X-ray and neutron scattering

Abstract:

Revealing the predominant driving force behind symmetry breaking in correlated materials is sometimes a formidable task due to the intertwined nature of different degrees of freedom. This is the case for La2−xSrxNiO4+δ, in which coupled incommensurate charge and spin stripes form at low temperatures. Here, we use resonant x-ray photon correlation spectroscopy to study the temporal stability and domain memory of the charge and spin stripes in La2−xSrxNiO4+δ. Although spin stripes are more spatially correlated, charge stripes maintain a better temporal stability against temperature change. More intriguingly, charge order shows robust domain memory with thermal cycling up to 250 K, far above the ordering temperature. These results demonstrate the pinning of charge stripes to the lattice and that charge condensation is the predominant factor in the formation of stripe orders in nickelates.

Abstract:

Spin-ices are frustrated magnets that support a particularly rich variety of emergent physics. Typically, it is the interplay of magnetic dipole interactions, spin anisotropy, and geometric frustration on the pyrochlore lattice that drives spin-ice formation. The relevant physics occurs at temperatures commensurate with the magnetic interaction strength, which for most systems is 1–5 K. Here, we show that non-magnetic cadmium cyanide, Cd(CN)2, exhibits analogous behaviour to magnetic spin-ices, but does so on a temperature scale that is nearly two orders of magnitude greater. The electric dipole moments of cyanide ions in Cd(CN)2 assume the role of magnetic pseudospins, with the difference in energy scale reflecting the increased strength of electric vs magnetic dipolar interactions. As a result, spin-ice physics influences the structural behaviour of Cd(CN)2 even at room temperature.

Abstract:

Defect engineering is a powerful tool that can be used to tailor the properties of metal–organic frameworks (MOFs). Here, we incorporate defects through ball milling to systematically vary the porosity of the giant pore MOF, MIL-100 (Fe). We show that milling leads to the breaking of metal–linker bonds, generating additional coordinatively unsaturated metal sites, and ultimately causes amorphisation. Pair distribution function analysis shows the hierarchical local structure is partially retained, even in the amorphised material. We find that solvents can be used to stabilise the MIL-100 (Fe) framework against collapse, which leads to a substantial retention of porosity over the non-stabilised material

Abstract:

This three-year Ph.D. research project is the result of the collaboration among the research group of Prof. Guido Ennas from University of Cagliari (Italy), expert in innovative and green synthesis approaches of micro- and nanomaterials, including mechanochemistry and sonochemistry, and the research group of Dr. Sarah Hudson from University of Limerick (Ireland), expert in biocatalysts and drug-delivery systems. The research focuses on the development of innovative synthesis routes for the preparation of proteins/enzymes – Metal Organic Frameworks (MOFs) hybrid composite materials for biomedical applications. In particular, alternative sonochemical and mechanochemical methods have been explored for the one-pot synthesis of glucose oxidase – iron(III) trimesate composites under eco- and bio-compatible conditions. Conventional harsh synthesis conditions have been overcome in order to minimise enzyme denaturation and activity loss, while retaining structural and textural features of the MOF. Beside their biocompatibility and low cost, one of the advantages of using iron(III) trimesate materials to immobilise biomolecules is their peroxidase-mimic behaviour. Indeed, such MOFs do not act as passive supports for the immobilisation of glucose oxidase, but also perform as enzyme-mimics, avoiding the immobilisation of additional peroxidase enzymes. Hybrid glucose oxidase – iron(III) trimesate composites prepared via mechanochemical and sonochemical approaches under mild conditions were applied as sensitive biosensors for glucose colorimetric detection