[Show abstract][Hide abstract] ABSTRACT: Selenium-rich Ge-Te-Se glasses have been synthesized along the GeSe4-GeTe4 pseudo-composition line and acquired by (77)Se Hahn echo magic-angle spinning NMR. The comparison with the GeSe4 spectrum shows a drastic modification of the typical double-resonance lineshape even at low Te concentrations (<10%). In order to rationalize this feature and to understand the effect of Te on the structure of our glasses, first-principles molecular dynamics simulations and gauge including projector augmented wave NMR parameter calculations have been performed. The distribution of the tellurium atoms in the selenium phase was shown to be mainly responsible for the (77)Se lineshape changes. Another possible factor related to the perturbation of the δiso value due to Te proximity appears to be much more limited in the bulk, while the results obtained using molecular models suggest shifts of several hundreds of ppm.
Physical Chemistry Chemical Physics 10/2015; DOI:10.1039/C5CP04416B · 4.49 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have used density-functional-theory methods and the ab initio random
structure searching (AIRSS) approach to predict stable structures and
stoichiometries of mixtures of iron and oxygen at high pressures. Searching was
performed for 12 different stoichiometries at pressures of 100, 350 and 500
GPa, which involved relaxing more than 32,000 structures. We find that
Fe$_2$O$_3$ and FeO$_2$ are the only phases stable to decomposition at 100 GPa,
while at 350 and 500 GPa several stoichiometries are found to be stable or very
nearly stable. We report a new structure of Fe$_2$O$_3$ with $P2_12_12_1$
symmetry which is found to be more stable than the known Rh$_2$O$_3$(II) phase
at pressures above $\sim$233 GPa. We also report two new structures of FeO,
with $Pnma$ and $R\bar{3}m$ symmetries, which are found to be stable within the
ranges 195-285 GPa and 285-500 GPa, respectively, and two new structures of
Fe$_3$O$_4$ with $Pca2_1$ and $P2_1/c$ symmetries, which are found to be stable
within the ranges 100-340 GPa and 340-500 GPa, respectively. Finally, we report
two new structures of Fe$_4$O$_5$ with $P4_2/n$ and $P\bar{3}m1$ symmetries,
which are found to be stable within the ranges 100-231 GPa and 231-500 GPa,
respectively. Our new structures of Fe$_3$O$_4$ and Fe$_4$O$_5$ are found to
have lower enthalpies than their known structures within their respective
stable pressure ranges.
[Show abstract][Hide abstract] ABSTRACT: Hydrogen sulfides have recently received a great deal of interest due to the
record high superconducting temperatures of up to 203 K observed on strong
compression of dihydrogen sulfide (H2S). A joint theoretical and experimental
study is presented in which decomposition products of compressed H2S are
characterized, and their superconducting properties are calculated. In addition
to the experimentally known H2S and H3S phases, our first-principles structure
searches have identified several energetically competitive stoichiometries that
have not been reported previously; H2S3, H3S2, and H4S3. In particular, H4S3 is
predicted to be thermodynamically stable within a large pressure range of
25-113 GPa. High-pressure room-temperature X-ray diffraction measurements
confirm the presence of an H4S3 phase that emerges at 27 GPa and coexists with
H3S and residual H2S, at least up to the highest pressure studied in our
experiments of 140 GPa. Electron-phonon coupling calculations show that H4S3
has a small Tc of below 2 K, and that H2S is mainly responsible for the
observed superconductivity for samples prepared at low temperature.
[Show abstract][Hide abstract] ABSTRACT: Despite relevance to disparate areas such as cloud microphysics and
tribology, major gaps in the understanding of the structures and phase
transitions of low-dimensional water ice remain. Here we report a first
principles study of confined 2D ice as a function of pressure. We find that at
ambient pressure hexagonal and pentagonal monolayer structures are the two
lowest enthalpy phases identified. Upon mild compression the pentagonal
structure becomes the most stable and persists up to \textit{ca.} 2 GPa at
which point square and rhombic phases are stable. The square phase agrees with
recent experimental observations of square ice confined within graphene sheets.
