37 research outputs found
Manipulation of the crystalline phase diagram of hydrogen through nanoscale confinement effects in porous carbons
Possible coupling between magnons and phonons in multiferroic CaMn7O12
Spin and lattice dynamics of CaMn7O12 ceramics were investigated using
infrared, THz and inelastic neutron scattering (INS) spectroscopies in the
temperature range 2 to 590 K, and, at low temperatures, in applied magnetic
fields of up to 12 T. On cooling, we observed phonon splitting accompanying the
structural phase transition at Tc = 450K as well as the onset of the
incommensurately modulated structure at 250 K. In the two antiferromagnetic
phases below T_N1 = 90K and T_N2 = 48 K, several infrared-active excitations
emerge in the meV range; their frequencies correspond to the maxima in the
magnon density of states obtained by INS. At the magnetic phase transitions,
these modes display strong anomalies and for some of them, a transfer of
dielectric strength from the higher-frequency phonons is observed. We propose
that these modes are electromagnons. Remarkably, at least two of these modes
remain active also in the paramagnetic phase; for this reason, we call them
paraelectromagnons. In accordance with this observation, quasielastic neutron
scattering revealed short-range magnetic correlations persisting within
temperatures up to 500K above T_N1
Effect of Pore Geometry on Ultra-Densified Hydrogen in Microporous Carbons
This is the final version. Available on open access from Elsevier via the DOI in this recordOur investigations into molecular hydrogen (H2) confined in microporous carbons with
different pore geometries at 77 K have provided detailed information on effects of pore shape on
densification of confined H2 at pressures up to 15 MPa. We selected three materials: a disordered,
phenolic resin-based activated carbon, a graphitic carbon with slit-shaped pores (titanium carbidederived carbon), and single-walled carbon nanotubes, all with comparable pore sizes of < 1 nm.
We show via a combination of in situ inelastic neutron scattering studies, high-pressure H2
adsorption measurements, and molecular modelling that both slit-shaped and cylindrical pores
with a diameter of ~0.7 nm lead to significant H2 densification compared to bulk hydrogen under
the same conditions, with only subtle differences in hydrogen packing (and hence density) due to
geometric constraints. While pore geometry may play some part in influencing the diffusion
kinetics and packing arrangement of hydrogen molecules in pores, pore size remains the critical
factor determining hydrogen storage capacities. This confirmation of the effects of pore geometry
and pore size on the confinement of molecules is essential in understanding and guiding the
development and scale-up of porous adsorbents that are tailored for maximising H2 storage
capacities, in particular for sustainable energy applications.Engineering and Physical Sciences Research Council (EPSRC
Modifying the properties of 4f single-ion magnets by peripheral ligand functionalisation
We study the ligand-field splittings and magnetic properties of three Er-III single-ion magnets which differ in the peripheral ligand sphere but exhibit similar first coordination spheres by inelastic neutron scattering (INS) and SQUID magnetometry. The INS spectra of the three compounds are profoundly different pointing at a strong response of the magnetic behavior tominor structural changes, as they are e. g. encountered when depositing molecules on surfaces. The observation of several magnetic excitations within the J = 15/2 ground multiplet together with single-crystal magnetic measurements allows for the extraction of the sign and magnitude of all symmetry-allowed Stevens parameters. The parameter values and the energy spectrum derived from INS are compared to the results of state-of-the-art ab initio CASSCF calculations. Temperature-dependent alternating current (ac) susceptibility measurements suggest that the magnetisation relaxation in the investigated temperature range of 1.9 K < T < 5 K is dominated by quantum tunnelling of magnetisation and two-phonon Raman processes. The possibility of observing electron paramagnetic resonance transitions between the ground-state doublet states, which can be suppressed in perfectly axial single-ion magnets, renders the studied systems interesting as representations of quantum bits