L Vocadlo's research while affiliated with University College London and other places

Publications (97)

Article
The regime governing the growth of Mercury's core is unknown, but the dynamics of core growth are vital to understanding the origin and properties of the planet's weak magnetic field. Here, we use advanced first-principles methods, which include a magnetic entropy contribution, to investigate the magnetic and thermo-elastic properties of liquid Fe-...
Article
Geophysical and cosmochemical constraints suggest the inner-core is mainly composed of iron with a few percent of light elements. However, despite extensive studies over many years, no single alloying light-element has been found that is able to simultaneously match the observed inner-core density and both seismic velocities. This has motivated a n...
Article
We have measured the thermal expansion of (Fe1-y Ni y )Si for y = 0, 0.1 and 0.2, between 40 and 1273 K. Above ~700 K the unit-cell volumes of the samples decrease approximately linearly with increasing Ni content. Below ~200 K the unit-cell volume of FeSi falls to a value between that of (Fe0.9Ni0.1)Si and (Fe0.8Ni0.2)Si. We attribute this extra c...
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The Fe–Ni–Si system is potentially a very important component of terrestrial planetary cores. However, at present, even the behaviour of the FeSi and NiSi end members is poorly understood, especially at low to moderate pressures—the data for FeSi are contradictory and NiSi has been little studied. For FeSi, there is general agreement that there is...
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It has recently been reported, on the basis of extrapolated experimental data, that the iron carbide, Fe7C3, has shear wave velocities and a Poisson's ratio consistent with the seismological values for the Earth's inner core, and thus that Fe7C3 is a strong candidate for the inner core composition. In this study, using ab initio molecular dynamics...
Chapter
This chapter looks at the ways in which mineral physics can help in our understanding of the structure, composition, and evolution of Earth's core. The only direct observations of Earth's core are from seismology, so it is the job of mineral physics to find compositional models that exactly match the seismological data. However, the conditions of p...
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Neutron powder diffraction data have been collected from Na2SO4·10D2O (the deuterated analogue of mirabilite), a highly hydrated sulfate salt that is thought to be a candidate rock-forming mineral in some icy satellites of the outer solar system. These measurements, made using the OSIRIS instrument on the ISIS neutron spallation source, covered the...
Article
Understanding the physical properties of the Earth's core is a key step in the study of the evolution and dynamics of our planet. For much of the last century, based on studies of meteorites [1], it was believed that Earth's core was predominantly a mixture of iron and nickel. More specifically, the Earth's inner core is a solid Fe-Ni alloy at high...
Article
The hexagonal close-packed structure (hcp) is the accepted stable form of pure iron (Fe) under Earth's core conditions. It has been proposed, however, that especially when alloyed with lighter elements, the body-centred (bcc) and face-centred (fcc) cubic structures cannot be excluded from the inner core [1]. At relatively modest conditions, experim...
Article
Ab initio finite temperature molecular dynamics simulations have been used to calculate the elastic constants of hexagonal-close-packed (hcp) Fe as a function of temperature at ~300 GPa. The longitudinal modulus C11 decreases with temperature, in stark contrast to previous calculations, but in agreement with experimental observations on other trans...
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The results are described of a series of neutron powder diffraction experiments over the pressure and temperature ranges 0 < P < 2 GPa, 150 < T < 240 K, which were carried out with the objective of determining the phase behaviour and thermoelastic properties of perdeuterated ammonia dihydrate (ND3·2D2O). In addition to the low-pressure cubic crysta...
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We have collected high resolution neutron powder diffraction patterns from Na2SO4·10D2O over the temperature range 4.2–300 K following rapid quenching in liquid nitrogen, and over a series of slow warming and cooling cycles. The crystal is monoclinic, space-group P21/c (Z = 4) with a = 11.44214(4) Å, b = 10.34276(4) Å, c = 12.75486(6) Å, β = 107.84...
