Phase stability, bonding mechanism, and elastic constants of Mo5Si3 by first-principles calculation

We present results of first-principles local-density-functional calculations of the structural and elastic properties of Mo5Si3. Among the three different structures (D8(m), D8(8), and D8(l)), the D8(m) structure (referred to as the T1 phase) has the greatest binding with a high heat of formation of -3.8 eV/formula unit. The bonding in Mo5Si3 is found to have pronounced covalent components, characterized by the planar Mo-Si-Mo triangular bonding units on the (001) plane and by the unusually short Mo-Mo bonds directly along the c-axis. The calculated six elastic constants of the D8(m) structure are in excellent agreement with the experimental values. While the bonding in the (001) basal plane is stronger than the bonding along the [001] direction (i.e. C-12 + C-12 > C-33 and C-66 > C-44), the crystal anharmonicity is found to be higher along the [001] direction. The implication of our results on the anisotropy of thermal expansion coefficients is briefly discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.