Lanthanum-fluoride addition to gallium-lanthanum-sulphide glasses

Lanthanum-fluoride (expressed as La2F6 for convenience) has been substituted for lanthanum sulphide in the glass system (mol%) (Ga2S3)(70)(La2S3)(30-x)(La2F6)(x) for 0 less than or equal to X less than or equal to 3. As fluorine substitutes for sulphur: (i) visible spectroscopy indicates that the absorption edge shifts to higher energies; (ii) infrared spectroscopy shows a small reduction in hydroxyl content; (iii) qualitatively the critical melt cooling rate decreases and (iv) the T-c-T-g gap increases by 24 degrees C at x=1.5. Isothermal penetration viscometry indicates that isoviscous points occur at lower temperatures as x increases. However, microscopic examination of samples after the viscosity measurements shows that above characteristic temperatures for each composition bulk crystallisation occurs, and a surface skin forms probably because of oxidation and/or hydrolysis.