Results of x-ray diffraction and nitrogen K-edge x-ray Raman scattering (XRS) investigations of the crystal and electronic structure of ionic compound Li3N across two high-pressure phase transitions [A. Lazicki , Phys. Rev. Lett. 95, 165503 (2005)] are interpreted using density-functional theory. A low-energy peak in the XRS spectrum which is observed in both low-pressure hexagonal phases of Li3N and absent in the high-pressure cubic phase is found to originate from an interlayer band similar to the important free-electron-like state present in the graphite and graphite intercalated systems, but not observed previously in ionic insulators. XRS detection of the interlayer state is made possible because of its strong hybridization with the nitrogen p bands. A pressure-induced increase in the band gap of the high-pressure cubic phase of Li3N is explained by the differing pressure dependencies of different quantum-number bands and is shown to be a feature of several low-Z closed-shell ionic materials.

Results of x-ray diffraction and nitrogen K-edge x-ray Raman scattering (XRS) investigations of the crystal and electronic structure of ionic compound Li3N across two high-pressure phase transitions [A. Lazicki , Phys. Rev. Lett. 95, 165503 (2005)] are interpreted using density-functional theory. A low-energy peak in the XRS spectrum which is observed in both low-pressure hexagonal phases of Li3N and absent in the high-pressure cubic phase is found to originate from an interlayer band similar to the important free-electron-like state present in the graphite and graphite intercalated systems, but not observed previously in ionic insulators. XRS detection of the interlayer state is made possible because of its strong hybridization with the nitrogen p bands. A pressure-induced increase in the band gap of the high-pressure cubic phase of Li3N is explained by the differing pressure dependencies of different quantum-number bands and is shown to be a feature of several low-Z closed-shell ionic materials.

The phonon density of states (DOS) and phonon entropy of B2 FeAl were determined as functions of the Fe site vacancy concentration using several scattering techniques and were computed from first principles. Measurements at elevated temperature and pressure were performed to explore volume effects, test the usefulness of the quasiharmonic (QH) approximation, and provide comparison for the first-principles calculations. The average temperature and pressure dependencies of phonons were consistent with the QH model. The decrease in specific volume associated with the introduction of vacancies causes a stiffening of the DOS that was captured well with the experimentally determined Gruumlneisen parameter. Features associated with vacancies in the DOS are not well explained by this model, however, especially in the gap between the acoustic and optic branches. First-principles calculations indicated that these modes are primarily associated with vibrations of Al atoms in the first-nearest-neighbor shell of the vacancy, with some vibration amplitude also involving the second-nearest-neighbor Fe atoms. At the vacancy concentrations of study, the phonon entropy of vacancy formation was found to be approximately -1.7k(B)/atom, about half as large and of opposite sign as the configurational entropy of vacancy formation.

The phonon density of states (DOS) and phonon entropy of B2 FeAl were determined as functions of the Fe site vacancy concentration using several scattering techniques and were computed from first principles. Measurements at elevated temperature and pressure were performed to explore volume effects, test the usefulness of the quasiharmonic (QH) approximation, and provide comparison for the first-principles calculations. The average temperature and pressure dependencies of phonons were consistent with the QH model. The decrease in specific volume associated with the introduction of vacancies causes a stiffening of the DOS that was captured well with the experimentally determined Gruumlneisen parameter. Features associated with vacancies in the DOS are not well explained by this model, however, especially in the gap between the acoustic and optic branches. First-principles calculations indicated that these modes are primarily associated with vibrations of Al atoms in the first-nearest-neighbor shell of the vacancy, with some vibration amplitude also involving the second-nearest-neighbor Fe atoms. At the vacancy concentrations of study, the phonon entropy of vacancy formation was found to be approximately -1.7k(B)/atom, about half as large and of opposite sign as the configurational entropy of vacancy formation.