We also find a double layer AA stacked square ice phase, which clarifies the
difference between experimental observations and earlier force field
simulations. This work provides a fresh perspective on 2D confined ice,
highlighting the sensitivity of the structures observed to both the confining
pressure and width.
[Show abstract][Hide abstract] ABSTRACT: Establishing the phase diagram of hydrogen is a major challenge for experimental and theoretical physics. Experiment alone cannot establish the atomic structure of solid hydrogen at high pressure, because hydrogen scatters X-rays only weakly. Instead, our understanding of the atomic structure is largely based on density functional theory (DFT). By comparing Raman spectra for low-energy structures found in DFT searches with experimental spectra, candidate atomic structures have been identified for each experimentally observed phase. Unfortunately, DFT predicts a metallic structure to be energetically favoured at a broad range of pressures up to 400 GPa, where it is known experimentally that hydrogen is non-metallic. Here we show that more advanced theoretical methods (diffusion quantum Monte Carlo calculations) find the metallic structure to be uncompetitive, and predict a phase diagram in reasonable agreement with experiment. This greatly strengthens the claim that the candidate atomic structures accurately model the experimentally observed phases.
[Show abstract][Hide abstract] ABSTRACT: [Mo6X14](2-) octahedral molybdenum clusters are the main building blocks of a large range of materials. Although (95)Mo nuclear magnetic resonance was proposed to be a powerful tool to characterize their structural and dynamical properties in solution, these measurements have never been complemented by theoretical studies which can limit their interpretation for complex systems. In this Article, we use quantum chemical calculations to evaluate the (95)Mo chemical shift of three clusters: [Mo6Cl14](2-), [Mo6Br14](2-), and [Mo6I14](2-). In particular, we test various computational parameters influencing the quality of the results: size of the basis set, treatment of relativistic and solvent effects. Furthermore, to provide quantum chemical calculations that are directly comparable with experimental data, we evaluate for the first time the (95)Mo nuclear magnetic shielding of the experimental reference, namely, MoO4(2-) in aqueous solution. This is achieved by combining ab initio molecular dynamics simulations with a periodic approach to evaluate the (95)Mo nuclear shieldings. The results demonstrate that, despite the difficulty to obtain accurate (95)Mo chemical shifts, relative values for a cluster series can be fairly well-reproduced by DFT calculations. We also show that performing an explicit solvent treatment for the reference compound improves by ∼50 ppm the agreement between theory and experiment. Finally, the standard deviation of ∼70 ppm that we calculate for the (95)Mo nuclear shielding of the reference provides an estimation of the accuracy we can achieve for the calculation of the (95)Mo chemical shifts using a static approach. These results demonstrate the growing ability of quantum chemical calculations to complement and interpret complex experimental measurements.
[Show abstract][Hide abstract] ABSTRACT: Ordinary materials can transform into novel phases at extraordinary high pressure and temperature. The recently developed method of ultrashort laser-induced confined microexplosions initiates a non-equilibrium disordered plasma state. Ultra-high quenching rates overcome kinetic barriers to the formation of new metastable phases, which are preserved in the surrounding pristine crystal for subsequent exploitation. Here we demonstrate that confined microexplosions in silicon produce several metastable end phases. Comparison with an ab initio random structure search reveals six energetically competitive potential phases, four tetragonal and two monoclinic structures. We show the presence of bt8 and st12, which have been predicted theoretically previously, but have not been observed in nature or in laboratory experiments. In addition, the presence of the as yet unidentified silicon phase, Si-VIII and two of our other predicted tetragonal phases are highly likely within laser-affected zones. These findings may pave the way for new materials with novel and exotic properties.
