At ambient pressure, sodium, chlorine, and their only known compound NaCl, have well-understood c... more At ambient pressure, sodium, chlorine, and their only known compound NaCl, have well-understood crystal structures and chemical bonding. Sodium is a nearly-free-electron metal with the bcc structure. Chlorine is a molecular crystal, consisting of Cl2 molecules. Sodium chloride, due to the large electronegativity difference between Na and Cl atoms, has highly ionic chemical bonding, with stoichiometry 1:1 dictated by charge balance, and rocksalt (B1-type) crystal structure in accordance with Pauling's rules. Up to now, Na-Cl was thought to be an ultimately simple textbook system. Here, we show that under pressure the stability of compounds in the Na-Cl system changes and new materials with different stoichiometries emerge at pressure as low as 25 GPa. In addition to NaCl, our theoretical calculations predict the stability of Na3Cl, Na2Cl, Na3Cl2, NaCl3 and NaCl7 compounds with unusual bonding and electronic properties. The bandgap is closed for the majority of these materials. Guided by these predictions, we have synthesized cubic NaCl3 at 55-60 GPa in the laser-heated diamond anvil cell at temperatures above 2000 K.
Due to the strongly reducing conditions (the presence of metallic iron was suggested both by expe... more Due to the strongly reducing conditions (the presence of metallic iron was suggested both by experiments [1] and theory [2]), diamond was believed to be the main host of carbon through most of the lower mantle [3]. We showed [4] that cementite Fe3C is another good candidate to be the main host of "reduced" carbon in the mantle, reinforcing an earlier hypothesis [5]. The fate of "oxidised" carbon (in subducted slabs) is of particular importance - if carbonates decompose producing fluid CO2, this would have important implications for the chemistry and rheology of the mantle. Knowledge of crystal structures and phase diagrams of carbonates is crucial here. The high-pressure structures of CaCO3 were predicted [6] and subsequently verified by experiments. For MgCO3, Isshiki et al. [7] found a new phase above 110 GPa, and several attempts were made to solve it [8,9]. Here [4], using an evolutionary algorithm for crystal structure prediction [10], we show that there are two post-magnesite phases at mantle-relevant pressure range, one stable at 82-138 GPa, and the other from 138 GPa to ~160 GPa. Both are based on threefold rings of CO4-tetrahedra and are more favourable than all previously proposed structures. We show that through most of the P-T conditions of the mantle, MgCO3 is the major host of oxidized carbon in the Earth. We predict the possibility of CO2 release at the very bottom of the mantle (in SiO2-rich basaltic part of subducted slabs), which could enhance partial melting of rocks and be related to the geodynamical differences between the Earth and Venus. 1.Frost D.J., Liebske C., Langenhorst F., McCammon C.A., Tronnes R.G., Rubie D.C. (2004). Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle. Nature 428, 409-412. 2.Zhang F., Oganov A.R. (2006). Valence and spin states of iron impurities in mantle-forming silicates. Earth Planet. Sci. Lett. 249, 436-443. 3.Luth R.W. (1999). Carbon and carbonates in the mantle. In: Mantle Petrology: Field Observations and High Pressure Experimentation: A Tribute to Francis R. (Joe) Boyd. Geochemical Soc., Special Publication No. 6. Eds: Y. Fei, C.M. Bertka, B.O. Mysen. 4.Oganov A.R., Ono S., Ma Y., Glass C.W., Garcia A. (2008). Novel high-pressure structures of MgCO3, CaCO3 and CO2 and their role in the Earth's lower mantle. Earth Planet. Sci. Lett. 273, 38-47 5.Scott H.P.,, Williams Q., Knittle E. (2001). Stability and equation of state of Fe3C to 73 GPa: Implications for carbon in the Earth's core. Geoph. Res. Lett. 28, 1875-1878. 6.Oganov A.R., Glass C.W., Ono S. (2006). High-pressure phases of CaCO3: crystal structure prediction and experiment. Earth Planet. Sci. Lett. 241, 95-103. 7.Isshiki M., Irifune T., Hirose K., Ono S., Ohishi Y., Watanuki T., Nishibori E., Takadda M., and Sakata M. (2004). Stability of Magnesite and its high-pressure form in the lowermost mantle. Nature 427, 60-63. 8.Skorodumova N.V., Belonoshko A.B., Huang L., Ahuja R., Johansson B. (2005) Stability of the MgCO3 structures under lower mantle conditions. Am. Mineral. 90, 1008-1011. 9.Panero W.R., Kabbes J.E. (2008). Mantle-wide sequestration of carbon in silicates and the structure of magnesite II. Geophys. Res. Lett. 35, L14307. 10.Oganov A.R., Glass C.W. (2006). Crystal structure prediction using ab initio evolutionary algorithms: principles and applications. J. Chem. Phys. 124, art. 244704.
