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The superconductivity of alkali-metal intercalated BC2, MxBC2 (M = Li, Na, and K; x = 0.5–1.5), has been studied using first-principles calculations. The calculated critical temperature (Tc) values are substantially high at x = 0.5 (49.8–57.1 K), which are higher than those for MgB2 and close to those predicted for LixByCz compounds. The Tc values at x = 1.5 are comparatively low (0.6–5.6 K) and close to those for graphite intercalation compounds. No superconductivity is observed at x = 1.0 for all alkali metals. An analysis of the electronic structures reveals that at x = 0.5, the state at the Fermi energy includes the σ bond character. In contrast, at x = 1.5, the state includes only π bonds comprising pz orbitals of B and C atoms. The σ bond character is essential for attaining high Tc values because the σ bond couples strongly with the bending-like phonon modes of the BC2 layer. However, the π bond couples weakly with the stretching-like phonon modes due to the small overlap of the pz orbitals, which results in a relatively low Tc for the material.

Tungsten diboride, WBj, usuafly has a hexagonal structure with the space group P63/mmc (number 194); and molybdenum diboride, MoB2, has a trigonal structure with R３̄m (number 166). Other than these phases, both diborides are reported to have a phase with an AIB2-type structure (P6/ mmm, number 191). AlB2-type MoB2 is easy to synthesize and has been extensively studied, whereas AlB2-type WB2 is very difficult to synthesize and has appeared only once in a report by Woods et al. in 1966 (Woods, H. P.; Wawner, Jr., F. E.; Fox, B. G. Science 1966, 151, 7S.) We have investigated these diborides by means of first-principles calculations and found that boron defects are responsible for the difference in their synthesizability. AlB2-type MoB2 became stable enough with some boron defects added, while AlB2-type WB2 became minimally stable, suggesting it may not actually exist. Following our calculations, we attempted to synthesize AlB2-type WB2 with the optimum our calculations, we attempted to syntnesize AIB2-type WB2 with the optimum quantity of boron defects but observed no trace of it We conclude, from both calculations and experiments, that AIB2-type WB2 does not exist stably in the W-B phase diagram and that the compound produced in Woods et al.'s report might have contained some impurities. [ABSTRACT FROM AUTHOR]

The stability of lithium atoms in α-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 α-rhombohedral boron would have to be very small below 1400 K. [ABSTRACT FROM AUTHOR]