Li doping to boron at high pressures

Abstract

Analogy with alkali-doped fulleranes suggests the advent of high-Tc superconductivity in Li-doped α-rhombohedral (R) boron. The difficulty in preparation of doped α-R boron hindered the potential of superconductivity. Recently, by improving the preparation method, doping of Li to α-R has been successful, showing superconductivity. Unfortunately, the Tc is not so high as expected, namely, around 5 K for a Li content of 2.5 at.% (B12Li0.3). Band calculations show that the DOS at the Fermi level increases as the Li concentration is increased, reaching the maximum at the Li content 7.7 at.% (B12Li), where the highest Tc is predicted. A practical problem is the difficulty of doping with such high concentrations. An idea of use of Li-doped β-R boron with the aid of a high-pressure technique has been proposed. High concentrations were already achieved for β-R boron up to the Li content 15 at.% (B105Li18), although these samples were nonmetal. The recently established phase diagram of boron shows that β-R boron is transformed to α-R boron at high pressures. From this, it may be possible that, starting from Li-doped β-R boron, subjecting it to high pressure, we will have a transition to Li-doped α-R boron. This paper presents a theoretical study to examine the possibility of this transformation. It is found that Li-doped β-R boron is an alloy system, while it has two stoichiometric compounds with the compositions B104Li8 and B104Li16. These compounds are stable semiconductors at ambient pressure, where hardening takes place on heavily doping. The transformation from β-R B104Li8 to α-R B12Li will occur at p>72 GPa, provided that a path to further high-concentration compound B104Li16 is avoided by some means.