Physical Review B Condensed Matter and Materials Physics, Feb 1, 2007
The evolution of Fermi surfaces and lattice dynamics for alkali metals in the fcc structure with ... more The evolution of Fermi surfaces and lattice dynamics for alkali metals in the fcc structure with pressure have been studied using ab initio calculations within the density functional theory. Fermi surface nesting features along the Γ-K symmetry direction in the Brillouin zone have been identified for Li, K, Rb, and Cs, while it is absent for Na. Moreover, a transverse acoustic phonon softening along the Γ-K with pressure is predicted for Li, Na, K, Rb, and Cs. This observation suggests a common phonon softening behavior in fcc alkali metals at high pressure. Analysis of the theoretical results suggests that the consideration of both phonon and electronic instabilities is crucial to the understanding of pressure-induced phase transitions in the fcc alkali metals.
Abundant evidence has shown the emergence of dramatic new chemical phenomena under pressure, incl... more Abundant evidence has shown the emergence of dramatic new chemical phenomena under pressure, including the formation of unexpected crystal structures and completely new counterintuitive compounds. In many cases, there is no convincing explanation for these phenomena and there are virtually no chemical rules or models capable of predicting or even rationalizing these phenomena. Here we consider two central chemical properties of atoms, electronegativity and chemical hardness, and determine them as a function of pressure up to 500 GPa. For elements without orbital transfer at high pressure, electronegativity first increases and then decreases, while chemical hardness monotonically decreases as pressure increases. For some active metals, the chemical hardness has a further increase at pressures of the order of tens-hundreds of gigapascals. Furthermore, we discover that orbital transfer, in particular s-d transfer, makes Ni a "pseudo-noble-gas", Fe and Co strong electron acceptors, while Cu and Zn become active metals. We show the explicative and predictive power of our electronegativity and chemical hardness scales under pressure.
Over the years, various models have been used for smoothing experimental PVT data and parametriza... more Over the years, various models have been used for smoothing experimental PVT data and parametrization of thermoelastic functions (see, e.g., Anderson et al., 1989, JAP, 65, 1534-1543; Saxena, Zhang, 1990, PCM, 17, 45-51; Jackson, Rigden, 1996, PEPI, 96, 85-112; Pavese, 2002, PCM, 29, 43-51). The Mie-Grüneisen- Debye formula for the thermal pressure can be obtained by differentiation of the Helmoholtz free energy, and is therefore a strictly thermodynamic formula. In the high-temperature limit it is easy to calculate the thermal part of the free energy by the Einstein model, keeping in mind that the Einstein temperature, TE, is related to the Debye temperature, TD, as TE = 0.775TD. Obtained by direct differentiation of the free energy, the bulk moduli, entropy, heat capacity and other thermodynamic functions, are by construction internally consistent. An important point is the choice of the analytical form of the volumetric dependence of the Einstein (or Debye) temperature, and correct account of intrinsic anharmonicity, which makes the Einstein temperature depend not only on volume, but also on temperature. Here we propose a simple analytical form for the pressure as a function of temperature and volume. This equation can be used for constructing a practical pressure scale, as has been verified on numerous examples (Dorogokupets, Oganov, 2007; Dorogokupets, Dewaele, 2007; Fei et al., 2007; Hirose et al., 2008; Matsui, 2008; Ueda et al., 2008; Sun et al., 2008; Ono et al., 2008; Wu et al., 2008; etc.).
We present a systematic search for low-energy metastable superhard carbon allotropes by using the... more We present a systematic search for low-energy metastable superhard carbon allotropes by using the recently developed evolutionary metadynamics technique. It is known that cold compression of graphite produces an allotrope at 15-20 GPa. Here we look for all low-enthalpy structures accessible from graphite. Starting from 2H- or 3R-graphite and applying the pressure of 20 GPa, a large variety of intermediate $sp^3$ carbon allotropes were observed in evolutionary metadynamics simulation. Our calculation not only found all the previous proposed candidates for `superhard graphite', but also predicted two allotropes (\emph{X}-carbon and \emph{Y}-carbon) showing novel 5+7 and 4+8 topologies. These superhard carbon allotropes can be classified into five families based on 6 (diamond/lonsdaleite), 5+7 (\emph{M/W}-carbon), 5+7 (\emph{X}-carbon), 4+8 (bct C$_4$), and 4+8 (\emph{Y}-carbon) topologies. This study shows evolutionary metadynamics is a powerful approach both to find the global minima and systematically search for low-energy metastable phases reachable from given starting materials.
