Theoretical study of metal borides stability

TL;DR

This paper uses ab initio calculations to explore the stability of metal borides, especially metal-sandwich structures, explaining experimental off-stoichiometry and predicting stability trends under pressure.

Contribution

It introduces a simple model for boron-deficiency in lithium monoboride and assesses the stability of metal-sandwich phases across various metals.

Findings

MS lithium monoboride phases are favored under high pressure.

Boron-deficient phases have lower formation enthalpy than stoichiometric ones.

Noble metal borides are less unstable than AlB₂-type phases but not stable at equilibrium.

Abstract

We have recently identified metal-sandwich (MS) crystal structures and shown with ab initio calculations that the MS lithium monoboride phases are favored over the known stoichiometric ones under hydrostatic pressure [Phys. Rev. B 73, 180501(R) (2006)]. According to previous studies synthesized lithium monoboride tends to be boron-deficient, however the mechanism leading to this phenomenon is not fully understood. We propose a simple model that explains the experimentally observed off-stoichiometry and show that compared to such boron-deficient phases the MS-LiB compounds still have lower formation enthalpy under high pressures. We also investigate stability of MS phases for a large class of metal borides. Our ab initio results suggest that MS noble metal borides are less unstable than the corresponding AlB$_2$-type phases but not stable enough to form under equilibrium conditions.