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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

Lattice thermal conductivity κlat of MgO at high pressures P and temperatures T up to 150 GPa and 4000 K are determined using lattice dynamics calculations and the linearized phonon Boltzmann transport equation (BTE) beyond the relaxation time approximation (RTA) from first principles. It is found that the complete solution of the linearized BTE substantially corrects values of κlat calculated with the RTA by ∼30%, from ∼42 to ∼54Wm-1K-1 under ambient conditions. The calculated values of κlat are in good agreement with those from the existing experiments. At conditions representative of the Earth's core-mantle boundary (P=136GPa and T=3800K), κlat is predicted to be ∼32 and ∼40Wm-1K-1 by RTA and the full solution of BTE, respectively. We report a detailed comparison of our study with earlier theoretical studies.

DOI： 10.1103/PhysRevB.95.184303

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics  

Pseudo-cubic tetragonal boron, which may be another form of boron allotropes, has recently been discovered under high pressure and high temperature conditions. In this paper, the structure of pseudo-cubic tetragonal boron is studied by density-functional-theory (DFT) calculation. The structure is abnormal compared with other boron allotropes in many respects, making it difficult to comprehend. The lattice is very close to a cubic lattice, such that the icosahedra are largely distorted along the c-axis. Such distortions are normally not favorable for boron crystals; in fact, the present calculations supported this. The reported positions of partially occupied interstitial sites render the intericosahedral bonds unusually long or short, which were again not supported by the present calculations. Furthermore, the potential of involving impurities is unlikely in terms of the formation energy and lattice parameters. Therefore, the structure of pseudo-cubic tetragonal boron was not proven by calculation, despite this extensive study. Something may be overlooked in the present structural model, or something unusual may have happened in this structure, the solution of which is left as an open question.

DOI： 10.7567/JJAP.56.05FB05

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

The MgSiO3 post-perovskite phase is the most abundant silicate phase in a super-Earth's mantle, although it only exists within the Earth's lowermost mantle. In this study, we established the thermal equation of state (EoS) of the MgSiO3 post-perovskite phase, which were determined by using both laser-heated diamond anvil cell and density-functional theoretical techniques, within a multi-megabar pressure range, corresponding to the conditions of a super-Earth's mantle. The Keane and AP2 EoS models were adopted for the first time to extract meaningful physical properties. The experimentally determined Gruneisen parameter, which is one of the thermal EoS parameters, and its volume dependence were found to be consistent with their theoretically obtained values. This reduced the previously reported discrepancy observed between experiment and theory. Both the experimental and theoretical EoS were also found to be in very good agreement for volumes at pressures and temperatures of up to 300 GPa and 5000 K, respectively. Our newly developed EoS should be applicable to a super-Earth's mantle, as well as the Earth's core-mantle boundary region.

DOI： 10.1038/srep22652

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

A post-cotunnite phase transition in zirconia (ZrO2) at high pressure was investigated by synchrotron X-ray diffraction measurements and ab initio calculations based on density functional theory. This study successfully demonstrated a cotunnite- to Fe2P-type phase transition. Static enthalpy difference (Delta H) calculations predicted the appearance of the Fe2P phase at 124 GPa (LDA) and 143 GPa (GGA), and experimental trials demonstrated the coexistence of the Fe2P and cotunnite phases at 175 GPa after heating to 3,000 K. Both phases were quenchable to ambient conditions. The volume of the Fe2P phase was slightly less (similar to Delta 0.6 %) than that of the cotunnite phase over the experimental pressure range, indicating that the Fe2P phase is the higher pressure phase. The coexistence of both phases in this study may be attributed to the slow kinetics of the phase transition resulting from the close structural relationship of the two phases. An Fe2P-type structural model can be derived by applying a simple operation to the cotunnite-type structure, consisting of a 1/2 shift of several zirconium arrangements parallel to the b-axis of the cotunnite-type unit cell. It is concluded that the high-pressure cotunnite-to-Fe2P phase transition may be a common trend in many dioxides.

