Direct observation of the double glass transition in a phase-separated glass

Heat capacity measurements have been performed for the phase-separated pseudobinary chalcogenide glass system X PbSe-(1− X )Ge 1.5 As 0.5 Se 3 in the composition range X = 0.0 to 0.6. The heat capacity versus temperature plots for the phase-separated glasses exhibit two glass transition breaks. The composition dependences of the heat capacity changes, ifΔC p , at the two transitions are shown to be in accord with the behavior predictable from the phase diagram of a glass-forming system exhibiting liquid-liquid immiscibility and with the structure of these glasses as determined by electron microscopy. The two composition points on the immiscibility dome at which the temperature is equal to the higher glass transition temperature have been determined from the composition dependence of the ΔC p 's and indicate that the dome spans almost the entire composition range for this system.