The Earth's lower mantle, the largest region within our planet (670-2890 km depths), is believed ... more The Earth's lower mantle, the largest region within our planet (670-2890 km depths), is believed to contain ˜75 vol.% of (Mg,Fe)SiO3 perovskite, ˜20% (Mg,Fe)O, and ˜5% CaSiO3. This mineralogy was unable to explain many unusual properties of the D'' layer, the lowermost ˜150 km of the mantle. Using ab initio simulations and high-pressure experiments we have demonstrated [1] that at pressures and temperatures of the D'' layer, MgSiO3 transforms from perovskite into a layered CaIrO3--type structure (space group Cmcm); this structure was also independently found in [2]. The elastic properties of the new phase and its stability field explain most of the previously puzzling properties of the D'' layer: its seismic anisotropy [3], strongly undulating shear-wave discontinuity at its top^ [4], and the anticorrelation between shear and bulk sound velocities [5]. This new phase is therefore likely to be a major Earth-forming mineral, and its discovery will change our understanding of the deep Earth's interior. Latest studies of the effects of impurities [6,7] on the stability of this phase, and similar phases of other compounds will be discussed. REFERENCES: 1. Oganov A.R., Ono S. (2004). Nature 430, 445-448. 2. Murakami M., et al. (2004). Science 304, 855-858. 3. Panning M., Romanowicz B. (2004). Science 303, 351-353. 4. Sidorin I., et al. D.V. (1999). Science 286, 1326-1331. 5. Su W.J., Dziewonski A.M. (1997). Phys. Earth Planet. Inter. 100, 135-156. 6. Mao W.L., et al. (2004). Proc. Natl. Acad. Sci. 101, 15867-15869. 7. Ono S., Oganov A.R., Ohishi Y. (2004). Submitted.
A novel stable crystallographic structure is discovered in a variety of ABO3, ABF3 and A2O3 compo... more A novel stable crystallographic structure is discovered in a variety of ABO3, ABF3 and A2O3 compounds (including materials of geological relevance, prototypes of multiferroics, exhibiting strong spin-orbit effects, etc...), via the use of first principles. This novel structure appears under hydrostatic pressure, and is the first "post-post-perovskite" phase to be found. It provides a successful solution to experimental puzzles in important systems, and is characterized by one-dimensional chains linked by group of two via edge-sharing oxygen/fluorine octahedra. Such unprecedented organization automatically results in anisotropic elastic properties and new magnetic arrangements. Depending on the system of choice, this post-post-perovskite structure also possesses electronic band gaps ranging from zero to ~ 10 eV being direct or indirect in nature, which emphasizes its "universality" and its potential to have striking, e.g., electrical or transport phenomena.
The lower mantle of the Earth extends from about 670 km to 2980 km and consists mainly of MgSiO3-... more The lower mantle of the Earth extends from about 670 km to 2980 km and consists mainly of MgSiO3-perovskite (~ 75 vol%), (Mg,Fe)O magnesiowüstite (~ 20 vol%) and CaSiO3-perovskite (~ 5 vol%). It is possible to calculate thermodynamic properties, structures and energetics of the individual minerals at extreme conditions of the mantle using ab initio methods, such as density functional theory. To obtain a more realistic picture of the lower mantle it is necessary to not only investigate chemically pure minerals, but to consider minerals as solid solutions, as they are in nature. The density functional theory with the generalized gradient approximation (GGA) and the projector augmented wave (PAW) method, as implemented in the VASP code, was used to calculate the structure and stability of CaSiO3 perovskite in the pressure range of the Earth's mantle (0-150 GPa), no post-perovskite structure has been found [1]. Here we focus on the two perovskite solvus. We use a subregular solid solution model together with point defect calculations to model the solvus at different pressures in the lower mantle regime. Additionally, the effect of the different symmetries ( Pbnm and I4/ mmm) of the perovskites has to be included. This is important especially for the Ca-perovskite, since the energy differences of the two phases are very small and thus likely to have an influence on the solvus. We investigated the solvus at different pressures of the lower mantle. At pressures and temperatures of the lower mantle, the solvus in the (Ca,Mg)SiO3 system remains wide open and solubilities of Ca in Mg-perovskite and Mg in Ca-perovskite low. From these results in the simple system it is highly unlikely that Ca-perovskite will disappear (i.e. fully dissolve in Mg-perovskite) with depths in the lower mantle. Information of the solubility of Ca in MgSiO3 in more complex systems will elucidate the mineralogical composition of the lower mantle of the Earth. This is the first work to treat this subject with ab initio methods. Presently, calculations on the Ca-Mg-perovskite solvus with aluminium impurities are in progress. [1] Jung D.Y., Oganov A.R. (2005) Phys. Chem. Minerals 32, 146-153
