Investigations on the Mechanical threshold in IV-V-VI Glasses

Abstract

The main features observed in the DSC studies of Ge–As–Te glasses are the following: (i) The T? versus ?r? plot for the two families of Ge–As–Te glasses shows a linear increase with ?r?, with a small change in slope at ?r? = 2.60. (ii) The T? and activation energy of crystallisation of both sets of glasses show trends indicating a minimum and maximum, respectively, in the ?r? range 2.40 to 2.55. There are indications of a minimum in the T? versus ?r? plots near ?r? = 2.45 and 2.50 (glass compositions outside the present GFR). Similarly, the activation energy versus ?r? plots seem to indicate a maximum occurring around these compositions. Since the stoichiometric compositions of the Ge–As–Te glass family are Ge??As???Te? (?r? = 2.46) and Ge?????.43, the anomalies indicated by T? and the activation energy at these compositions may be signatures of the chemical threshold occurring at these ?r? values. This is plausible because in an experiment such as DSC, performed under non?isothermal conditions where the sample is heated beyond its glass transition temperature, the molecular structural units and their crystallisation kinetics play a greater role than the network topology. Since the chemical threshold depends on molecular structural units, DSC experiments may be more sensitive to this threshold. However, good?quality glasses with ?r? = 2.45 and 2.50 could not be obtained using the present preparative conditions. This raises an interesting question—namely, the ease of preparation of glasses at stoichiometric compositions. The present study suggests that these compositions require a higher cooling rate to form, which is surprising. The photoacoustic data, on the other hand, show an anomaly at ?r? = 2.60. The ? versus ?r? plots show a saturation behaviour beyond ?r? = 2.60. The ? data show sharp maxima at ?r? = 2.60. These photoacoustic properties clearly indicate an anomaly at this coordination number. Although glasses with ?r? = 2.45 and 2.55 could not be prepared, the trend in the photoacoustic data suggests that an anomaly of comparable magnitude to that observed at ?r? = 2.60 does not occur at ?r? = 2.40. The high?pressure resistivity studies on Ge–As–Te glasses also show anomalous behaviour at ?r? = 2.60. These studies focus on the ?r??dependence of the pressure derivative of the logarithm of the normalised resistivity. It is interesting to note that these results support Tanaka’s hypothesis regarding the formation and development of a layered structure in these glasses. It is clear from the present photoacoustic and high?pressure resistivity studies that the mechanical threshold in Ge–As–Te glasses occurs at ?r? = 2.60. The DSC experiments show possible indications of an anomaly occurring near the chemical threshold of these glasses. The deviation between the observed threshold value 2.60 and Tanaka’s predicted value 2.67 could be due to the partial covalent bonding interactions of the heavier element Te.