Article
Ammonia remains amongst the most plausible planetary "antifreeze" agents, and its physical properties in hydrate compounds under the appropriate conditions (roughly 0 - 5 GPa, 100 - 300 K) must be known in order for it to be accommodated in planetary models. The pressure melting curve, and the expected polymorphism of the stoichiometric ammonia hyd...
Article
The ability to perform neutron diffraction studies at simultaneous high pressures and high temperatures is a relatively recent development. The suitability of this technique for determining P-V-T equations of state has been investigated by measuring the lattice parameters of Mg1-xFexO ( x = 0.2, 0.3, 0.4), in the range P < 10.3 GPa and 300 < T < 98...
Article
We model the thermal evolution of a subsurface ocean of aqueous ammonium sulfate inside Titan using a parameterized convection scheme. The cooling and crystallization of such an ocean depends on its heat flux balance, and is governed by the pressure-dependent melting temperatures at the top and bottom of the ocean. Using recent observations and pre...
Article
We report results of the first neutron powder diffraction study of sulfuric acid tetrahydrate (SAT); D(2)SO(4)4D(2)O is tetragonal, space group P42(1)c, with two formula units per unit cell. At 1.7 K the unit-cell dimensions are a=b=7.475 12(6) A, c=6.324 66(5) A and V=353.405(5) A(3). At 225 K the unit-cell dimensions are a=b=7.4833(1) A, c=6.4103...
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We have simulated the evolution of gypsum and ice diapiric bodies within the martian crust in order to determine the likely spatial scale of feature produced at the surface, and the timescale for ascent of the diapirs to the surface.
Article
This chapter looks at the ways in which mineral physics can help in our understanding of the structure, composition, and evolution of the Earth's core. The only direct observations of the Earth's core are from seismology, so it is the job of mineral physics to find compositional models which exactly match the seismological data. However, the condit...
Article
Using a suite of techniques and equipment, we conduct several different types of experiments on planetary ices at UCL. Samples are prepared in the Ice Physics Laboratory, which consists of a 5 chamber complex of inter-connected cold rooms, controllable from +30 to -30 deg C. Within this laboratory we have a functioning triaxial deformation cell ope...
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Detailed neutron powder diffraction measurements have been carried out on two polymorphs of anhydrous magnesium sulfate, α-MgSO4 and β-MgSO4. α-MgSO4 is orthorhombic, space group Cmcm (Z = 4); at 4.2 K the unit-cell dimensions are a = 5.16863 (3), b = 7.86781 (5), c = 6.46674 (5) Å, V = 262.975 (2) Å3 [ρcalc = 3040.16 (2) kg m−3], and at 300 K, a =...
Article
Detailed neutron powder diffraction measurements have been carried out on two polymorphs of anhydrous magnesium sulfate, alpha-MgSO4 and beta-MgSO4. alpha-MgSO4 is orthorhombic, space group Cmcm (Z = 4); at 4.2 K the unit-cell dimensions are a = 5.16863 (3), b = 7.86781 (5), c = 6.46674 (5) angstrom, V = 262.975 (2) angstrom(3) [rho(calc) = 3040.16...
Article
The presence of an ocean inside Titan has yet to be confirmed, but is not unexpected. If gravity data do confirm a subsurface ocean, then what might its composition be? A recent model [1] predicts that it will be made of aqueous ammonium sulfate solution (AS), rather than an ammonia-water (AW) mixture [e.g. 2], formed by the leaching of sulfates fr...
Article
We model the chemical evolution of Titan, wherein primordial NH3 reacts with sulfate-rich brines leached from the silicate core during its hydration. The resulting differentiated body consists of a serpentinite core overlain by a high-pressure ice VI mantle, a liquid layer of aqueous ammonium sulfate, and a heterogeneous shell of methane clathrate,...
Article
The presence of an ocean inside Titan has yet to be confirmed, but is not unexpected. If gravity data do confirm a subsurface ocean, then what might its composition be? A recent model [1] predicts that it will be made of aqueous ammonium sulfate solution (AS), rather than an ammonia-water (AW) mixture [e.g. 2], formed by the leaching of sulfates fr...