Using ab initio evolutionary methodology for crystal structure prediction, we explore metallic st... more Using ab initio evolutionary methodology for crystal structure prediction, we explore metallic structures of oxygen at pressures in the range of 100 250GPa . Two energetically competitive monoclinic structures of C2/c (4mol/cell) and C2/m (8mol/cell) were found as candidates for ζ-O2 . The C2/m structure is our preferred solution, providing slightly lower enthalpy and better agreement with experimental x-ray diffraction (XRD) pattern and Raman frequencies. Our theoretical prediction supports the isosymmetric nature of the ɛ-ζ transition, in accordance with the suggestion based on the powder XRD measurements [Y. Akahama , Phys. Rev. Lett. 74, 4690 (1995)]. Moreover, we find that at least up to 250GPa , oxygen remains a molecular solid and there is no post- ζ -phase in this pressure range.
Synthesis of novel superhard materials, frequently achieved with high-pressure experimental techn... more Synthesis of novel superhard materials, frequently achieved with high-pressure experimental techniques, is a difficult task. Study of such materials, usually first obtained in very small quantities, is complicated, and this field is full of controversies and artefacts (see, e.g. [1]). Hardness was long believed to be an exceedingly difficult property to model or predict; however, a number of simple models have recently been proposed and shown to yield surprisingly accurate results (see reviews in [2]). We have found [3] a way to further improve these models, by augmenting them with bond-valence model and graph theory. Combining such models with our structure prediction method [4], we have developed a powerful [5] approach for computational design of novel superhard materials. Using this method, we recently addressed [3] the previously proposed [6] possibility that C3N4 may be harder than diamond, and the claim [7] that TiO2-cotunnite is the hardest oxide with the Vickers hardness of 38 GPa. Our results unequivocally suggest that the latter suggestion is incorrect. No TiO2 polymorph can attain hardness greater than 17 GPa [3], i.e. all possible structures of TiO2 are softer than common corundum (Al2O3). Furthermore, TiO2-cotunnite is dynamically unstable at atmospheric pressure [8]. REFERENCES: [1] Oganov A.R., Solozhenko V.L., Kurakevych O.O., Gatti C., Ma Y., Chen J., Liu Z., and Hemley R.J., http://arxiv.org/abs/ 0908.2126 (2009). [2] Theory of Superhard Materials (Special Issue), J. Superhard Materials, issue 3 (2010). [3] Lyakhov A.O., Oganov A.R., in prep. (2010). [4] Oganov A.R., Glass C.W., J. Chem. Phys. 124, 244704 (2006). [5] Oganov A.R., Lyakhov A.O., J. Superhard Mater. 32, 143-147 (2010). [6] Liu A.Y., Cohen M.L., Science 245, 841-842 (1989). [7] Dubrovinsky L.S., Dubrovinskaia N.A., et al., Nature 410, 653-654 (2001). [8] Kim D.Y., et al., Appl. Phys. Lett. 90, 171903 (2007).
Methane is an extremely important energy source with a great abundance in nature and plays a sign... more Methane is an extremely important energy source with a great abundance in nature and plays a significant role in planetary physics, being one of the major constituents of giant planets Uranus and Neptune. The stable crystal forms of methane under extreme conditions are of great fundamental interest. Using the ab initio evolutionary algorithm for crystal structure prediction, we found three novel insulating molecular structures with P212121, Pnma, and Cmcm space groups. Remarkably, under high pressure, methane becomes unstable and dissociates into ethane (C2H6) at 95 GPa, butane (C4H10) at 158 GPa, and further, carbon (diamond) and hydrogen above 287 GPa at zero temperature. We have computed the pressure-temperature phase diagram, which sheds light into the seemingly conflicting observations of the unusually low formation pressure of diamond at high temperature and the failure of experimental observation of dissociation at room temperature. Our results support the idea of diamond formation in the interiors of giant planets such as Neptune.
