Structural and Mechanical Properties of Cubic Na2O: First-Principles Calculations
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Abstract
This work presents the investigation of the structural and mechanical properties of Na2O in cubic structure (hereafter denoted as cubic Na2O) by using the first-principles calculations within the local density approximation (LDA) and the generalized gradient approximation (GGA) in the functional form by Perdew-Wang 91 (PW91). The structural parameters such as the equilibrium volume (V0), the bulk modulus (B0), and its pressure derivative (B’0) can be obtained by fitting the calculated energy (E) and volume (V) data with the third-order Birch - Murnaghan’s equation of state whereas the elastic constants can be obtained from the stress-strain approach. The calculated equilibrium structural parameters and all three elastic constants (C11, C12, and C44) show a good agreement with the experimental and other theoretical values. All elastic constants and bulk moduli calculated from LDA are slightly larger than those values calculated from GGA, by the reason of the lattice constant calculated from LDA is slightly smaller than that calculated from GGA. By using the calculated elastic constants and Born’s criteria, the cubic Na2O was found to be mechanically stable at ambient pressure. Consistent with the calculated elastic constants (Cij), other elastic moduli such as bulk modulus (B), shear modulus (G), Young’s modulus (Y), and Poisson ratio (v) of cubic Na2O can be obtained by Voigt–Reuss–Hill (VRH) method. Because the B/G and v values can be used to determine the brittleness and ductility of materials, our calculated B/G and v values indicate that cubic Na2O behaves as a brittle material at ambient pressure. Furthermore, the hardness of cubic Na2O can also be obtained by using G/B and G values. The details of the structural and mechanical properties calculations for cubic Na2O were presented and discussed.
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