DOI： 10.1007/s00269-014-0728-3

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The lattice thermal conductivity (κlat) of MgSiO 3 perovskite (Mg-Pv) under high-pressure and high-temperature conditions was computed based on the ab initio anharmonic lattice dynamics method with the density functional perturbation theory. κlat of Mg-Pv is found to increase with increasing pressure from 9.8 (at 23.5 GPa) to 43.6 W m-1 K-1 (at 136 GPa) at 300 K, while decreasing with increasing temperature from 28.1 (at 300 K) to 2.3 W m-1 K -1 (at 4000 K) at 100 GPa. A multiphase composite average yielded a mantle Rayleigh number adequate to promote the vigorous thermal convection of the mantle that is expected geophysically. © 2013 American Physical Society.

DOI： 10.1103/PhysRevLett.110.025904

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The phase relations in boron have been investigated at high pressure and high temperature using a multianvil apparatus, and the quenched sample has been analyzed by x-ray diffraction, Raman spectra, and transmission electron microscopy. We demonstrate that gamma-B-28 can be synthesized over a wide pressure and temperature range, and alpha-tetragonal B-50 (T-B-50) is obtained at higher temperatures and similar pressures. The phase boundary of the beta-B-106, gamma-B-28, and T-B-50 is determined at pressures between 7.0 and 18GPa and the temperatures of 500-2200 degrees C. The results suggest that T-B-50 might be an intermediate phase formed for kinetic reasons (Ostwald rule) on the way from beta-B-106 to beta-tetragonal B-192 (T-192) and gamma-B-28 to T-192.

DOI： 10.1103/PhysRevB.85.014107

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Li doping is a promising method for achieving metallization of alpha-rhombohedral boron (alpha-boron for short), which is a potential candidate for a high-T(c) superconducting material. Toward this end, a serious drawback has been the difficulty of doping alpha-boron, even though there are theoretical predictions claiming that it should be easy. This discrepancy has been systematically studied by the ab initio pseudopotential method through calculations of various structural and phonon properties of the material. For this study, a comparison with beta-boron is important because experimental data are available in this case. The present results demonstrate that while Li doping is difficult for alpha-boron under normal conditions, it is easy for beta-boron, which is completely consistent with experiments. The difference between these crystals originates from the contrasting characteristics of the bonding. For alpha-boron, the bonding requirement of the host crystal is fulfilled so well that the only way for a Li atom to enter the crystal is through the antibonding states. Electronically, this is favorable because it causes an almost perfect rigid-band shift without modifying the bonding nature of the host crystal. In terms of structural effects, Li doping causes a slight decrease in the cell angle alpha(rh) as well as softening of the elastic properties. A striking effect of Li doping is manifested in substantial phonon softening of the librational mode. These changes can be regarded as reliable criteria for the experimental detection of Li inclusion. On the other hand, beta-boron can be characterized as a frustrated system, and the crystal has a propensity to welcome guest atoms in order to eliminate ill-connected bonds. As a result, even though Li is easily incorporated into beta-boron, the carriers are not activated for electrical conduction. The remaining problem is how to overcome the difficulty of Li doping of alpha-boron. The most important contribution of this study lies in demonstrating the usefulness of high-pressure synthesis as an efficient doping method.