At ambient pressure, sodium, chlorine, and their only known compound NaCl, have well-understood c... more At ambient pressure, sodium, chlorine, and their only known compound NaCl, have well-understood crystal structures and chemical bonding. Sodium is a nearly-free-electron metal with the bcc structure. Chlorine is a molecular crystal, consisting of Cl2 molecules. Sodium chloride, due to the large electronegativity difference between Na and Cl atoms, has highly ionic chemical bonding, with stoichiometry 1:1 dictated by charge balance, and rocksalt (B1-type) crystal structure in accordance with Pauling's rules. Up to now, Na-Cl was thought to be an ultimately simple textbook system. Here, we show that under pressure the stability of compounds in the Na-Cl system changes and new materials with different stoichiometries emerge at pressure as low as 25 GPa. In addition to NaCl, our theoretical calculations predict the stability of Na3Cl, Na2Cl, Na3Cl2, NaCl3 and NaCl7 compounds with unusual bonding and electronic properties. The bandgap is closed for the majority of these materials. Guided by these predictions, we have synthesized cubic NaCl3 at 55-60 GPa in the laser-heated diamond anvil cell at temperatures above 2000 K.
Due to the strongly reducing conditions (the presence of metallic iron was suggested both by expe... more Due to the strongly reducing conditions (the presence of metallic iron was suggested both by experiments [1] and theory [2]), diamond was believed to be the main host of carbon through most of the lower mantle [3]. We showed [4] that cementite Fe3C is another good candidate to be the main host of "reduced" carbon in the mantle, reinforcing an earlier hypothesis [5]. The fate of "oxidised" carbon (in subducted slabs) is of particular importance - if carbonates decompose producing fluid CO2, this would have important implications for the chemistry and rheology of the mantle. Knowledge of crystal structures and phase diagrams of carbonates is crucial here. The high-pressure structures of CaCO3 were predicted [6] and subsequently verified by experiments. For MgCO3, Isshiki et al. [7] found a new phase above 110 GPa, and several attempts were made to solve it [8,9]. Here [4], using an evolutionary algorithm for crystal structure prediction [10], we show that there are two post-magnesite phases at mantle-relevant pressure range, one stable at 82-138 GPa, and the other from 138 GPa to ~160 GPa. Both are based on threefold rings of CO4-tetrahedra and are more favourable than all previously proposed structures. We show that through most of the P-T conditions of the mantle, MgCO3 is the major host of oxidized carbon in the Earth. We predict the possibility of CO2 release at the very bottom of the mantle (in SiO2-rich basaltic part of subducted slabs), which could enhance partial melting of rocks and be related to the geodynamical differences between the Earth and Venus. 1.Frost D.J., Liebske C., Langenhorst F., McCammon C.A., Tronnes R.G., Rubie D.C. (2004). Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle. Nature 428, 409-412. 2.Zhang F., Oganov A.R. (2006). Valence and spin states of iron impurities in mantle-forming silicates. Earth Planet. Sci. Lett. 249, 436-443. 3.Luth R.W. (1999). Carbon and carbonates in the mantle. In: Mantle Petrology: Field Observations and High Pressure Experimentation: A Tribute to Francis R. (Joe) Boyd. Geochemical Soc., Special Publication No. 6. Eds: Y. Fei, C.M. Bertka, B.O. Mysen. 4.Oganov A.R., Ono S., Ma Y., Glass C.W., Garcia A. (2008). Novel high-pressure structures of MgCO3, CaCO3 and CO2 and their role in the Earth's lower mantle. Earth Planet. Sci. Lett. 273, 38-47 5.Scott H.P.,, Williams Q., Knittle E. (2001). Stability and equation of state of Fe3C to 73 GPa: Implications for carbon in the Earth's core. Geoph. Res. Lett. 28, 1875-1878. 6.Oganov A.R., Glass C.W., Ono S. (2006). High-pressure phases of CaCO3: crystal structure prediction and experiment. Earth Planet. Sci. Lett. 241, 95-103. 7.Isshiki M., Irifune T., Hirose K., Ono S., Ohishi Y., Watanuki T., Nishibori E., Takadda M., and Sakata M. (2004). Stability of Magnesite and its high-pressure form in the lowermost mantle. Nature 427, 60-63. 8.Skorodumova N.V., Belonoshko A.B., Huang L., Ahuja R., Johansson B. (2005) Stability of the MgCO3 structures under lower mantle conditions. Am. Mineral. 90, 1008-1011. 9.Panero W.R., Kabbes J.E. (2008). Mantle-wide sequestration of carbon in silicates and the structure of magnesite II. Geophys. Res. Lett. 35, L14307. 10.Oganov A.R., Glass C.W. (2006). Crystal structure prediction using ab initio evolutionary algorithms: principles and applications. J. Chem. Phys. 124, art. 244704.
