Prediction of different crystal structure phases in metal borides: A lithium monoboride analog to $\mathrm{Mg}{\mathrm{B}}_2$

Modern compound prediction methods can efficiently screen large numbers of crystal structure phases and direct the experimental search for new materials. One of the most challenging problems in alloy theory is the identification of stable phases with a never seen prototype; such predictions do not always follow rational strategies. While performing ab initio data mining of intermetallic compounds we made an unexpected discovery: even in such a well-studied class of systems as metal borides there are previously unknown layered phases comparable in energy to the existing ones. With ab initio calculations we show that the new metal-sandwich (MS) lithium monoboride phases are marginally stable under ambient conditions but become favored over the known stoichiometric compounds under moderate pressures. The MS lithium monoboride exhibits electronic features similar to those in magnesium diboride and is expected to be a good superconductor.

Figure 1

(Color online) Known ($\mathrm{Al}{\mathrm{B}}_2$, $V2$) and proposed metal-sandwich (MS1, MS2) prototypes. The hexagonal layers of boron (gray) are separated by triangular layers of metal (yellow, only four atoms per layer are shown).

Figure 2

(Color online) Calculated relative enthalpy, $\Delta H\left(P\right)\equiv H\left(P\right)-H_{\alpha \text{−}\mathrm{Li}\mathrm{B}}\left(P\right)$, as a function of pressure for lithium monoboride phases. The points are connected with splines. The larger yellow spheres are lithium atoms and the smaller black spheres connected with sticks are boron atoms.

Figure 3

(Color online) Slices of the potential-energy surface in MS2-LiB along the eigenvectors of the softest phonon modes at $\Gamma$. The modes represent the interlayer sliding in the $x$ (see the [010] view) and $y$ directions of undistorted boron layers rigidly locked with the adjacent lithium layers. The inset demonstrates two shallow minima at small displacements. Ab initio data points are connected with splines.

Figure 4

(Color online) Band structure and partial density of states (PDOS) in MS2-LiB, calculated in $\mathrm{APW}+\mathrm{lo}$ (Refs. 21, 38). PDOS units are states/(eV∙spin∙boron atom). The thickness of band structure lines is proportional to boron $p_{x,y}$ (red) and $p_z$ (green) characters.