1,636 research outputs found
Extent of stacking disorder in diamond
Hexagonal diamond has been predicted computationally to display extraordinary
physical properties including a hardness that exceeds cubic diamond. However, a
recent electron microscopy study has shown that so-called hexagonal diamond
samples are in fact not discrete materials but faulted and twinned cubic
diamond. We now provide a quantitative analysis of cubic and hexagonal stacking
in diamond samples by analysing X-ray diffraction data with the DIFFaX software
package. The highest fractions of hexagonal stacking we find in materials which
were previously referred to as hexagonal diamond are below 60%. The remainder
of the stacking sequences are cubic. We show that the cubic and hexagonal
sequences are interlaced in a complex way and that naturally occurring
Lonsdaleite is not a simple phase mixture of cubic and hexagonal diamond.
Instead, it is structurally best described as stacking disordered diamond. The
future experimental challenge will be to prepare diamond samples beyond 60%
hexagonality and towards the so far elusive 'perfect' hexagonal diamond
Hydrogen mean force and anharmonicity in polycrystalline and amorphous ice
The hydrogen mean force from experimental neutron Compton profiles is derived
using deep inelastic neutron scattering on amorphous and polycrystalline ice.
The formalism of mean force is extended to probe its sensitivity to
anharmonicity in the hydrogen-nucleus effective potential. The shape of the
mean force for amorphous and polycrystalline ice is primarily determined by the
anisotropy of the underlying quasi-harmonic effective potential. The data from
amorphous ice show an additional curvature reflecting the more pronounced
anharmonicity of the effective potential with respect to that of ice Ih.Comment: 12 pages, 7 figures, original researc
Benchmarking acid and base dopants with respect to enabling the ice V to XIII and ice VI to XV hydrogen-ordering phase transitions
Doping the hydrogen-disordered phases of ice V, VI and XII with hydrochloric
acid (HCl) has led to the discovery of their hydrogen-ordered counterparts ices
XIII, XV and XIV. Yet, the mechanistic details of the hydrogen-ordering phase
transitions are still not fully understood. This includes in particular the
role of the acid dopant and the defect dynamics that it creates within the
ices. Here we investigate the effects of several acid and base dopants on the
hydrogen ordering of ices V and VI with calorimetry and X-ray diffraction. HCl
is found to be most effective for both phases which is attributed to a
favourable combination of high solubility and strong acid properties which
create mobile H3O+ defects that enable the hydrogen-ordering processes.
Hydrofluoric acid (HF) is the second most effective dopant highlighting that
the acid strengths of HCl and HF are much more similar in ice than they are in
liquid water. Surprisingly, hydrobromic acid doping facilitates hydrogen
ordering in ice VI whereas only a very small effect is observed for ice V.
Conversely, lithium hydroxide (LiOH) doping achieves a performance comparable
to HF-doping in ice V but it is ineffective in the case of ice VI. Sodium
hydroxide, potassium hydroxide (as previously shown) and perchloric acid doping
are ineffective for both phases. These findings highlight the need for future
computational studies but also raise the question why LiOH-doping achieves
hydrogen-ordering of ice V whereas potassium hydroxide doping is most effective
for the 'ordinary' ice Ih.Comment: 18 pages, 7 figures, 1 tabl
Ammonium Fluoride as a Hydrogen-disordering Agent for Ice
The removal of residual hydrogen disorder from various phases of ice with
acid or base dopants at low temperatures has been a focus of intense research
for many decades. As an antipode to these efforts, we now show using neutron
diffraction that ammonium fluoride (NH4F) is a hydrogen-disordering agent for
the hydrogen-ordered ice VIII. Cooling its hydrogen-disordered counterpart ice
VII doped with 2.5 mol% ND4F under pressure leads to a hydrogen-disordered ice
VIII with ~31% residual hydrogen disorder illustrating the long-range
hydrogen-disordering effect of ND4F. The doped ice VII could be supercooled by
~20 K with respect to the hydrogen-ordering temperature of pure ice VII after
which the hydrogen-ordering took place slowly over a ~60 K temperature window.
