First-Principles Local-Stress Calculations for Oxide Glass Materials

In glass materials, local physical properties vary due to distributions of coordination numbers, interatomic distances and bond angles. Therefore, it is essential to understand local atomic configurations in glass materials. Classical molecular-dynamics simulations have been widely performed. Recently, first-principles simulations based on density functional theory (DFT) have also been applied to glass models. However, by plane-wave based DFT methods, a total energy and a stress tensor can be obtained as quantities integrated or averaged in an entire supercell. To understand on the nature of local structures in glass materials, the analysis of local distribution of energy and stress is important. In this study, we analyzed atomic stresses in oxide glass materials using first-principles local-stress calculations [1] developed by one of our groups.<br/>Generally, in oxide materials, cations show tensile stresses and anions show compressive stresses. For a SiO2 glass model [2], atomic stresses of four-coordinated Si and two-coordinated O have a wide distribution due to the deviation in local atomic environments, but average values are close to those of the quartz-type SiO2 crystal. We found that formation energies of oxygen vacancies depend on sites and have strong correlations with atomic stresses. In a P-Si-Na-O glass model [3], the coordination number of Si is distributed from 4 to 6, and oxygen atoms can be classified into bridging oxygens (BO) and non-bridging oxygens (NBO). Tensile stresses of six-coordinated Si are higher than those of 4-coordinated Si and are close to those of the stishovite SiO2 crystal. We found that compressive stresses of NBO are smaller than those of BO and has a strong correlation with adsorption energies of protons. From these results, the analysis of atomic stresses is effective in predicting defect formation processes.<br/><br/>References: [1] M. Kohyama et al., Mater. Trans. 62, 1 (2021). [2] T. Tamura et al., Phys. Rev. B 69, 195204 (2004). [3] K. Takada et al., Phys. Chem. Chem. Phys. 23, 14580 (2021).