Anomalously strong viscosity behavior in mixed oxy-sulfide Na4P2S7-xOx invert glasses

Abstract In this report, the first observations of the anomalously strong viscosity behavior in the melt quenched (MQ) invert mixed oxy-sulfide Na4P2S7-xOx, 0 ≤ x ≤ 5, (NaPSO) glass series. We find that these glasses even with 67 mole% modifier, Na2S, exhibit a calorimetric fragility index, mcal, as small as 29 compared to typical values of ~ 60 to 80 for alkali silicate glasses with > 50 mole% SiO2. We find that the calorimetric fragility of these glasses varies systematically with the extent of network forming bridging oxygens (BOs) in the glasses. The mcal values were converted to kinetic fragility (mvis) values and used with the measured Tgs to produce viscosity curves for the glasses using the Mauro-Yue-Ellison-Gupta-Allan (MYEGA) model. The viscosity of the glass with a minimum mcal value of x = 2.5 was then shown to exhibit a viscosity that was as strong as some alkali aluminosilicate glasses. In this report, the first observations have been made of anomalously strong viscosity behavior and correspondingly strong bulk glass formation and resistance to crystallization in a melt quenched (MQ) prepared series of invert, modifier content greater than the glass former content, mixed oxy-sulfide Na4P2S7-xOx, 0 ≤ x ≤ 5, (NaPSO) glasses. We find that these glasses even with 67 mole % modifier, Na2S, and only 33 mole% glass former P2S5-xOx, exhibit a calorimetric fragility index, mcal, as small as 29 compared to typical values of ~ 60 to 80 for alkali silicate glasses with > 50 mole% SiO2 and the theoretical minimum mcal of ~ 15. The mcal values were converted to kinetic viscosity fragility, mvis, values and used with the measured glass transition temperatures, Tgs, to produce viscosity curves for the glasses using the Mauro-Yue-Ellison-Gupta-Allan (MYEGA) model. The viscosity of the glass with a minimum mcal value of x = 2.5 was then shown to exhibit a viscosity that was as strong as, and for some compositions stronger than, the viscosity of alkali aluminosilicate glasses which have three dimensional network structures compared to the one dimensional chain structures observed in these MOS invert glasses. Consistent with these small values of fragility indices and therefore predicted higher viscosity above the glass transition temperature, Tg, in the supercooled liquid regime, these glasses are also particularly resistant to crystallization. For example, the x = 2.5 glass composition exhibits a crystallization temperature, TC, more than 150K above its Tg. We find that the calorimetric fragility of these glasses varies systematically with the extent of network forming bridging oxygens (BOs) in the glasses. The structural change caused by incrementally replacing sulfur with oxygen has been found to have consistent, large effects on the thermal properties.