The average and local structure of the oxides Ba2SiO4, BaAl2O4, SrAl2O4, and Y2SiO5 are examined to evaluate crystal rigidity in light of recent studies suggesting that highly connected and rigid structures yield the best phosphor hosts. Simultaneous momentum-space refinements of synchrotron X-ray and neutron scattering yield accurate average crystal structures, with reliable atomic displacement parameters. The Debye temperature ΘD, which has proven to be a useful proxy for structural rigidity, is extracted from the experimental atomic displacement parameters and compared with predictions from density functional theory calculations and experimental low-temperature heat capacity measurements. The role of static disorder on the measured displacement parameters, and the resulting Debye temperatures, are also analyzed using pair distribution function of total neutron scattering, as refined over varying distance ranges of the pair distribution function. The interplay between optimal bonding in the structure, structural rigidity, and correlated motion in these structures is examined, and the different contributions are delineated.

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Chemie CommunicationsPowerful neutron and high-energy X-ray diffraction methods have been used to elucidate the partial structure factors of liquid hydrogen fluoride. Structural details of the short, strongly hydrogen-bonded chains provide a rigorous test for models of this fundamentally important solvent. For more information see the Communication by J. F. C. Turner and co-workers on the following pages.

Short-range investigation by means of variable-temperature neutron total scattering and pair distribution function analysis revealed that the local environment around the methylammonium (MA) cation in MAPbBr hybrid perovskite is maintained through the different phase transitions observed as a function of temperature. In addition, the orthorhombic distortion of the lattice is present at any temperature. Local structure around MA changes from static to configurationally averaged or dynamic with temperature but the local structure of the low-temperature orthorhombic phase is preserved.

Spallation neutron diffraction and high-energy x-ray diffraction methods have been used to study CaO:Al2O3  glasses at the 64:36 mol% eutectic and 50:50 mol% compositions. The samples were produced by the containerless cooling of liquid droplets heated by a laser beam and suspended in an aerodynamic levitator. The results show aluminium on average to be surrounded by 4.0(1) oxygen atoms at a distance of 1.76(1) Å in CaAl2O4, which increases to 4.8(1) at the eutectic composition. The two techniques have also been combined to reveal the local structure of the calcium atoms in the glass. In CaAl2O4  the calcium is found to be surrounded, on average, by 5.6(2) oxygen atoms at a distance of 2.38 Å. The Ca coordination decreases to the unusually low value of 3.9(2) oxygen atoms at a distance of 2.40 Å at the eutectic composition. No additional Ca–O correlations are observed up to 2.7 Å, but longer bonds cannot be ruled out. The higher-r correlations are shown to be similar in the two glasses, suggesting that both Al and Ca may act as network formers. The results are compared to previous studies on splat-quenched glasses with compositions near the eutectic.