Effects of aluminum substitution by potassium in the P2O5–Al2O3–Na2O–K2O phosphate glasses

Abstract P2O5–Al2O3–Na2O–K2O phosphate glasses with various K2O/Al2O3 ratios were synthesized using the melt quenching technique. Raman and absorption spectra were measured to investigate the effects of these ratios on the structures and spectroscopic properties of the glasses. As the K2O/Al2O3 ratio increases, the Raman spectra show shifts of some of the scattering bands to lower wavenumbers, the broadening of some bands and the tendency of decreasing the overlap between them, especially in the regions 400-700 cm−1 and 850-1300 cm−1. On the other hand, the absorption spectra of these phosphate glasses showed no marked changes in the optical band gap energy. Also, optical and thermal properties were measured to evaluate their potential for athermalization. As the ratio K2O/Al2O3 increases, the refractive index and glass transition temperature decrease, while the thermal expansion coefficient (α) increases. The existence of a negative thermo-optic coefficient (dn/dT) is important to obtain the temperature coefficient of the optical path (ds/dT) equal to −0.06·10−6/K for PAN10K glass. The results showed that increasing the K2O/Al2O3 ratio in phosphate glasses can control the value of ds/dT to achieve the athermalization.