses such as refrigeration and air conditioning. One of the primary replacements for CFCs such as CFC-12 (CF2CI2) for use in automobile air conditioners is HFC 134a (CF3CFH2). HFC 134 (CF2HCF2H), an isomer of 134a, is also a potentially useful HFC. A number of routes in the syntheses of these HFCs involve the use of heterogeneous catalysts to improve the efficiency and specificity of the chemical reaction sequences. Since the HFCs interact with the catalyst surface, it is of general interest to understand the nature of these molecule-surface interactions. For this reason we have undertaken this study of the adsorption of HFCs 134, 134a, and 143 (CF2HCFH2) on the zeolite NaX (Na86Al86Si106O384). Zeolites have found wide application as heterogeneous catalysts and sorbs in many industrial chemical processes, and from our perspective they have the advantages of large surface areas and relatively weak Raman scattering. These features make zeolites ideal hosts for studies of molecule-surface interactions. Furthermore, NaX binds the HFC molecules nondissociatively at room temperature with high enough number densities to produce reasonably intense Raman bands.