Physical Review B Condensed Matter and Materials Physics, Feb 1, 2007
The evolution of Fermi surfaces and lattice dynamics for alkali metals in the fcc structure with ... more The evolution of Fermi surfaces and lattice dynamics for alkali metals in the fcc structure with pressure have been studied using ab initio calculations within the density functional theory. Fermi surface nesting features along the Γ-K symmetry direction in the Brillouin zone have been identified for Li, K, Rb, and Cs, while it is absent for Na. Moreover, a transverse acoustic phonon softening along the Γ-K with pressure is predicted for Li, Na, K, Rb, and Cs. This observation suggests a common phonon softening behavior in fcc alkali metals at high pressure. Analysis of the theoretical results suggests that the consideration of both phonon and electronic instabilities is crucial to the understanding of pressure-induced phase transitions in the fcc alkali metals.
Abundant evidence has shown the emergence of dramatic new chemical phenomena under pressure, incl... more Abundant evidence has shown the emergence of dramatic new chemical phenomena under pressure, including the formation of unexpected crystal structures and completely new counterintuitive compounds. In many cases, there is no convincing explanation for these phenomena and there are virtually no chemical rules or models capable of predicting or even rationalizing these phenomena. Here we consider two central chemical properties of atoms, electronegativity and chemical hardness, and determine them as a function of pressure up to 500 GPa. For elements without orbital transfer at high pressure, electronegativity first increases and then decreases, while chemical hardness monotonically decreases as pressure increases. For some active metals, the chemical hardness has a further increase at pressures of the order of tens-hundreds of gigapascals. Furthermore, we discover that orbital transfer, in particular s-d transfer, makes Ni a "pseudo-noble-gas", Fe and Co strong electron acceptors, while Cu and Zn become active metals. We show the explicative and predictive power of our electronegativity and chemical hardness scales under pressure.
Over the years, various models have been used for smoothing experimental PVT data and parametriza... more Over the years, various models have been used for smoothing experimental PVT data and parametrization of thermoelastic functions (see, e.g., Anderson et al., 1989, JAP, 65, 1534-1543; Saxena, Zhang, 1990, PCM, 17, 45-51; Jackson, Rigden, 1996, PEPI, 96, 85-112; Pavese, 2002, PCM, 29, 43-51). The Mie-Grüneisen- Debye formula for the thermal pressure can be obtained by differentiation of the Helmoholtz free energy, and is therefore a strictly thermodynamic formula. In the high-temperature limit it is easy to calculate the thermal part of the free energy by the Einstein model, keeping in mind that the Einstein temperature, TE, is related to the Debye temperature, TD, as TE = 0.775TD. Obtained by direct differentiation of the free energy, the bulk moduli, entropy, heat capacity and other thermodynamic functions, are by construction internally consistent. An important point is the choice of the analytical form of the volumetric dependence of the Einstein (or Debye) temperature, and correct account of intrinsic anharmonicity, which makes the Einstein temperature depend not only on volume, but also on temperature. Here we propose a simple analytical form for the pressure as a function of temperature and volume. This equation can be used for constructing a practical pressure scale, as has been verified on numerous examples (Dorogokupets, Oganov, 2007; Dorogokupets, Dewaele, 2007; Fei et al., 2007; Hirose et al., 2008; Matsui, 2008; Ueda et al., 2008; Sun et al., 2008; Ono et al., 2008; Wu et al., 2008; etc.).
We present a systematic search for low-energy metastable superhard carbon allotropes by using the... more We present a systematic search for low-energy metastable superhard carbon allotropes by using the recently developed evolutionary metadynamics technique. It is known that cold compression of graphite produces an allotrope at 15-20 GPa. Here we look for all low-enthalpy structures accessible from graphite. Starting from 2H- or 3R-graphite and applying the pressure of 20 GPa, a large variety of intermediate $sp^3$ carbon allotropes were observed in evolutionary metadynamics simulation. Our calculation not only found all the previous proposed candidates for `superhard graphite', but also predicted two allotropes (\emph{X}-carbon and \emph{Y}-carbon) showing novel 5+7 and 4+8 topologies. These superhard carbon allotropes can be classified into five families based on 6 (diamond/lonsdaleite), 5+7 (\emph{M/W}-carbon), 5+7 (\emph{X}-carbon), 4+8 (bct C$_4$), and 4+8 (\emph{Y}-carbon) topologies. This study shows evolutionary metadynamics is a powerful approach both to find the global minima and systematically search for low-energy metastable phases reachable from given starting materials.
