Temperature dependence of structural, elastic and thermodynamic properties of X2PtH6 (X=Li and Na) from first principles calculation

Abstract

X₂PtH₆ (X = Li, Na) are promising materials for hydrogen storage. For this, their structural, elastic and thermodynamic properties at different temperatures were studied. Our calculations are carried out within the framework of the full potential-linearized augmented plane wave method. Our obtained data show that the stiffness of Li₂PtH₆ compound material is higher than that of Na₂PtH₆. The elastic constants of the compound material Li₂PtH₆ are higher than those of Na₂PtH₆. Increasing the temperature from 0 to 1500 K, decreases the Debye temperature, indicating the decrease in its higher thermal conductivity. The same behavior has been reported for the Gibbs free energy with temperature. Among these materials, Na₂PtH₆ has the smallest Gibbs free energy, suggesting its superior forming ability. Li₂PtH₆ has the highest shear and Young's modulus due to its strong chemical bonding. Li₂PtH₆ exhibits the lowest degree of anisotropy due to the lack of strong direction in the bond. However, at constant volume, the specific heat capacities, the coefficient of thermal expansion and the change in entropy increase with increasing temperature.