Structure, Stability and Mechanical Properties of Boron-Rich Mo-B Phases: A Computational Study

TL;DR

This computational study investigates the crystal structures, stability, and mechanical properties of boron-rich Mo-B phases, revealing stable structures, phase behavior, and potential superhardness of MoB5.

Contribution

The paper introduces a detailed lattice model for boron-rich Mo-B phases and identifies the most stable structures and compositions, including the potential superhardness of MoB5.

Findings

Stable structures of Mo-B phases closely match experimental data.

Boron-rich phases like MoB5 are potentially superhard with hardness 37-39 GPa.

Disordered nonstoichiometric phases form due to uniform distribution of B3 units.

Abstract

Molybdenum borides were studied theoretically using first-principles calculations, empirical total energy model and global optimization techniques to determine stable crystal structures. Our calculations reveal the structures of known Mo-B phases, attaining close agreement with experiment. Following our developed lattice model, we describe in detail the crystal structure of boron-rich $MoB_x$ phases with 3<x<9 as the hexagonal $P6_3/mmc$-$MoB_3$ structure with Mo atoms partially replaced by triangular boron units. The most energetically stable arrangement of these $B_3$ units corresponds to their uniform distribution in the bulk of the crystal structure, which leads to the formation of a disordered nonstoichiometric phase, with ordering arising at compositions close to x=5 due to a strong repulsive interaction between neighboring $B_3$ units. The most energetically favorable structures of $MoB_x$ correspond to the compositions 4<x<5, with $MoB_5$ being the boron-richest stable phase. The estimated hardness of $MoB_5$ is 37-39 GPa, suggesting that the boron-rich phases are potentially superhard.