Bond deformation paths and electronic instabilities of ultraincompressible transition metal diborides: Case study ofOsB2andIrB2

The energetically most stable orthorhombic structure of ${\mathrm{OsB}}_{2}$ and ${\mathrm{IrB}}_{2}$ is dynamically stable for ${\mathrm{OsB}}_{2}$ but unstable for ${\mathrm{IrB}}_{2}$. 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 ${\mathrm{OsB}}_{2}$ and by an in-plane bond splitting instability in ${\mathrm{IrB}}_{2}$. 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 ${\mathrm{OsB}}_{2}$. In contrast, an in-plane electronic instability in ${\mathrm{IrB}}_{2}$ 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 ${\mathrm{IrB}}_{2}$ nor ${\mathrm{OsB}}_{2}$ can be intrinsically superhard.