Nature of low compressibility and anisotropic elasticity in YbB2

Recent high-resolution synchrotron X-ray diffraction experiments (Kalkan et al., 2012) investigated bulk modulus and compressibility of high purity YbB2 and found that its compressibility can be compared even to that of diamond. In this paper we investigate the structural, compressibility and electronic properties of rare-earth YbB2 using density functional theory to illuminate the nature of its low compressibility and anisotropic elasticity. The calculated results are in agreement with the available theoretical and experimental data and confirm YbB2 as a hard material. The nature of YbB2 hardness and low compressibility is revealed by analyses of its electronic and directional bonding properties: it lies in very strong in-layer and strong enough between-layer bonds. The isotropic bulk modulus, shear modulus, Young's modulus, elastic anisotropy, wave velocities and Debye temperature of YbB2, under ambient and pressures up to 300 GPa are also discussed. Pressure induces expected behavior in most of these properties (monotonic increase), except in elastic constant c44, which softens and could lead to elastic instability. Furthermore, anisotropic factors, which are very different at zero pressure, give similar values at the pressure of 300 GPa, closing the gap between compressional and shear anisotropy. Ductility unexpectedly increases with pressure.