A nanocrystalline face-centered cubic (fcc) solid solution of 6% Fe in Cu was prepared by high-energy ball milling, and annealed at temperatures from 200 to 360 degrees C to induce chemical unmixing. The chemical state of the material was characterized by three-dimensional atom probe microscopy, Mossbauer spectrometry and X-ray powder diffractometry. The unmixing was heterogeneous, with iron atoms forming iron-rich zones that thicken with further annealing. The phonon partial density of states (pDOS) of Fe-57 was measured by nuclear resonant inelastic X-ray scattering, showing the pDOS of the as-prepared material to be that of an fcc crystal. The features of this pDOS became broader in the early stages of unmixing, but only small changes in average phonon frequencies occurred until the body-centered cubic (bcc) phase began to form. The vibrational entropy calculated from the pDOS underwent little change during the early stage of annealing, but decreased rapidly when the bcc phase formed in the material. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Two sp + sp(3)-hybridized yne-diamond (YD) allotropes are designed by employing first-principle calculations. The YDs are constructed by replacing half carbon single bonds (C-C) along the <001> direction in 2H-diamond and 4H-diamond with acetylenic linkages (C-C=C-C). Both YDs are energetically more favorable than experimental graphdiyne, theoretical graphynes (e.g., alpha-, beta-, and 6,6,12-graphyne), and T-carbon. The YDs are confirmed to be mechanically and dynamically stable. Different from the recently proposed semiconductive YD based on cubic diamond (i.e. Y-carbon), electronic band structure calculations show that both YDs we proposed are semimetals. Mechanically, two YDs inherit the superhardness and high tensile strength from the parent diamonds. We hope that our present findings can be useful in guiding the design and syntheses of superhard and semimetallic carbon materials. (C) 2014 Elsevier B.V. All rights reserved.

Two sp + sp(3)-hybridized yne-diamond (YD) allotropes are designed by employing first-principle calculations. The YDs are constructed by replacing half carbon single bonds (C-C) along the <001> direction in 2H-diamond and 4H-diamond with acetylenic linkages (C-C=C-C). Both YDs are energetically more favorable than experimental graphdiyne, theoretical graphynes (e.g., alpha-, beta-, and 6,6,12-graphyne), and T-carbon. The YDs are confirmed to be mechanically and dynamically stable. Different from the recently proposed semiconductive YD based on cubic diamond (i.e. Y-carbon), electronic band structure calculations show that both YDs we proposed are semimetals. Mechanically, two YDs inherit the superhardness and high tensile strength from the parent diamonds. We hope that our present findings can be useful in guiding the design and syntheses of superhard and semimetallic carbon materials. (C) 2014 Elsevier B.V. All rights reserved.

Earth's inner core is known to consist of crystalline iron alloyed with a small amount of nickel and lighter elements, but the shear wave (S wave) travels through the inner core at about half the speed expected for most iron-rich alloys under relevant pressures. The anomalously low S-wave velocity (v(S)) has been attributed to the presence of liquid, hence questioning the solidity of the inner core. Here we report new experimental data up to core pressures on iron carbide Fe7C3, a candidate component of the inner core, showing that its sound velocities dropped significantly near the end of a pressure-induced spin-pairing transition, which took place gradually between 10 GPa and 53 GPa. Following the transition, the sound velocities increased with density at an exceptionally low rate. Extrapolating the data to the inner core pressure and accounting for the temperature effect, we found that low-spin Fe7C3 can reproduce the observed vS of the inner core, thus eliminating the need to invoke partial melting or a postulated large temperature effect. The model of a carbon-rich inner core may be consistent with existing constraints on the Earth's carbon budget and would imply that as much as two thirds of the planet's carbon is hidden in its center sphere.

Earth's inner core is known to consist of crystalline iron alloyed with a small amount of nickel and lighter elements, but the shear wave (S wave) travels through the inner core at about half the speed expected for most iron-rich alloys under relevant pressures. The anomalously low S-wave velocity (v(S)) has been attributed to the presence of liquid, hence questioning the solidity of the inner core. Here we report new experimental data up to core pressures on iron carbide Fe7C3, a candidate component of the inner core, showing that its sound velocities dropped significantly near the end of a pressure-induced spin-pairing transition, which took place gradually between 10 GPa and 53 GPa. Following the transition, the sound velocities increased with density at an exceptionally low rate. Extrapolating the data to the inner core pressure and accounting for the temperature effect, we found that low-spin Fe7C3 can reproduce the observed vS of the inner core, thus eliminating the need to invoke partial melting or a postulated large temperature effect. The model of a carbon-rich inner core may be consistent with existing constraints on the Earth's carbon budget and would imply that as much as two thirds of the planet's carbon is hidden in its center sphere.

A new synchrotron radiation experimental capability of coupling nuclear resonant inelastic X-ray scattering with the cryogenically cooled high-pressure diamond anvil cell technique is presented. The new technique permits measurements of phonon density of states at low temperature and high pressure simultaneously, and can be applied to studies of phonon contribution to pressure-and temperature-induced magnetic, superconducting and metal-insulator transitions in resonant isotope-bearing materials. In this report, a pnictide sample, EuFe2As2, is used as an example to demonstrate this new capability at beamline 3-ID of the Advanced Photon Source, Argonne National Laboratory. A detailed description of the technical development is given. The Fe-specific phonon density of states and magnetism from the Fe sublattice in (EuFe2As2)-Fe-57 at high pressure and low temperature were derived by using this new capability.