Comparison of the optical cutoffs of bismuth borate and bismuth germanate glasses

Glasses containing heavy metal oxides are often candidates for linear and n onlinear optical applications but the relationship between the optical prop erties and structure has not been thoroughly investigated. We report a stud y of the optical mobility gaps and mobility gap tails of bismuth borate and bismuth germanate glasses in the range from J=0 to J=3 for the JBi(2)O(3 . )B(2)O(3) system and from D=0.0 to D=1.7 for the DBi2O3 .GeO2 system (where J and D refer to the molar ratios of bismuth to each network former). The mobility gaps ranged from 4.0 to 2.8 eV for the borates and from 3.7 to 3.1 eV for the germanates. In this work we analysed the optical absorption of the glasses in the near ultraviolet and visible and calculated mobility gap s and tail widths using models proposed by Urbach & Tauc. The analysis reve aled relationships between optical behaviour and structural changes (e.g. m olar volume) occurring in the glass. In particular the compacting of the ne twork was used to explain a decrease in the mobility gap while the differen t electron environments affected the tail of the density of states.