X-ray and neutron scattering

Abstract:

We report inelastic neutron scattering results that reveal an hour-glass magnetic excitation spectrum in La1.75Sr0.25CoO 4. The magnetic spectrum is similar to that observed previously in La1.67Sr0.33CoO4, but the spectral features are broader. We show that the spectrum of La1.75Sr 0.25CoO4 can be modelled by the spin dynamics of a system with a disordered cluster spin glass ground state. Bulk magnetization measurements are presented that support the proposed glassy ground state. The observations reiterate the importance of quasi-one-dimensional magnetic correlations and disorder for the hour-glass spectrum, and suggest that disordered spin and charge stripes exist at lower doping in La 2-xSrxCoO4 than previously thought. © 2013 American Physical Society.

Abstract:

The zeolitic imidazolate framework ZIF-4 undergoes an amorphization transition at about 600 K, and then transforms at about 700 K to ZIF-zni, the densest of the crystalline ZIFs. This series of long-range structural rearrangements must give a corresponding series of changes in the local structure, but these have not previously been directly investigated. Through analysis of neutron total diffraction data by reverse Monte Carlo modelling, we assess the changes in flexibility across this series, identifying the key modes of flexibility within ZIF-4 and the amorphous phase. We show that the ZnN 4 tetrahedra remain relatively rigid, albeit less so than SiO 4 tetrahedra in silicates. However, the extra degrees of freedom afforded by the imidazolate ligand, compared to silicate networks, vary substantially between phases, with a twisting motion out of the plane of the ligand being particularly important in the amorphous phase. Our results further demonstrate the feasibility of reverse Monte Carlo simulations for studying intermolecular interactions in solids, even in cases, such as the ZIFs, where the pair distribution function is dominated by intramolecular peaks. © 2013 IOP Publishing Ltd.

Abstract:

Neutron time-of-flight spectroscopy has been employed to study the crystal-field splitting of Pr3+ in the pyrochlore stannate Pr 2Sn2O7. The crystal field has been parameterized from a profile fit to the observed neutron spectrum. The single-ion ground state is a well-isolated non-Kramers doublet of Γ3+ symmetry with a large Ising-like anisotropy, χzz/ χ⊥≈60 at 10 K, but with a significant admixture of terms |MJ≠±J, which can give rise to quantum zero-point fluctuations. This magnetic state satisfies the requirements for quantum spin-ice behavior. © 2013 American Physical Society.