DOI： 10.1103/PhysRevB.84.094117

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Recently, it has been shown that, like beta-rhombohedral boron (beta-boron), alpha-rhombohedral boron (alpha-boron) exhibits superconductivity at high pressure. The transition pressure is similar to that of beta-boron, that is, about 160 GPa. This paper presents a theoretical and experimental study of the structural change in alpha-boron at high pressures. High-pressure (up to 200 GPa) X-ray experiments were performed at a synchrotron-radiation facility. At ambient temperature, the crystal of alpha-boron is stable over the whole range of pressures examined. There is no phase transitions, such as to alpha-Ga type, which were previously predicted. All the structural parameters vary continuously with pressure. The cause of metallization is a significant contraction of the intericosahedral three-center bond, which brings a higher coordination to the icosahedron and a transfer of charge from the intraicosahedoral bond to the three-center bond. Although everything is continuous, a careful analysis of the pressure dependence of the structural parameters by comparing experiment with ab initio calculation, reveals a characteristic change in the angle of the rhombohedral lattice and some of the internal coordinates of atoms at about 50 GPa. This is the pressure at which the metallization process begins. This structural change can be observed as the step in the pressure dependence of the electrical resistivity and the phonon softening. The interrelationships underlying these changes have been established on theoretical grounds. The long-debated issue of whether the intericosahedral bond is stronger than intraicosahedral bond has been resolved: the traditional understanding that the former is stronger has been proven. A direct piece of evidence for this is the initial deviation of the intericosahedral bond from the lattice vector.

DOI： 10.1143/JPSJ.80.084601

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Two high-pressure phase transitions in GeO2 have been discovered through first-principles computer simulations: the first is a transition from the pyrite-type (FeS2) to cotunnite-type (alpha-PbCl2) structure predicted to occur at a pressure of similar to 300 GPa, and the second is a transition from the cotunnite-type to the hexagonal Fe2P-type structure at similar to 600 GPa. The former is accompanied by a remarkable volume reduction of 5.4%, while the latter has a distinctive but quite small volume change of 0.3%. The post-pyrite transition to the cotunnite-type structure is expected from known high-pressure behavior of other dioxides, while the post-cotunnite transition to an Fe2P-type structure is quite unexpected, with no report in any dioxides so far except for a recent study on SiO2. The Fe2P-type phase has higher effective coordination numbers of Ge atoms, which contributes to stabilizing this structure relative to cotunnite. The results obtained extend our knowledge of the ultrahigh-pressure crystallography of dioxide materials.

DOI： 10.1103/PhysRevB.83.134114

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The Raman spectra of numerous dodecaborides have been measured on high-quality single crystals at ambient conditions with high spectral resolution and signal-to-noise ratio. Besides the strong Raman-active modes, numerous Raman-inactive modes occur in the spectra, indicating distortions of the structures. Ab initio calculation of the phonon spectra on ZrB12 excellently agrees with the experimental results. Force constants are theoretically calculated and force parameters are estimated from the Raman frequencies. The influence of the surface range on the Raman spectra is evident. The different isotopic effects (virtual crystal approximation, the polarization effect and the effect of isotopic disorder) on the phonon frequencies are determined, separated and discussed.

DOI： 10.1088/0953-8984/23/6/065403

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We report the discovery of a post-cotunnite phase of TiO2 by both density-functional ab initio calculations and high-pressure experiments. A pressure-induced phase transition to a hexagonal Fe2P-type structure (space group P6̄2m) was predicted to occur at 161 GPa and 0 K and successfully observed by in situ synchrotron x-ray diffraction measurements at 210 GPa and 4000 K with a significant increase in opacity. This change in opacity is attributed to a reduction of band gap from 3.0 to 1.9 eV across the phase change. The Fe2P-type structure is proved to be the densest phase in major metal dioxides. © 2011 American Physical Society.

DOI： 10.1103/PhysRevLett.107.045701

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Authorship：Lead author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Electronic structure of semiconducting boron carbide at high pressure has been theoretically investigated, because of interests in the positive pressure dependence of resistivity, in the gap closure, and in the phase transition. The most simplest form B-12(CCC) is assumed. Under assumptions of hydrostatic pressure and neglecting finite-temperature effects, boron carbide is quite stable at high pressure. The crystal of boron carbide is stable at least until a pressure higher than previous experiments showed. The gap closure occurs only after p = 600 GPa on the assumption of the original crystal symmetry. In the low pressure regime, the pressure dependence of the energy gap almost diminishes, which is an exceptional case for semiconductors, which could be one of reasons for the positive pressure dependence of resistivity. A monotonous increase in the apex angle of rhombohedron suggests that the covalent bond continues to increase. The C chain inserted in the main diagonal of rhombohedral structure is the chief reason of this stability.

