Structure-property relationships in critically connected (GeTe₄)_100-(As₂Se₃) glasses

Thermal, optical, mechanical and structural studies were carried out on glasses in the pseudo-binary joint GeTe₄-As₂Se₃ prepared by a melt quenching method. (GeTe₄)_100-(As₂Se₃) glasses in the entire composition range of 0 ≤ x ≤ 100 have an average coordination number (Z_av) = 2.4, where the glass forming ability is found to be maximum. In general, for Z_av ≤ 2.4, the glass transition is found to be dominated by the network connectivity and the chemical composition effects are minimal. Although Z_av of Ge₂₀Te₈₀ (GeTe₄) and As₂Se₃ (As₄₀Se₆₀) is 2.4, GeTe₄ is a poor glass former and As₂Se₃ is an excellent glass former. The glass-forming ability is expected to increase with the addition of As₂Se₃. Surprisingly, the glass forming ability is found to decrease with the initial addition of As₂Se₃ and then shows an increasing trend. Glass transition (T_g) shows a large variation from 175 °C for x = 0 to 108 °C for x = 30. Based on the variation in the properties, the tie-line can be divided into three regions: region I (0 ≤ x ≤ 20) where T_g shows a decreasing trend, region II (25 ≤ x ≤ 55) where T_g remains almost constant and region III (60 ≤ x ≤ 100) where T_g shows an increasing trend. Hardness measurement also shows a similar trend in the three regions. Thermal stability shows a continuous increase with the increase of As₂Se₃. The fragility index varies between 15 and 30 for all these glasses except for x = 0 (GeTe₄) indicating the strong nature of the melts containing As₂Se₃. Raman studies indicate that the glassy network is dominated mainly by GeTe_(4/2) in region I and in region III the network is dominated by AsSe_(3/2) based structures. Glasses in region II are found to be dominated by AsTe_3/2 based structures. This study brings out the dominance of chemical composition effects over the network connectivity in a critically coordinated network. These glasses are also found to transmit IR light up to 18 μm and offer a wide composition range to prepare bulk glasses to be useful for infrared applications.