[Show abstract][Hide abstract] ABSTRACT: Searches for low-energy tetrahedral polymorphs of carbon and silicon have been performed using density functional theory computations and the ab initio random structure searching approach. Several of the hypothetical phases obtained in our searches have enthalpies that are lower or comparable to those of other polymorphs of group 14 elements that have either been experimentally synthesized or recently proposed as the structure of unknown phases obtained in experiments, and should thus be considered as particularly interesting candidates. A structure of $Pbam$ symmetry with 24 atoms in the unit cell was found to be a low-energy, low-density metastable polymorph in carbon, silicon, and germanium. In silicon, $Pbam$ is found to have a direct band gap at the zone center with an estimated value of 1.4 eV, which suggests applications as a photovoltaic material. We have also found a low-energy chiral framework structure of $P{4}_{1}{2}_{1}2$ symmetry with 20 atoms per cell containing fivefold spirals of atoms, whose projected topology is that of the so-called Cairo-type two-dimensional pentagonal tiling. We suggest that $P{4}_{1}{2}_{1}2$ is a likely candidate for the structure of the unknown phase XIII of silicon. We discuss $Pbam$ and $P{4}_{1}{2}_{1}2$ in detail, contrasting their energetics and structures with those of other group 14 elements, particularly the recently proposed $P{4}_{2}/ncm$ structure, for which we also provide a detailed interpretation as a network of tilted diamondlike tetrahedra.
Physical Review B 06/2015; 91(21). DOI:10.1103/PhysRevB.91.214104 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We predict by first-principles methods a phase transition in TiO2 at 6.5 Mbar from the Fe2P-type polymorph to a ten-coordinated structure with space group I4/mmm. This is the first report, to our knowledge, of the pressure-induced phase transition to the I4/mmm structure among all dioxide compounds. The I4/mmm structure was found to be up to 3.3% denser across all pressures investigated. Significant differences were found in the electronic properties of the two structures, and the metallization of TiO2 was calculated to occur concomitantly with the phase transition to I4/mmm. The implications of our findings were extended to SiO2, and an analogous Fe2P-type to I4/mmm transition was found to occur at 10 TPa. This is consistent with the lower-pressure phase transitions of TiO2, which are well-established models for the phase transitions in other AX2 compounds, including SiO2. As in TiO2, the transition to I4/mmm corresponds to the metallization of SiO2. This transformation is in the pressure range reached in the interiors of recently discovered extrasolar planets and calls for a reformulation of the equations of state used to model them.
Proceedings of the National Academy of Sciences 05/2015; 112(22). DOI:10.1073/pnas.1500604112 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We use first-principles calculations to study structural, vibrational, and superconducting properties of H_{2}S at pressures P≥200 GPa. The inclusion of zero-point energy leads to two different possible dissociations of H_{2}S, namely 3H_{2}S→2H_{3}S+S and 5H_{2}S→3H_{3}S+HS_{2}, where both H_{3}S and HS_{2} are metallic. For H_{3}S, we perform nonperturbative calculations of anharmonic effects within the self-consistent harmonic approximation and show that the harmonic approximation strongly overestimates the electron-phonon interaction (λ≈2.64 at 200 GPa) and T_{c}. Anharmonicity hardens H─S bond-stretching modes and softens H─S bond-bending modes. As a result, the electron-phonon coupling is suppressed by 30% (λ≈1.84 at 200 GPa). Moreover, while at the harmonic level T_{c} decreases with increasing pressure, the inclusion of anharmonicity leads to a T_{c} that is almost independent of pressure. High-pressure hydrogen sulfide is a strongly anharmonic superconductor.
[Show abstract][Hide abstract] ABSTRACT: Alkali metals exhibit unexpected structures and electronic behavior at high
pressures. Compression of metallic sodium (Na) to 200 GPa leads to the
stability of a wide-band-gap insulator with the double hexagonal hP4 structure.