Article
Planetary bodies are essentially crystalline aggregates. Underlying the structure and evolution of all such bodies is the microscopic behaviour of the component crystals - their equilibrium structures, elasticity, and transport properties, for example. Determining the magnitudes of these properties for the different 'ices' and for the highly hydrat...
Article
Primordial ammonia accreted into Titan is unlikely to remain chemically stable in aqueous solution, most likely reacting with sulfate brines leached out of the core. Pockets of ammonium sulfate brine trapped in a methane- clathrate crust as Titan formed are a likely source for cryomagmas which are apparently erupted to the surface. In rising to the...
Article
Hydrated sulfate salts are important evaporite minerals on the Earth, and on Mars too, as well as being a candidate rock-forming mineral inside the icy moons of the outer solar system. In order to be able to model the interiors of these icy moons, we are carrying out a program of investigation of epsomite and related hydrates at high pressure. Prev...
Article
Planetary bodies are essentially crystalline aggregates. The structure and evolution of all such bodies are thus fundamentally due to the microscopic behaviour of the component crystals - equilibrium structures, elasticity, and transport properties for example. Such properties can be calculated with high precision from quantum mechanical first prin...
Article
The presence of salts such as MgSO4 and Na2SO4 in chondritic meteorites has led to the suggestion that the water-rich icy moons of the Gas Giant planets are likely to be dominated by multiply hydrated salts such as Fritzsche's salt (MgSO4.11H2O), epsomite (MgSO4.7H2O) and mirabilite (Na2SO4.10H2O). Moreover, hydrated sulfates are likely to be impor...
Article
Although density functional theory (DFT) calculations have been widely used in many areas of the geosciences for the last 15 years, arguably the most successful application of these methods has been when they are used to understand the properties of minerals and melts in the Earth's interior. This is simply because the temperatures and pressures of...
Article
Atomic-scale materials modelling based on first-principles quantum mechanics is playing an important role in the science of the Earth and the other planets. We outline the basic theory of this kind of modelling and explain how it can be applied in a variety of different ways to probe the thermodynamics, structure and transport properties of both so...
Article
The melting temperature, T m , of copper has been determined from ambient pressure to 16 GPa using multi-anvil techniques. The melting curve obtained ( T m =1355(5)+44.5(31) P −0.61(21) P 2 , with T m in Kelvin and P in GPa) is in good agreement with both the previous experimental studies and with recent ab initio calculations.
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1] Using first-principles molecular dynamics (MD) simulations, we study the solubility of argon in molten iron at high pressures and temperatures. In particular we explore whether the low pressure immiscibility of liquid Fe and Ar persists to high pressure (130 GPa) and temperature (4500K), or whether they mix. Starting from a variety of Fe/ Ar mix...
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The oxygen balance of primordial mars: oxygen fugacity of selected SNC meteorites, sub-surface H2O inventory, the martian Fe3+/Fe2+ ratio and the implications for biogenic influences.
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We describe the polymorphism of epsomite from a high-pressure powder neutron diffraction study, and relate these findings to the interiors of icy moons and to impact metamorphism.
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We present the results of simulations of magnesium, sodium and ammonium sulfates and their hydrates, for comparison with experimental studies, and incorporation into planetary models.
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Using high-resolution neutron powder diffraction, the molar volume of a pure sample of D2O ice II has been measured, within its stability field, at 225 K, over the pressure range 0.25 < P < 0.45 GPa. Ar gas was used as the pressure medium, to avoid the formation of `stuffed ice' gas hydrates encountered when using He. The third-order Birch–Murnagha...
Article
It is well established that the inner core exhibits significant anisotropy with compressional P-wave velocities which are \(\sim 3\)~% faster along the polar axis than in the equatorial plane. Interpretation of this seismic anisotropy is hampered by lack of clear data on the physical properties of core phases at simultaneously high pressures and hi...