DOI： 10.1088/1742-6596/215/1/012117

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Recent discovery of a compound BC5, whose structure is most likely diamond, invoked intensive interests in the superconductivity research. Little is known not only for its properties but for the preparation methods. For synthesizing BC5, it is very important to know under what conditions this compound can be obtained. In this paper, density-functional-theoretic study has been made on the stability of BC5 relative to a mixture of BC3 and graphite as a function of pressure. It is shown that BC5 becomes stable above 2 GPa. Therefore, high pressure synthesize is very useful.

DOI： 10.1088/1742-6596/215/1/012116

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER SCIENTIFIC PUBLISHERS  

An accurate calculation method for Fermi surfaces is presented in order to study the electronic doping effect to semiconductor alpha-boron. Given the eigenvalues of energy at a limited set of k points, the present interpolation method retains the original accuracy of the energy eigenvalues, which are provided by a band calculation program. A significant contribution of the present method is to eliminate spurious components in the interpolation which are present in other methods. This is achieved by removal of redundancy in the basis functions. An application is given for doped-semiconductor alpha-boron, which has recently attracted a growing interest in connection to superconductivity research.

DOI： 10.1166/jctn.2009.1326

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Recently, it has been discovered that the semiconductor alpha-boron becomes metallic at high pressures and finally undergoes a superconducting transition at 160 GPa, without causing any phase transition. Before the superconducting transition, a step is often observed in the pressure dependence of electrical resistivity in this class of boron crystals, which possess icosahedron-based structures. This step structure used to be thought to occur due to a phase transitions. In the present paper, we show that the step of alpha-boron at 50 GPa is not due to a phase transition. It is caused by a gradual change in the bonding character from semiconductor to metal. The increase in the metallic character is caused by the shortening of the three-center bond, which is a characteristic feature of icosahedron-based boron crystals. This shortening of the three-center bond enhances the bonding character of the conduction bottom band and finally closes the band gap. However, even far before the gap closing, the shortening has important consequences for the crystal properties of alpha-boron: for the changes in the deformation of the lattice and in the librational phonon mode at approximately 50 GPa. This change in structure affects the change in band structure, that is, the bottom of the conduction band initially moves from the Gamma point toward the Z point until p = 50 GPa and then almost terminates in the middle point. The step in resistivity is a direct consequence of the change in this energy gap. This provides a coherent understanding of the characteristic changes in resistivity, the deformation of the lattice, and the librational mode.

DOI： 10.1143/JPSJ.78.084714

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

Use of a high-pressure process is proposed for synthesizing highly doped semiconductors. Working on a real example in a recent experiment by Ekimov on B-doped diamond synthesis, a theoretical investigation has been made of an efficient doping method. The underlying principle is simple: stiffer materials are energetically favorable over softer ones under high pressure. Softer impurities are easily dissolved in stiff crystals. In Ekimov's method, the energy required for a B atom to transfer from boron carbide to diamond comes from the energy gain by converting graphite to diamond, in addition to the entropy effect of dissolving the B atom in diamond. On the basis of this analysis, a potential of more than 10 at% of doping, which would improve T-c, by high-pressure processes is suggested. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssb.200880522

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Authorship：Lead author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

A theoretical assessment of doping alpha-boron with hydrogen is presented aiming at high T-c superconductor. Dissimilar to other impurities, hydrogen is unique because T site is the most stable site. This position is well suited to the three-center bond. The bottom of conduction band has a large component at this center. Insertion of H atom increases the bonding character of this band through the three-center bond, which is otherwise a weak bond. Enhancement of bonding character of the three-center bond merges the bottom conduction and the top valence bands, yielding a relatively large density of states (DOS) at the Fermi level. This large DOS suggests that H doped alpha-boron is a promising high T-c material. When the gaseous state H-2 is used as the starting material, the formation energy is above 2 eV at the normal condition, so that heavy doping is not easy. High-pressure process can avoid this difficulty. It is predicted that hydrogen is incorporated easily at p > 18 GPa. A thermodynamic investigation has been made for high-pressure doping method when gases are used as the starting materials.