Post-hP4 structures remain unexplored, but they are important for addressing
the question of the pressure at which Na reverts to a metal. Here we report the
reentrant metallicity of Na at the very high pressure of 15.5 terapascal (TPa),
predicted using first-principles structure searching simulations. Na is
therefore insulating over the large pressure range of 0.2-15.5 TPa. Unusually,
Na adopts an oP8 structure at pressures of 117-125 GPa, and the same oP8
structure at 1.75-15.5 TPa. Metallization of Na occurs on formation of a stable
and striking body-centered cubic cI24 electride structure consisting of Na12
icosahedra, each housing at its center about one electron which is not
associated with any Na ions.
[Show abstract][Hide abstract] ABSTRACT: Structures of calcium peroxide (CaO2) are investigated in the pressure range 0-200 GPa using the ab initio random structure searching (AIRSS) method and density functional theory (DFT) calculations. At 0 GPa, there are several CaO2 structures very close in enthalpy, with the ground-state structure dependent on the choice of exchange-correlation functional. Further stable structures for CaO2 with C2/c, I4/mcm and P21/c symmetries emerge at pressures below 40 GPa. These phases are thermodynamically stable against decomposition into CaO and O2. The stability of CaO2 with respect to decomposition increases with pressure, with peak stability occurring at the CaO B1-B2 phase transition at 65 GPa. Phonon calculations using the quasiharmonic approximation show that CaO2 is a stable oxide of calcium at mantle temperatures and pressures, highlighting a possible role for CaO2 in planetary geochemistry. We sketch the phase diagram for CaO2, and find at least five new stable phases in the pressure-temperature ranges 0 ≤ P ≤ 60 GPa, 0 ≤ T ≤ 600 K, including two new candidates for the zero-pressure ground state structure.
[Show abstract][Hide abstract] ABSTRACT: We use first principles calculations to study structural, vibrational and
superconducting properties of H$_2$S at pressures $P\ge 200$ GPa. The inclusion
of zero point energy leads to two different possible dissociations of H$_2$S,
namely 3H$_2$S $\to$ 2H$_3$S + S and 5H$_2$S $\to$ 3H$_3$S + HS$_2$, where both
H$_3$S and HS$_2$ are metallic. For H$_3$S, we perform non-perturbative
calculations of anharmonic effects within the self-consistent harmonic
approximation and show that the harmonic approximation strongly overestimates
the electron-phonon interaction ($\lambda\approx 2.64$ at 200 GPa) and T$_c$.
Anharmonicity hardens HS bond-stretching modes and softens H--S bond-bending
modes. As a result, the electron-phonon coupling is suppressed by $30\%$
($\lambda\approx 1.84$ at 200 GPa). Moreover, while at the harmonic level T$_c$
decreases with increasing pressure, the inclusion of anharmonicity leads to a
T$_c$ that is almost independent of pressure. High pressure hydrogen sulfide is
a strongly anharmonic superconductor.
[Show abstract][Hide abstract] ABSTRACT: Metallic germanium is a promising anode material in secondary lithium-ion batteries (LIBs) due to its high theoretical capacity (1623 mAh/g) and low operating voltage, coupled with the high lithium-ion diffusivity and electronic conductivity of lithiated Ge. Here, the lithiation mechanism of micron-sized Ge anodes has been investigated with X-ray diffraction (XRD), pair distribution function (PDF) analysis, and in-/ex-situ high-resolution 7Li solid-state nuclear magnetic resonance (NMR), utilizing the structural information and spectroscopic fingerprints obtained by characterizing a series of relevant LixGey model compounds. In contrast to previous work, which postulated the formation of Li9Ge4 upon initial lithiation, we show that crystalline Ge first reacts to form a mixture of amorphous and crystalline Li7Ge3 (space group P3212). Although Li7Ge3 was proposed to be stable in a recent theoretical study of the Li-Ge phase diagram (Morris, A. J.; Grey, C. P.; Pickard, C. J. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 054111), it had not been identified in prior experimental studies. Further lithiation results in the transformation of Li7Ge3, via a series of disordered phases with related structural motifs, to form a phase that locally resembles Li7Ge2, a process that involves the gradual breakage of the Ge-Ge bonds in the Ge-Ge dimers (dumbbells) on lithiation. Crystalline Li15Ge4 then grows, with an overlithiated phase, Li15+δGe4, being formed at the end of discharge. This study provides comprehensive experimental evidence, by using techniques that probe short-, medium-, and long-range order, for the structural transformations that occur on electrochemical lithiation of Ge; the results are consistent with corresponding theoretical studies regarding stable lithiated LixGey phases.
