Prediction of stable insulating intermetallic compounds

We explore the stability of structures exhibiting hybridization gaps across a broad range of binary and ternary intermetallic compositions by means of band structure and total energy calculations. This search reveals previously unknown metal-based insulators, some with large gaps exceeding 1 eV, such as Al${}_2$Fe and Al${}_4$IrRe. We confirm large gaps using a hybrid density functional including exact exchange, and predict a gap of 2.2 eV for AlMnSi in the Pearson type tP6 structure, which is a chemically ordered ternary variant of the prototype MoSi${}_2$ (Pearson type tI6) structure.

Figure 1

Structures of the class. Colors indicate atomic species, size indicates vertical height. (a) Al${}_2$Os in the Pearson tI6 (MoSi${}_2$) structure. TM hexagons in alternate layers outlined in solid and dashed lines. (b) AlMnSi in tP6 structure. View along (001) axis reveals uniform chemical identity of layers, and relationship to cP2 (CsCl) structure. (c) CrSi${}_2$ in the hP9 structure revealing three stacked hexagons. (d) Generic hexagon motif in single layer. (e) Al${}_2$Ru in oF24 (Si${}_2$Ti) structure reveals four stacked hexagon motifs. (f) Ternary decoration of Al${}_4$IrRe in oF24 structure.

Figure 2

Phase diagram of Al-Mn-Si. Heavy circles indicate known stable phases, light circles indicate known high-temperature phases, squares indicate unknown structures or compositions. $\Delta H$ values are enthalpy of formation (meV/atom) of stable structures. Positive $\Delta E$ values are energies above the convex hull for unstable structures, while negative $\Delta E$ is the formation enthalpy with respect to adjacent compositions for stable structures.

Figure 3

(a) Electronic density of states of AlMnSi in the MoSi${}_2$.tI6 structure. Shown are DOS for the PBE and HSE06 functionals with $E_F=0$. A threshold of 0.2 states/eV/atom defines the pseudogap. (b) Band structure of AlMnSi in the MoSi${}_2$.tI6 structure calculated using the HSE06 functional.