The Earth's lower mantle, the largest region within our planet (670-2890 km depths), is believed ... more The Earth's lower mantle, the largest region within our planet (670-2890 km depths), is believed to contain ˜75 vol.% of (Mg,Fe)SiO3 perovskite, ˜20% (Mg,Fe)O, and ˜5% CaSiO3. This mineralogy was unable to explain many unusual properties of the D'' layer, the lowermost ˜150 km of the mantle. Using ab initio simulations and high-pressure experiments we have demonstrated [1] that at pressures and temperatures of the D'' layer, MgSiO3 transforms from perovskite into a layered CaIrO3--type structure (space group Cmcm); this structure was also independently found in [2]. The elastic properties of the new phase and its stability field explain most of the previously puzzling properties of the D'' layer: its seismic anisotropy [3], strongly undulating shear-wave discontinuity at its top^ [4], and the anticorrelation between shear and bulk sound velocities [5]. This new phase is therefore likely to be a major Earth-forming mineral, and its discovery will change our understanding of the deep Earth's interior. Latest studies of the effects of impurities [6,7] on the stability of this phase, and similar phases of other compounds will be discussed. REFERENCES: 1. Oganov A.R., Ono S. (2004). Nature 430, 445-448. 2. Murakami M., et al. (2004). Science 304, 855-858. 3. Panning M., Romanowicz B. (2004). Science 303, 351-353. 4. Sidorin I., et al. D.V. (1999). Science 286, 1326-1331. 5. Su W.J., Dziewonski A.M. (1997). Phys. Earth Planet. Inter. 100, 135-156. 6. Mao W.L., et al. (2004). Proc. Natl. Acad. Sci. 101, 15867-15869. 7. Ono S., Oganov A.R., Ohishi Y. (2004). Submitted.
A novel stable crystallographic structure is discovered in a variety of ABO3, ABF3 and A2O3 compo... more A novel stable crystallographic structure is discovered in a variety of ABO3, ABF3 and A2O3 compounds (including materials of geological relevance, prototypes of multiferroics, exhibiting strong spin-orbit effects, etc...), via the use of first principles. This novel structure appears under hydrostatic pressure, and is the first "post-post-perovskite" phase to be found. It provides a successful solution to experimental puzzles in important systems, and is characterized by one-dimensional chains linked by group of two via edge-sharing oxygen/fluorine octahedra. Such unprecedented organization automatically results in anisotropic elastic properties and new magnetic arrangements. Depending on the system of choice, this post-post-perovskite structure also possesses electronic band gaps ranging from zero to ~ 10 eV being direct or indirect in nature, which emphasizes its "universality" and its potential to have striking, e.g., electrical or transport phenomena.
The lower mantle of the Earth extends from about 670 km to 2980 km and consists mainly of MgSiO3-... more The lower mantle of the Earth extends from about 670 km to 2980 km and consists mainly of MgSiO3-perovskite (~ 75 vol%), (Mg,Fe)O magnesiowüstite (~ 20 vol%) and CaSiO3-perovskite (~ 5 vol%). It is possible to calculate thermodynamic properties, structures and energetics of the individual minerals at extreme conditions of the mantle using ab initio methods, such as density functional theory. To obtain a more realistic picture of the lower mantle it is necessary to not only investigate chemically pure minerals, but to consider minerals as solid solutions, as they are in nature. The density functional theory with the generalized gradient approximation (GGA) and the projector augmented wave (PAW) method, as implemented in the VASP code, was used to calculate the structure and stability of CaSiO3 perovskite in the pressure range of the Earth's mantle (0-150 GPa), no post-perovskite structure has been found [1]. Here we focus on the two perovskite solvus. We use a subregular solid solution model together with point defect calculations to model the solvus at different pressures in the lower mantle regime. Additionally, the effect of the different symmetries ( Pbnm and I4/ mmm) of the perovskites has to be included. This is important especially for the Ca-perovskite, since the energy differences of the two phases are very small and thus likely to have an influence on the solvus. We investigated the solvus at different pressures of the lower mantle. At pressures and temperatures of the lower mantle, the solvus in the (Ca,Mg)SiO3 system remains wide open and solubilities of Ca in Mg-perovskite and Mg in Ca-perovskite low. From these results in the simple system it is highly unlikely that Ca-perovskite will disappear (i.e. fully dissolve in Mg-perovskite) with depths in the lower mantle. Information of the solubility of Ca in MgSiO3 in more complex systems will elucidate the mineralogical composition of the lower mantle of the Earth. This is the first work to treat this subject with ab initio methods. Presently, calculations on the Ca-Mg-perovskite solvus with aluminium impurities are in progress. [1] Jung D.Y., Oganov A.R. (2005) Phys. Chem. Minerals 32, 146-153