Chemistry of Materials 01/2015; 27(3):150126094946004. DOI:10.1021/cm504312x · 8.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The only known compound of sodium and hydrogen is archetypal ionic NaH.
Application of high pressure is known to promote states with higher atomic
coordination, but extensive searches for polyhydrides with unusual
stoichiometry remain unsuccessful in spite of several theoretical predictions.
Here we report the first observation of formation of polyhydrides of Na (NaH3
and NaH7) above 40 GPa and 2000 K. We combined synchrotron x-ray diffraction
and Raman spectroscopy in a laser heated diamond anvil cell and theoretical
Ab-Initio Random Structure search, which both agree in stable structures and
compositions. Our results support the formation of multicenter bonding in a
material with unusual stoichiometry. These results are applicable to the design
of new energetic solids and high-temperature superconductors based on
hydrogenrich materials.
[Show abstract][Hide abstract] ABSTRACT: The cubic antifluorite structure comprises a face-centered cubic sublattice
of anions with cations on the tetrahedral sites. The voids in the antifluorite
structure that are crucial for superionicity in Li2O might also act as atomic
traps. Trapping of guest atoms and small molecules within voids of a host
structure leads to the formation of what are known as clathrate compounds. Here
we investigate the possibility of trapping helium or larger neon guest atoms
under pressure within alkali metal oxide and sulfide structures and find stable
helium and neon bearing compounds even at pressures below 1 GPa. These
structures are stabilized by a reduction in volume from incorporation of helium
or neon atoms within the antifluorite structure. Our study suggests a novel
class of alkali oxide and sulfide materials incorporating noble gas atoms that
might potentially be useful for gas storage.
[Show abstract][Hide abstract] ABSTRACT: Optical and synchrotron x-ray diffraction diamond anvil cell experiments have
been combined with first principles theoretical structure predictions to
investigate mixed N2 and H2 up to 55 GPa. We found the formation of oligomeric
NxH (x>1) compounds using mechano- and photochemistry at pressures above 47 and
10 GPa, respectively, and room temperature. These compounds can be recovered to
ambient pressure at T<130 K, whereas at room temperature, they can be
metastable down to 3.5 GPa. Our results suggest new pathways for synthesis of
environmentally benign high energy-density materials and alternative planetary
ice.
The Journal of Chemical Physics 09/2014; 142(21). DOI:10.1063/1.4922051 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study the effects of atomic vibrations on the solid-state chemical
shielding tensor using first principles density functional theory calculations.
At the harmonic level, we use a Monte Carlo method and a perturbative
expansion. The Monte Carlo method is accurate but computationally expensive,
while the perturbative method is computationally more efficient, but
approximate. We find excellent agreement between the two methods for both the
isotropic shift and the shielding anisotropy. The effects of zero-point quantum
mechanical nuclear motion are important up to relatively high temperatures: at
500 K they still represent about half of the overall vibrational contribution.
We also investigate the effects of anharmonic vibrations, finding that their
contribution to the zero-point correction to the chemical shielding tensor is
small. We exemplify these ideas using magnesium oxide and the molecular
crystals L-alanine and beta-aspartyl-L-alanine. We therefore propose as the
method of choice to incorporate the effects of temperature in solid state
chemical shielding tensor calculations the perturbative expansion within the
harmonic approximation. This approach is accurate and requires a computational
effort that is about an order of magnitude smaller than that of dynamical or
Monte Carlo approaches, so these effects might be routinely accounted for.
The Journal of Chemical Physics 08/2014; 141(13). DOI:10.1063/1.4897261 · 2.95 Impact Factor