Theoretical study of metal borides stability

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 (LiBy) tends to be boron deficient (y=0.8-1.0), however, the mechanism leading to this phenomenon is not fully understood. We use a simple model to simulate this compound with ab initio methods and discover that the boron-deficient lithium monoboride is a remarkable adaptive binary alloy: it has virtually no energy barriers to change its composition post synthesis within a small but finite range of y at zero temperature. Having demonstrated that the model well explains the experimentally observed off-stoichiometry, we next compare the LiBy and MS-LiB phases and find that the latter have lower formation enthalpy under high pressures. We also systematically investigate the stability of MS phases for a large class of metal borides. Our results suggest that MS noble-metal borides are less unstable than the corresponding AlB2-type phases but not stable enough to form under equilibrium conditions.