We present a study of the thermodynamic and physical properties of ${\text{Ta}}_5{\text{Si}}_3$ compounds by means of density-functional theory based calculations. Among the three different structures $\left(\text{D}{8}_{\text{m}},\text{D}{8}_{\text{l}},\text{D}{8}_8\right)$, the $\text{D}{8}_{\text{l}}$ structure (${\text{Cr}}_5{\text{B}}_3$ prototype) is the low-temperature phase with a high formation enthalpy of $-449.20\text{kJ}/\text{mol}$, the $\text{D}{8}_{\text{m}}$ structure (${\text{W}}_5{\text{Si}}_3$ prototype) is the high-temperature phase with a formation enthalpy of $-419.36\text{kJ}/\text{mol}$, and the $\text{D}{8}_8$ structure (${\text{Mn}}_5{\text{Si}}_3$ prototype) is a metastable phase. The optimized lattice constants of the different ${\text{Ta}}_5{\text{Si}}_3$ compounds are also in good agreement with the experimental data. The electronic density of states and the bonding charge density have also been calculated to elucidate the bonding mechanism in these compounds and the results indicate that bonding is mostly of covalent nature. The elastic constants of the $\text{D}{8}_{\text{m}}$ and $\text{D}{8}_{\text{l}}$ structures have been calculated together with the different moduli. Finally, by using a quasiharmonic Debye model, the Debye temperature, the heat capacity, the coefficient of thermal expansion, and the Grüneisen parameter have also been obtained in the present work. The transformation temperature (2303.7 K) between the $\text{D}{8}_{\text{m}}$ and the $\text{D}{8}_{\text{l}}$ structures has been predicted by means of the Gibbs energy, and this predicted temperature (2303.7 K) is close to the experimental value (2433.5 K).

• Received 9 May 2009

DOI:https://doi.org/10.1103/PhysRevB.80.104103