Article
An investigation of the relative stabilities and equations of state of possible Fe3S polymorphs was conducted using first-principles pseudopotential calculations. These calculations were based on density functional theory and performed using ultrasoft Vanderbilt pseudopotentials within the generalized gradient approximation. In accord with experime...
Article
A number of melting curves of various materials have recently been measured experimentally and calculated theoretically, but the agreement between different groups is not always good. We discuss here some of the problems which may arise in both experiments and theory. We also report the melting curves of Fe and Al calculated recently using quantum...
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35th Lunar and Planetary Science Conference, League City, Texas, 2004. Abstract #1252
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Ab initio calculations of the melting properties of copper in the pressure range 0-100 GPa are reported. The ab initio total energies and ionic forces of systems representing solid and liquid copper are calculated using the projector augmented wave implementation of density functional theory with the generalized gradient approximation for exchange-...
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35th Lunar and Planetary Science Conference, League City, Texas, 2004. Abstract #1254
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The Earth’s core is largely composed of iron (Fe), alloyed with less dense elements such as sulphur, silicon and/or oxygen. The phase relations and physical properties of both solid and liquid Fe-alloys are therefore of great geophysical importance. As a result, over the past fifty years the properties of Fe and its alloys have been extensively stu...
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We have measured the thermal expansivity of ammonia dihydrate (ND3.2D2O) phase I from 4.2 to 174 K at ambient pressure, and the incompressibility at 174 K from 0 to 0.45 GPa, using time-of-flight neutron powder diffraction. The unit cell volume as a function of temperature, V(T), was fitted with a Grüneisen approximation to the zero-pressure equati...
Article
The Earth’s core is largely composed of iron (Fe). The phase relations and physical properties of both solid and liquid Fe are therefore of great geophysical importance. As a result, over the past 50 years the properties of Fe have been extensively studied experimentally. However, achieving the extreme pressures (up to 360 GPa) and temperatures (∼6...
Article
We have carried out ab initio simulations on the high-pressure polymorph of solid water, ice II, a phase for which there is a surprising lack of experimental data. We report our calculated third-order Birch–Murnaghan equation of state for ice II: the zero pressure and temperature density, ρ0 = 1240.27±0.62 kg m−3, bulk modulus, K0 = 16.18±0.12 GPa,...
Article
The fact that the core is largely composed of iron (Fe) was firmly established as a result of Birch's analysis of materials-density/sound-wave velocity systematics. Today we believe that the outer core is about 6 to 10% less dense than pure liquid Fe, while the solid inner core is a few percent less dense than pure Fe. From cosmochemical and other...
Article
We have carried out ab initio simulations on the ambient pressure phase I of solid ammonia, and on the high-pressure phase IV. Our plane-wave pseudopotential calculations yield very good agreement with existing structural data, lattice energies, and equations of state. We have also studied the tendency toward symmetrization of the hydrogen bonds at...
Article
We present the first ab initio simulations of the low-pressure phase of ammonia dihydrate (NH3 · 2H2O), ADH I, a likely constituent of many volatile-rich solid bodies in the outer Solar System (e.g., Saturn’s moons). Ordered monoclinic (space group P21) and orthorhombic (space group P212121) variants of the experimentally observed cubic cell (space...
Article
We have measured the compressibility of CsCl-structured FeSi to 40 GPa by X-ray powder diffraction using synchrotron radiation. A third order Birch-Murnaghan equation of state fit to the experimental data yields Vo = 21.74 ± 0.02 Å, with a Ko of 184 ± 5 GPa and K1o = 4.2 ± 0.3. The measured density and elastic properties of CsCl-FeSi are consistent...
Article
In his 1952 paper, Birch observed that the interior of the Earth is a problem at once fascinating and baffling. This is especially true of the Earth's inner core, and in the fifty years since Birch's work, many subsequent papers have been published with the aim of constraining its nature and composition. Birch concluded the inner core is most simpl...