Miguel Martinez-Canales

Decomposition and Terapascal phases of water ice.

Computational searches for stable and metastable structures of water ice and other H:O compositions at TPa pressures have led us to predict that H(2)O decomposes into H(2)O(2) and a hydrogen-rich phase at pressures of a little over 5 TPa. The hydrogen-rich phase is stable over a wide range of hydrogen contents, and it might play a role in the erosion of the icy component of the cores of gas giants as H(2)O comes into contact with hydrogen. Metallization of H(2)O is predicted at a higher pressure of just over 6 TPa, and therefore H(2)O does not have a thermodynamically stable low-temperature metallic form. We have also found a new and rich mineralogy of complicated water ice phases that are more stable in the pressure range 0.8-2 TPa than any predicted previously.

Persistence and Eventual Demise of Oxygen Molecules at Terapascal Pressures

Computational searches for structures of solid oxygen under high pressures in the multi-TPa range are carried out using density-functional-theory methods. We find that molecular oxygen persists to about 1.9 TPa at which it transforms into a semiconducting square-spiral-like polymeric structure (I4(1)/acd) with a band gap of ~3.0 eV. Solid oxygen forms a metallic zigzag chainlike structure (Cmcm) at about 3.0 TPa, but the chains in each layer gradually merge as the pressure is increased and a structure of Fmmm symmetry forms at about 9.3 TPa in which each atom has four nearest neighbors. The superconducting properties of molecular oxygen do not vary much with compression, although the structure becomes more symmetric. The electronic properties of oxygen have a complex evolution with pressure, swapping between insulating, semiconducting, and metallic.

Stable all-nitrogen metallic salt at terapascal Pressures

The phase diagram and equation of state of dense nitrogen are of interest in understanding the fundamental physics and chemistry under extreme conditions, including planetary processes, and in discovering new materials. We predict several stable phases of nitrogen at multi-TPa pressures, including a P4/nbm structure consisting of partially charged N(2)(δ+) pairs and N(5)(δ-) tetrahedra, which is stable in the range 2.5-6.8 TPa. This is followed by a modulated layered structure between 6.8 and 12.6 TPa, which also exhibits a significant charge transfer. The P4/nbm metallic nitrogen salt and the modulated structure are stable at high pressures and temperatures, and they exhibit strongly ionic features and charge density distortions, which is unexpected in an element under such extreme conditions and could represent a new class of nitrogen materials. The P-T phase diagram of nitrogen at TPa pressures is investigated using quasiharmonic phonon calculations and ab initio molecular dynamics simulations.

Fermi surface nesting and phonon instabilities in simple cubic calcium

Phonon instabilities and Fermi surface nesting are studied in the high-pressure simple cubic phase of calcium by means of ab initio calculations. We have focused on nesting along Γ X, which could be responsible for some of the anomalies observed in the phonon spectrum. Phonon frequencies calculated with the density functional perturbation theory are imaginary at several Brillouin-zone points (e.g. at M). However, including anharmonic contributions to the potential might be crucial to stabilize simple cubic calcium, as solving the Schrödinger equation associated to the transversal unstable mode at M gives a positive frequency.

Quantum and isotope effects in lithium metal

Lithium gets a new ground state For the past 70 years, the lowest-energy crystal structure of lithium was believed to be a relatively complex one called the 9R structure. Ackland et al. show that this is incorrect. The actual lowest-energy structure for lithium is the much simpler closest-packed face-centered cubic form. In addition, 6Li and 7Li isotopes have crystal phase transitions at slightly different pressures and temperatures. This difference is chalked up to large quantum mechanical effects between the isotopes. Lithium is the only metal that shows this type of quantum effect and presents a challenge for theoreticians to explain. Science, this issue p. 1254 Lithium’s ground state has a face-centered cubic structure, and quantum effects alter the phase diagram between the 6Li and 7Li isotopes. The crystal structure of elements at zero pressure and temperature is the most fundamental information in condensed matter physics. For decades it has been believed that lithium, the simplest metallic element, has a complicated ground-state crystal structure. Using synchrotron x-ray diffraction in diamond anvil cells and multiscale simulations with density functional theory and molecular dynamics, we show that the previously accepted martensitic ground state is metastable. The actual ground state is face-centered cubic (fcc). We find that isotopes of lithium, under similar thermal paths, exhibit a considerable difference in martensitic transition temperature. Lithium exhibits nuclear quantum mechanical effects, serving as a metallic intermediate between helium, with its quantum effect–dominated structures, and the higher-mass elements. By disentangling the quantum kinetic complexities, we prove that fcc lithium is the ground state, and we synthesize it by decompression.

Postaragonite phases of CaCO3 at lower mantle pressures

The stability, structure and properties of carbonate minerals at lower mantle conditions has significant impact on our understanding of the global carbon cycle and the composition of the interior of the Earth. In recent years, there has been significant interest in the behavior of carbonates at lower mantle conditions, specifically in their carbon hybridization, which has relevance for the storage of carbon within the deep mantle. Using high-pressure synchrotron X-ray diffraction in a diamond anvil cell coupled with direct laser heating of CaCO

No evidence of metallic methane at high pressure

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Anomalous static electronic screening in compressed lithium

using a CO

No experimental Fermi surface measurements have been reported or made on low-temperature martensitic lithium

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Dirac cones in two-dimensional borane

laser, we identify a crystalline phase of the material above 40 GPa