DOI： 10.1088/1742-6596/176/1/012005

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Authorship：Lead author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Recently, we have shown by density functional theory calculations that alpha-boron is the most stable phase at low temperature among icosahedron-based boron crystals. This is consistent with low-temperature crystal growth of alpha-boron. A problem of the calculation is, however, that the conclusion would be changed if defect states for beta-boron, which are observed in experiment, were taken into account. This urges us to suspect the perfectness of alpha-boron crystal. In this paper, we investigate possibility of imperfections of alpha-boron from the standpoint of crystal stability. Intrinsic defects of vacancy and self-interstitial have been examined. Based on the total energy calculations, we conclude that intrinsic defects are not present at a level above 0.1 at%.

DOI： 10.1088/1742-6596/176/1/012004

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Boron solids are, similar to diamond, strong electron-phonon coupling systems and therefore potentially have high-Tc superconductivity. For realizing high transition temperature Tc, however, careful control of the valence electrons is necessary, which requires detailed understanding of the electronic structure. By critically reviewing recent development in this field, along with the situation in heavily doped diamond and clathrates, we show a promising route for high Tc in boron system. Although both α- and β-boron become metallic and exhibit superconductivity at high pressure, the strong covalent character still remains, which significantly limits its Tc achieved by applying high pressure. Heavy doping of related compounds is desirable, as long as the basic electronic structure is retained, but it is extremely difficult at normal conditions. A promising method to achieve this is high pressure synthesis. Similar technique is useful for heavy doping of diamond. To design high-pressure experiment, the role of theory is important through predicting the phase diagram, which was not available for boron previously. © 2009 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/176/1/012001

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Theoretical investigation has been made for the possibility of efficient doping to a boron, aiming at superconducting application. The idea is that application of high pressure makes it easier to dope soft impurities to stiff host crystals. Along with this line of thought, several of the dopants, Na, Ca, C, H and Li, have been examined. For Na, and H as well as Li, this can be achieved at moderated pressures. In terms of the density of states, Li is the best for superconducting purpose. However, H may also have potential to this application. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2007.09.056

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

It was previously predicted that doping Li into semiconducting boron (alpha-rhombohedral) brought metallic character to the matrix and possibility a high- Tc superconductor. However, experiments show that Li doping of alpha-rhombohedral boron is difficult. In this paper, the potential for Li doping of boron is reexamined using the ab initio pseudopotential method. Based on the calculated formation enthalpy, an efficient method for doping is proposed. The method utilizes high pressure, such as 10 GPa. Slight changes in the structural parameters for Li insertion are also resolved, which may be useful for the experimental detection of Li in boron. The stability of alpha-rhombohedral boron at high pressures is also compared to that of Ga- type structure, which has been put forth as a candidate for the high-pressure phase. The present study gives further confirmation of the stability of alpha-rhombohedral boron at least up to 70 GPa.

DOI： 10.1080/0953-8984/19/36/365241

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Recently, an elastic softening at low temperatures has been observed in Si by an ultrasonic measurement. By assuming that an intrinsic defect of vacancy causes this softening, which is suggested by the experiment, the effects of monovacancy on the elastic properties have been theoretically studied. The primal concern of this paper is to survey existence of a low-energy excitation, which would be expected commonly for such an abnormal behavior. Although the static ground-state of monovacancy is stiff owing to the strong Jahn-Teller distortion, it is found that a low-energy excitation really exists in the motion of the neighboring Si atoms surrounding the vacancy.

DOI： 10.1088/1742-6596/92/1/012063

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

It is believed that hydrogen in silicon is located at the bond center. But, the hydrogen atom is actually deviated significantly from the bond center dynamically, which is illustrated by the present calculations. The fast relaxation to bending causes a large decay rate of the H vibration.

DOI： 10.1088/1742-6596/92/1/012147

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