| 11. |
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| 12. |
- Grechnev, Alexei, et al.
(författare)
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Layered compound Nb3SiC2 predicted from first-principles theory
- 2004
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 85:15, s. 3071-3073
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Tidskriftsartikel (refereegranskat)abstract
- A previously unobserved ternary carbide, Nb3SiC2, belonging to the family of the so-called Mn+1AXn or MAX phases is predicted from first-principles calculations. It has a theoretical bulk modulus of 296 Gpa, wich is much higher than that of Ti3SiC2. The new phase is metastable with a formation energy of +0.02 eV/atom. We suggest that the phase may possibly be synthesized using thin film technology. The chemical binding of Nb3SiC2 is investigated using the balanced crystal orbital overlap population indicator and it is found to be dominated by the formation of Nb4D-C 2p covalent bonds.
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| 13. |
- Grechnev, G. E., et al.
(författare)
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Magnetic susceptibility of hcp iron and the seismic anisotropy of Earth's inner core
- 2003
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Ingår i: PHYSICAL REVIEW B. - 1098-0121. ; 68:6, s. 064414-
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Tidskriftsartikel (refereegranskat)abstract
- The seismic anisotropy of the Earth's core is believed to be due to a preferred orientation of hexagonal close packed (hcp) iron crystals that constitute the dominating element in the inner core. In this connection, the magnetic properties of the hcp iron in an external magnetic field are very interesting and are studied here by employing an ab initio full-potential linear muffin tin orbital method. By this means the magnetic susceptibility chi of hcp iron and its anisotropy energy for pressures and temperatures corresponding to the Earth's inner core conditions have been evaluated in the framework of the local spin density approximation. The accuracy of this method has been validated by calculating the anisotropic susceptibility of paramagnetic transition metals that form in the hcp crystal structure at ambient conditions. Our calculations demonstrate that for hcp iron the anisotropy of chi is dependent on the c/a ratio. In conjunction with recent data on the c/a ratio and elastic constants of hcp iron, the magnetic anisotropy can explain the seismic anisotropy of the Earth's inner core.
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| 14. |
- Kollar, J., et al.
(författare)
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Calculation of surface stress for fcc transition metals
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 68:24, s. 245417-
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Tidskriftsartikel (refereegranskat)abstract
- Using the density functional theory, formulated within the framework of the exact muffin-tin orbitals method, we have calculated the surface stress for the (111) free surfaces of the fcc 4d and 5d transition metals. Good agreement is obtained with the available ab initio data for Pd, Ir, and Au, while for Pt we predict a surface stress, which is about 33% lower compared to former theoretical results. The present surface stress values for the 4d and 5d fcc metals show the typical trend characteristic for the cohesive or surface energies of d series.
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| 15. |
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| 16. |
- Li, S., et al.
(författare)
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Crystallographic structures of PbWO4
- 2003
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Ingår i: High Pressure Research. - : Informa UK Limited. - 0895-7959 .- 1477-2299. ; 23:3, s. 343-347
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Tidskriftsartikel (refereegranskat)
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| 17. |
- Li, Sa, 1975-
(författare)
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Materials Design from ab initio Calculations
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents a theoretical study of bulk materials using ab initio methods based on the density functional theory (DFT).Crystallographic structural phase transformations and phase stability for 5f-dioxides, ABO3 perovskites, and ABO4 compounds have been extensively studied. Different approaches such as static total energy calculations, elastic stability and dynamical stability (phonon calculations) criteria have been used to determine the phase stability. As a special case, the lattice dynamics of solid Xe has been studied as a function of pressure.Dielectric functions and optical constants have been calculated for solar energy cell system CuIn1-xGaxSe2 with concentrations x=0, 0.25, 0.5 and 1.0 as well as for C60, PbWO4 and δ-AlOOH. The absorption coefficient provides information about the optimum solar energy conversion efficiency. We have derived absorption coefficients for a number of compounds. Comparisons between the calculated and experimental dielectric functions and absorption coefficients have been made.The main part of this thesis focuses on the nanolayered ternary compounds M N+1AXN (MAX), where N = 1, 2 or 3, M is an early transition metal, A is an A-group (mostly IIIA and IVA) element, and X is either C and/or N. These ternary carbides and nitrides combine unusual properties of both metals and ceramics. They exhibit high hardness, but fully reversible plasticity, and negligible thermoelectric power. These excellent properties make the MAX phases another new class of materials with versatile technological applications. Our work presents a systematic study of the electronic, bonding, elastic and optical properties of the MAX phases. A new MAX phase-Ti4SiC3, is calculated to be stable, and at the same time also been synthesized by experimentalists. Surface energy calculations have also been performed for the (0001) surface of the Ti-Si-C system. The general relations between the electronic structure and materials properties of the MAX phases have been elaborated in the thesis.
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| 18. |
- Marqués, M., et al.
(författare)
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Pressure effects on the structure and vibrations of β- and γ-C3N4
- 2004
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 70:10, s. 104114-
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of an investigation on the pressure behavior of structural, bonding and vibrational properties of β- and γ-C3N4 phases. Emphasis is focused on the trends of the calculated properties along the A3N4 (A: C, Si, Ge) family. Geometry optimizations and electronic structure calculations are carried out in the framework of the local density functional theory using a planewave-pseudopotential scheme. The equilibrium cell geometry, the isothermal bulk modulus and its pressure derivatives have been evaluated for the two phases in a pressure range up to 400 GPa. The Bader’s Atoms in Molecules formalism is applied to characterize the chemical bonding and the atomic contributions to the bulk compressibility in β- and γ-A3N4 crystals. The calculated stability diagram reveals the occurrence of a hypothetical β-C3N4→γ-C3N4 phase transition around 370 GPa. Γ-point harmonic vibrational modes are computed at selected pressures within the density functional perturbation theory approach. In agreement with recent theoretical calculations, the comparison of the computed Raman vibrational frequencies with experimental estimations for β-C3N4 raises doubts on a previously reported synthesis of this structure. Pressure effects on the vibrational frequencies inform of a reduction of the P63∕m symmetry of the β phase at pressures around 60 GPa and contribute to look into the mechanical stability of the γ phase.
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| 19. |
- Meresse, Y, et al.
(författare)
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X-ray diffraction studies of AuCu3-type neptunium compounds under pressure
- 2000
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Ingår i: JOURNAL OF ALLOYS AND COMPOUNDS. - : ELSEVIER SCIENCE SA. - 0925-8388. ; 296:1-2, s. 27-32
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Tidskriftsartikel (refereegranskat)abstract
- NpX3 (AuCu3-type structure) compounds with X=Al, Ga, Ge, In and Sn were studied up to 50 GPa using the energy dispersive X-ray diffraction (EDXRD) technique. No structural phase transitions were observed for any of the compounds studied up to the highest
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| 20. |
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