We have examined the crystal structures and structural phase transitions of the deuterated, partially deuterated and hydrogenous organic-inorganic hybrid perovskite methyl ammonium lead iodide (MAPbI3) using time-of-flight neutron and synchrotron X-ray powder diffraction. Near 330 K the high temperature cubic phases transformed to a body-centered tetragonal phase. The variation of the order parameter Q for this transition scaled with temperature T as Q ∼ (Tc−T)β, where Tc is the critical temperature and the exponent β was close to ¼, as predicted for a tricritical phase transition. However, we also observed coexistence of the cubic and tetragonal phases over a range of temperature in all cases, demonstrating that the phase transition was in fact first-order, although still very close to tricritical. Upon cooling further, all the tetragonal phases transformed into a low temperature orthorhombic phase around 160 K, again via a first-order phase transition. Based upon these results, we discuss the impact of the structural phase transitions upon photovoltaic performance of MAPbI3 based solar cells.

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We present zero-field muon spin relaxation ͑SR͒ measurements of La 1.6Ϫx Nd 0.4 Sr x CuO 4 with xϭ0.125,0.15,0.2; La 1.475 Nd 0.4 Ba 0.125 CuO 4 , La 1.875 Ba 0.125 CuO 4 , and La 1.875 Ba 0.125Ϫy Sr y CuO 4 with y ϭ0.025,0.065. All of the samples with dopant concentrations xϩyр0.15 show similar static magnetic order with coherent precession of the muon spins below T N Ϸ30 K, with a T→0 ordered Cu moment Ϸ0.3 B . The samples with xϭ0.20 show no coherent precession but manifest two distinct relaxation regimes, typical of quasistatic magnetism. We then present transverse-field SR hysteresis measurements of the La 1.45 Nd 0.4 Sr 0.15 CuO 4 and La 1.4 Nd 0.4 Sr 0.2 CuO 4 systems that show a large superconducting response below approximately 7 K and 12 K, respectively. We argue that superconductivity and magnetic order coexist in the xϭ0.15 system.

The effect of nanoparticles (NP) on chain dimensions in polymer melts has been the source of considerable theoretical and experimental controversy. We exploit our ability to ensure a spatially uniform dispersion of 13 nm silica NPs miscible in polystyrene melts, together with neutron scattering, x-ray scattering, and transmission electron microscopy, to show that there is no measurable change in the polymer size in miscible mixtures, regardless of the relative sizes of the chains and the nanoparticles, and for NP loadings as high as 32.7 vol%. Our results provide a firm basis from which to understand the properties of polymer nanocomposites.

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Chain behavior has been determined in polymer nanocomposites (PNCs) comprised of well-dispersed 12 nm diameter silica nanoparticles (NPs) in poly­(methyl methacrylate) (PMMA) matrices by Small-Angle Neutron Scattering (SANS) measurements under the Zero Average Contrast (ZAC) condition. In particular, we directly characterize the bound polymer layer surrounding the NPs, revealing the bound layer profile. The SANS spectra in the high-q region also show no significant change in the bulk polymer radius of gyration on the addition of the NPs. We thus suggest that the bulk polymer conformation in PNCs should generally be determined using the high q region of SANS data.

We have studied the various aspects of the homogeneous acetoxylation of ethylene to vinyl acetate by palladium acetate using ultraviolet−visible (UV/vis), Raman, and infrared spectroscopic methods. Vibrational assignments for the Raman and the infrared spectra of crystalline palladium acetate have been made. Density functional theoretical calculations were used to help with our assignments. Raman spectroscopy demonstrated that Pd acetate solvated in glacial acetic acid maintains a trimeric structure, similar to that of the crystal. Upon addition of potassium acetate in concentrations between 0.10 and 0.20 M, the palladium acetate trimers decompose to dimers. It remains, however, an open question whether monomers, or perhaps dimers that are fully coordinated by acetate ligands, form with increasing potassium acetate concentration. In the presence of ethylene, dimeric species appear to form by decomposition of the trimers, and it is the dimers which are the catalytically active and selective species in the formation of vinyl acetate. During reaction with ethylene, palladium(II) in palladium acetate is reduced to palladium black, and the hydrocarbon products vinyl acetate and acetaldehyde are formed. Monomeric palladium acetate species are apparently not formed by this reaction.