Bond deformation paths and electronic instabilities of ultraincompressible transition metal diborides: Case study of OsB2 and IrB2

The energetically most stable orthorhombic structure of OsB₂ and IrB₂ is dynamically stable for OsB₂ but unstable for IrB₂. Both diborides have substantially lower shear strength in their easy slip systems than their metal counterparts. This is attributed to an easy sliding facilitated by out-of-plane weakening of metallic Os-Os bonds in OsB₂ and by an in-plane bond splitting instability in IrB₂. A much higher shear resistance of Os-B and B-B bonds than Os-Os ones is found, suggesting that the strengthened Os-B and B-B bonds are responsible for hardness enhancement in OsB₂. In contrast, an in-plane electronic instability in IrB₂ limits its strength. The electronic structure of deformed diborides suggests that the electronic instabilities of 5d orbitals are their origin of different bond deformation paths. Neither IrB₂ nor OsB₂ can be intrinsically superhard.