paper

Thermodynamic stabilities of ternary metal borides: An ab initio guide for synthesizing layered superconductors

arXiv:0806.0061 · doi:10.1103/PhysRevB.78.094520

Abstract

Density functional theory calculations have been used to identify stable layered Li- $M$ -B crystal structure phases derived from a recently proposed binary metal-sandwich (MS) lithium monoboride superconductor. We show that the MS lithium monoboride gains in stability when alloyed with electron-rich metal diborides; the resulting ordered Li $_{2 (1 - x)} M_{x}$ B $_{2}$ ternary phases may form under normal synthesis conditions in a wide concentration range of $x$ for a number of group-III-V metals $M$ . In an effort to pre-select compounds with the strongest electron-phonon coupling we examine the softening of the in-plane boron phonon mode at $I^$ in a large class of metal borides. Our results reveal interesting general trends for the frequency of the in-plane boron phonon modes as a function of the boron-boron bond length and the valence of the metal. One of the candidates with a promise to be an MgB $_{2}$ -type superconductor, Li $_{2}$ AlB $_{4}$ , has been examined in more detail: according to our {\it ab initio} calculations of the phonon dispersion and the electron-phonon coupling $I^»$ , the compound should have a critical temperature of $\sim 4$ K.

10 pages, 9 figures, submitted to PRB