First principles study of the structural, electronic, and optical properties of Sn2+-doped ZnO-P2O5 glasses

The atomic-structural, electronic, and optical properties of Sn-doped ZnO-P2O5 glasses are examined using combined semi-empirical and density functional theory calculations. From model structures for two different ZnO-P2O5 glass compositions, it is found that these glasses consist of nearly ideal tetrahedral PO4 unit and largely distorted ZnO4 and ZnO5 units. The concentration of ZnO5 units is calculated to increase and exceed that of ZnO4 as the amount of ZnO in the glass is increased. Also, the concentration of terminal oxygen atoms shared by two Zn neighbors is largely increased for 70ZnO:30P2O5 (70:30) glass compared to 60:40 glass, which can be rationalized by electrostatic bond strength calculations. Regarding medium-range order, calculations show that Q1 and Q2 structures (where Qn refers to structures containing n bridging oxygen atoms on PO4 tetrahedra) are dominant for the 60:40 glass, while Q0 and Q1 prevail for the 70:30 glass, in good agreement with experimental observations. For the Sn-doped ZnO-P2O5 glass, various Sn coordination geometries are found, with trigonal pyramidal SnO3 and largely distorted pyramidal SnO4 configurations being dominant and a small fraction of two-fold and five-fold coordinated Sn configurations also present. From combined cluster and glass model calculations, two-fold coordinated Sn are found to exhibit slightly smaller optical transition energy than three-fold and four-fold coordinated Sn. However, local variations in coordination structures throughout the glass likely result in a broad singlet transition (S0→S1) around 5.4 eV for all the different Sn configurations. A triplet transition (S0→T1) driven by spin-orbit coupling can occur around 4.5 - 4.8 eV, but the magnitude of this transition is predicted to be much smaller than the spin-allowed singlet transition.