46 research outputs found
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
Comprehensive determination of the high-pressure structural behaviour of BaTiO<sub>3</sub>
We have mapped the phase diagram of BaTiO3 more extensively than previous attempts using high-pressure neutron-powder diffraction. The mapping of the phase diagram has been performed using isothermal compression at fixed temperatures (175, 225, 290, 480 K) within each of the known crystallographic phases, up to âŒ6 GPa using a large volume press. The crystallographic structure of each phase has been measured, and the determined absolute atomic displacements of all atoms within the cell have provided detailed information on the order of the phase transitions observed, and the behaviour of the ferroelectric dipole moment.Publisher PDFPeer reviewe
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Low-intermediate-temperature, high-pressure thermoelastic and crystallographic properties of thermoelectric clausthalite (PbSe-I)
The thermoelastic properties of the rock-salt structured thermoelectric lead selenide
(clausthalite, PbSe-I) have been determined using neutron powder diffraction techniques for
the temperature interval 10 - 500 K at ambient pressure, and 0 - 5.2 GPa at 298, and 150 K.
Within this temperature range, lead selenide can be described using the same selfconsistent
phenomenological model developed for the isostructural phases lead sulfide
(PbS) and lead telluride (PbTe) in which the cations and anions behave as independent
Debye oscillators (vibrational Debye temperatures of PbSe-I: Pb 111(1) K, Se 205(1) K).
Simultaneous fitting of the unit cell volume and isochoric heat capacity to a two-term Debye
internal energy function gives characteristic temperatures of 104(3) K and 219(5) K in
excellent agreement with the two vibrational Debye temperatures derived from fitting the
individual atomic displacement parameters. GrĂŒneisen constants for the two term fits are
1.79 and 2.28 for the lower and upper characteristic temperature respectively. The
calculated thermodynamic GrĂŒneisen parameter increases monotonically from 2.03 at 10 K,
to a maximum 2.22 at 100 K before decreasing back to 2.00 at 298 K and is broadly in
agreement with the average of the two GrĂŒneisen parameters associated with the two-term
internal energy function. Despite the simplicity of the model, the calculated phonon density of
states that is implicit within the two-term Debye model is found to show fair agreement with
the full and partial vibrational densities of states derived from density functional theory
(DFT). The bulk modulus and its pressure derivative at 298 K are 47.9(4) GPa and 5.4(2)
respectively by fitting the pressure dependence of the unit cell volume to a 3rd order Birch-
Murnaghan equation-of-state. For lower temperatures (T < 300 K) the high-pressure
transition to PbSe-II is associated with a steep initial Clapeyron slope of 151 K GPa-1
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Structure and spectroscopy of methionyl-methionine for aquaculture
The amino acid L-methionine is an essential amino acid and is commonly used as a feed supplement in terrestrial animals. It is less suitable for marine organisms because it is readily excreted. It is also highly water soluble and this results in loss of the feed and eutrophication of the water. To address these problems, the dipeptide DL-methionyl-DL-methionine (trade name: AQUAVI Met-Met) has been developed as a dedicated methionine source for aquaculture. The commercial product is a mixture of a racemic crystal form of D-methionyl-D-methionine/L-methionyl-L-methionine and a racemic crystal form of D-methionyl-L-methionine/L-methionyl-D-methionine. In this work, we have computationally, structurally, spectroscopically and by electron microscopy characterised these materials. The microscopy and spectroscopy demonstrate that there is no interaction between the DDâLL and DLâLD racemates on any length scale from the macroscopic to the nanoscal
Recovering local structure information from highâpressure total scattering experiments
High pressure is a powerful thermodynamic tool for exploring the structure and the phase behaviour of the crystalline state, and is now widely used in conventional crystallographic measurements. Highâpressure local structure measurements using neutron diffraction have, thus far, been limited by the presence of a strongly scattering, perdeuterated, pressureâtransmitting medium (PTM), the signal from which contaminates the resulting pair distribution functions (PDFs). Here, a method is reported for subtracting the pairwise correlations of the commonly used 4:1 methanol:ethanol PTM from neutron PDFs obtained under hydrostatic compression. The method applies a molecularâdynamicsâinformed empirical correction and a nonânegative matrix factorization algorithm to recover the PDF of the pure sample. Proof of principle is demonstrated, producing corrected highâpressure PDFs of simple crystalline materials, Ni and MgO, and benchmarking these against simulated data from the average structure. Finally, the first local structure determination of αâquartz under hydrostatic pressure is presented, extracting compression behaviour of the realâspace structure