These findings demonstrate that ND4F-doping slows down the hydrogen-ordering
kinetics quite substantially. The partial hydrogen order of the doped sample is
consistent with the antiferroelectric ordering of pure ice VIII. Yet, we argue
that local ferroelectric domains must exist between ionic point defects of
opposite charge. In addition to the long-range effect of NH4F-doping on
hydrogen-ordered water structures, the design principle of using topological
charges should be applicable to a wide range of other 'ice-rule' systems
including spin ices and related polar materials.Comment: 23 pages, 4 figures, 2 table
Canyon Diablo lonsdaleite is a nanocomposite containing c/h stacking disordered diamond and diaphite
In 1967, a diamond polymorph was reported from hard, diamond-like grains of the Canyon Diablo iron meteorite and named lonsdaleite. This mineral was defined and identified by powder X-ray diffraction (XRD) features that were indexed with a hexagonal unit cell. Since 1967, several natural and synthetic diamond-like materials with XRD data matching lonsdaleite have been reported and the name lonsdaleite was used interchangeably with hexagonal diamond. Its hexagonal structure was speculated to lead to physical properties superior to cubic diamond, and as such has stimulated attempts to synthesize lonsdaleite. Despite numerous reports, several recent studies have provided alternative explanations for the XRD, transmission electron microscopy and Raman data used to identify lonsdaleite. Here, we show that lonsdaleite from the Canyon Diablo diamond-like grains are a nanocomposite material dominated by subnanometre-scale cubic/hexagonal stacking disordered diamond and diaphite domains. These nanostructured elements are intimately intergrown, giving rise to structural features erroneously associated with h diamond. Our data suggest that the diffuse scattering in XRD and the hexagonal features in transmission electron microscopy images reported from various natural and laboratory-prepared samples that were previously used for lonsdaleite identification, in fact arise from cubic/hexagonal stacking disordered diamond and diaphite domains. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'
Thermal desorption of Hâ‚‚O ice: from nanoscale films to the bulk
The desorption properties of H2O films are investigated across a wide range of film thicknesses from 53 nanometres (nm) to 101 micrometres (μm) using a quartz-crystal microbalance (QCM) and temperature-programmed desorption. Three desorption stages are observed belonging to amorphous solid water (ASW), stacking disordered ice I (ice Isd), and hexagonal ice I (ice Ih). The desorption of ASW is only detectable for the ≥10 μm films and is separated from the ice I desorption by 10–15 K with an associated desorption energy of ∼64 kJ mol−1. The desorption energy of the 53-nm film was found to be near 50 kJ mol−1 as also noted in the literature, but with increasing film thickness, the desorption energy of ice I rises, reaching a plateau around 65–70 kJ mol−1. The reason for the increased desorption energy is suggested to be due to molecules unable to desorb due to the thick covering layer of H2O and possibly re-adsorption events. Before complete desorption of ice I at around 220 K for the 101 μm film, a two-stage ice I desorption is observed with the QCM for the ≥10 μm films near 200 K. This event corresponds to the desorption of ice Isd as corroborated by X-ray diffraction patterns collected upon heating from 92 to 260 K at ambient pressure. Cubic ice is not observed as is commonly stated in the literature as resulting from the crystallization of ASW. Therefore, ice Isd is the correct terminology for the initial crystallization product of ASW
Local structure and orientational ordering in liquid bromoform
The neutron diffraction data of liquid bromoform (CHBr3) at 25°C was analysed using the Empirical Potential Structure Refinement technique in combination with H/D isotopic substitution. Compared to liquid chloroform (CHCl3), CHBr3 displays more spatially defined intermolecular contacts. A preference for polar stacking with collinear alignment of dipole moments is observed for the most closely approaching CHBr3 molecules, although to a lesser extent than in chloroform. Consistent with this, and in line with dielectric spectroscopy, the Kirkwood correlation factor from the structural model of CHBr3 is smaller than that of CHCl3. The net antiparallel alignment of dipole moments in CHBr3, as suggested by dielectric spectroscopy, must be due to weak but persistent long-range orientation correlations in CHBr3, which counteract the local polar stacking
Microfield distributions in strongly coupled two-component plasmas
The electric microfield distribution at charged particles is studied for
two-component electron-ion plasmas using molecular dynamics simulation and
theoretical models. The particles are treated within classical statistical
mechanics using an electron-ion Coulomb potential regularized at distances less
than the de Broglie length to take into account the quantum-diffraction
effects. The potential-of-mean-force (PMF) approximation is deduced from a
canonical ensemble formulation. The resulting probability density of the
electric microfield satisfies exactly the second-moment sum rule without the
use of adjustable parameters. The correlation functions between the charged
radiator and the plasma ions and electrons are calculated using molecular
dynamics simulations and the hypernetted-chain approximation for a
two-component plasma. It is shown that the agreement between the theoretical
models for the microfield distributions and the simulations is quite good in
general.Comment: 18 figures. Submitted to Phys. Rev.
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