Using ab initio evolutionary methodology for crystal structure prediction, we explore metallic st... more Using ab initio evolutionary methodology for crystal structure prediction, we explore metallic structures of oxygen at pressures in the range of 100 250GPa . Two energetically competitive monoclinic structures of C2/c (4mol/cell) and C2/m (8mol/cell) were found as candidates for ζ-O2 . The C2/m structure is our preferred solution, providing slightly lower enthalpy and better agreement with experimental x-ray diffraction (XRD) pattern and Raman frequencies. Our theoretical prediction supports the isosymmetric nature of the ɛ-ζ transition, in accordance with the suggestion based on the powder XRD measurements [Y. Akahama , Phys. Rev. Lett. 74, 4690 (1995)]. Moreover, we find that at least up to 250GPa , oxygen remains a molecular solid and there is no post- ζ -phase in this pressure range.
Synthesis of novel superhard materials, frequently achieved with high-pressure experimental techn... more Synthesis of novel superhard materials, frequently achieved with high-pressure experimental techniques, is a difficult task. Study of such materials, usually first obtained in very small quantities, is complicated, and this field is full of controversies and artefacts (see, e.g. [1]). Hardness was long believed to be an exceedingly difficult property to model or predict; however, a number of simple models have recently been proposed and shown to yield surprisingly accurate results (see reviews in [2]). We have found [3] a way to further improve these models, by augmenting them with bond-valence model and graph theory. Combining such models with our structure prediction method [4], we have developed a powerful [5] approach for computational design of novel superhard materials. Using this method, we recently addressed [3] the previously proposed [6] possibility that C3N4 may be harder than diamond, and the claim [7] that TiO2-cotunnite is the hardest oxide with the Vickers hardness of 38 GPa. Our results unequivocally suggest that the latter suggestion is incorrect. No TiO2 polymorph can attain hardness greater than 17 GPa [3], i.e. all possible structures of TiO2 are softer than common corundum (Al2O3). Furthermore, TiO2-cotunnite is dynamically unstable at atmospheric pressure [8]. REFERENCES: [1] Oganov A.R., Solozhenko V.L., Kurakevych O.O., Gatti C., Ma Y., Chen J., Liu Z., and Hemley R.J., http://arxiv.org/abs/ 0908.2126 (2009). [2] Theory of Superhard Materials (Special Issue), J. Superhard Materials, issue 3 (2010). [3] Lyakhov A.O., Oganov A.R., in prep. (2010). [4] Oganov A.R., Glass C.W., J. Chem. Phys. 124, 244704 (2006). [5] Oganov A.R., Lyakhov A.O., J. Superhard Mater. 32, 143-147 (2010). [6] Liu A.Y., Cohen M.L., Science 245, 841-842 (1989). [7] Dubrovinsky L.S., Dubrovinskaia N.A., et al., Nature 410, 653-654 (2001). [8] Kim D.Y., et al., Appl. Phys. Lett. 90, 171903 (2007).
Methane is an extremely important energy source with a great abundance in nature and plays a sign... more Methane is an extremely important energy source with a great abundance in nature and plays a significant role in planetary physics, being one of the major constituents of giant planets Uranus and Neptune. The stable crystal forms of methane under extreme conditions are of great fundamental interest. Using the ab initio evolutionary algorithm for crystal structure prediction, we found three novel insulating molecular structures with P212121, Pnma, and Cmcm space groups. Remarkably, under high pressure, methane becomes unstable and dissociates into ethane (C2H6) at 95 GPa, butane (C4H10) at 158 GPa, and further, carbon (diamond) and hydrogen above 287 GPa at zero temperature. We have computed the pressure-temperature phase diagram, which sheds light into the seemingly conflicting observations of the unusually low formation pressure of diamond at high temperature and the failure of experimental observation of dissociation at room temperature. Our results support the idea of diamond formation in the interiors of giant planets such as Neptune.
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