| dc.description.abstract | Cu-Ge-Te Glasses
(i) Bulk, meltquenched CuGeTe glasses are found to exhibit a single glass transition and a singlestage crystallization upon heating.
(ii) The composition dependence of the glasstransition temperature, crystallization temperature, activation energy for crystallization, and glassforming ability of CuGeTe glasses shows unusual variations at the composition x = 5, which corresponds to the average coordination number Z = 2.4. The anomalies observed in the thermal properties at x = 5 (Z = 2.4) can be associated with rigidity percolation.
(iii) Xray diffraction studies on CuGeTe samples annealed at the crystallization temperature for 72 hours indicate crystallization into hexagonal Te and rhombohedral GeTe phases. No binary or ternary crystalline phases involving Cu are seen in the XRD patterns. Samples melted under vacuum and cooled to room temperature also show only hexagonal Te and rhombohedral GeTe phases. It is likely that the added Cu randomly replaces Ge and is dispersed in the Ge-Te glass matrix.
(iv) Highpressure electricalresistivity studies reveal that CuGeTe glasses undergo a continuous semiconductortometal transition near 4 GPa. The ambient resistivity () and the activation energy for electrical conduction (E) exhibit a maximum at x = 5 (Z = 2.40), where rigidity percolation occurs.
(v) The thermal and electrical properties of network glasses are determined mainly by the nature of added impurities and how they enter the parent glass matrix, which can drastically alter network connectivity. The present results indicate that the initial addition of Cu decreases the overall connectivity of the Ge-Te network. Cu enters tetrahedrally at Ge sites in a random manner, and the system undergoes rigidity percolation at x = 5.
Ag-Ge-Te Glasses
(i) AgGeTe glasses, prepared over a wide composition range, exhibit a single T and a single T. A minimum in T, T, E, and T occurs at x = 5, where network rigidity percolates. A maximum in T and minima in T, E, and T occur at the chemical threshold, where the network is maximally chemically ordered.
(ii) Devitrification of these glasses yields crystalline phases AgGeTe, GeTe, and Te, depending on the silver content. These results indicate that the structural network of Ag-Ge-Te glasses consists of flexible Te chains and AgTe and GeTe tetrahedral units.
(iii) AgGeTe glasses undergo a semiconductor-metal transition at about 4-5 GPa. Glasses with x < 10 metallize around 4.0 GPa, while those with x > 10 metallize around 4.5 GPa.
(iv) The activation energy for electrical conduction exhibits anomalies at x = 5 and x = 20. Samples recovered after pressure release remain amorphous.
Pressure Dependence of Glass Transition (CuGeTe)
(i) The glasstransition temperature of CuGeTe decreases with increasing pressure. The value of dT/dP = -5.8 °C·kbar¹.
(ii) The model relating T - E - Z is used to explain this result. The conductivity activation energy decreases with pressure, suggesting a reduction in optical bandgap. This bandgap reduction shifts T to lower values.
(iii) It is likely that strong glasses, which follow Arrhenius behaviour, show negative dT/dP values, whereas fragile glasses, which deviate from Arrhenius behaviour, show positive dT/dP values.
(iv) The application of high pressure to CuGeTe glass drives the material closer to its equilibrium state. | |
