First-principles study on metal-doped icosahedral B12 solids

Valence control of α-rhombohedral boron by electronic doping

It was previously predicted that doping Li into semiconducting boron (α-rhombohedral) brought metallic character to the matrix and possibility a high-T(c) superconductor. However, experiments show that Li doping of α-rhombohedral boron is difficult. In this paper, the potential for Li doping of boron is re-examined 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 α-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 α-rhombohedral boron at least up to 70 GPa.

Synthesis, crystal structure, and properties of two modifications of MgB(12)C(2)

Single crystals of two modifications of the new magnesium boride carbide MgB(12)C(2) were synthesized from the elements in a metallic melt by using tantalum ampoules. Crystals were characterized by single-crystal X-ray diffraction and electron microprobe analysis (energy-dispersive (EDX) and wavelength-dispersive (WDX) X-ray spectroscopy). Orthorhombic MgB(12)C(2) is formed in a Cu/Mg melt at 1873 K. The crystal structure of o-MgB(12)C(2) (Imma, Z=4, a=5.6133(10), b=9.828(2), c=7.9329(15) A, 574 reflections, 42 variables, R(1)(F)=0.0208, wR(2)(I)=0.0540) consists of a hexagonal primitive array of B(12) icosahedra with Mg atoms and C(2) units in trigonal-prismatic voids. Each icosahedron has six exohedral B--B and six B--C bonds. Carbon is tetrahedrally coordinated by three boron atoms and one carbon atom with a remarkably long C--C distance of 1.727 A. Monoclinic MgB(12)C(2) is formed in an Al/Mg melt at 1573 K. The structure of m-MgB(12)C(2) (C2/c, Z=4, a=7.2736(11), b=8.7768(13), c=7.2817(11) A, beta=105.33(3) degrees , 1585 reflections, 71 variables, R(1)(F)=0.0228, wR(2)(I)=0.0610) may be described as a distorted cubic close arrangement of B(12) icosahedra. Tetrahedral voids are filled by C atoms and octahedral voids are occupied by Mg atoms. The icosahedra are interconnected by four exohedral B--B bonds to linear chains and by eight interstitial C atoms to form a three-dimensional covalent network. Both compounds fulfill the electron-counting rules of Wade and Longuet-Higgins.

Theoretical Study of the Stability of Lithium Atoms in α-Rhombohedral Boron

The stability of lithium atoms in alpha-rhombohedral boron was investigated by first-principles calculations of total energies and molecular dynamics (MD) simulations. In the case of a low concentration (1.03 at. %), Li at the center of the icosahedral B12 site (the I-site) had a negative binding energy, which suggests Li at the I-site is unstable. However, MD simulations at temperatures below 750 K indicated that Li is still confined in the B12 cage under these conditions, which means Li at the I-site is metastable. Over 800 K, Li began to move away from the B12 site and settled at the tetrahedral site (the T-site) or at the octahedral site (the O-site). Li at the T-site also had a negative binding energy, but MD simulations indicated it was metastable up to 1400 K and did not move to other sites. Li at the O-site was energetically the most favorable, having a positive binding energy. In the case of a high concentration (7.69 at. %), the I-site changed to an unstable saddle point. At this concentration, the T-site was metastable and the O-site became the most stable. In MD simulations at 1400 K, Li atoms at the O-site never jumped to other sites regardless of concentration. Considering these facts, the diffusion coefficient of Li in alpha-rhombohedral boron would have to be very small below 1400 K.

Development of a high energy resolution electron energy-loss spectroscopy microscope

We have developed a high energy resolution electron energy-loss spectroscopy (EELS) microscope, which can take spectra from specified small specimen areas and specified small reciprocal space areas to investigate detailed electronic structures. The EELS microscope is equipped with retarding Wien filters as the monochromator and the analyser. The filters are designed to achieve a stigmatic focus. The energy resolutions are 12 meV and 25 meV for cases without and with a specimen, respectively. Spatial and momentum resolutions are 30-110 nm in diameter and 1.1 nm-1 in angular diameter, respectively. EELS spectra are presented to show the performance of this instrument.