ELASTIC AND VISCOELASTIC PROPERTIES OF AMORPHOUS SELENIUM AND IDENTIFICATION OF THE PHASE-TRANSITION BETWEEN RING AND CHAIN STRUCTURES

Using a broadband mechanical spectrometer, the tensile modulus and stress relaxation of amorphous selenium were studied in the glass transformation range. Bimodal spectra were recorded under isothermal conditions in the time range 10(-1) s < t < 10(5) s. It was found that the structural equilibrations of the two contributions to the stress decay proceed independent of one another. The mechanical correlation functions of stabilized supercooled liquid Se can be described using a weighted sum of two stretched exponentials. The temperature dependences of the relative relaxation strengths are compared with results from a rotational isomeric state model. Our results are consistent with the existence of a cooperative transition, near 300 K, between ringlike and chain elements in the disordered chain model of Misawa and Suzuki. This transition has been kinetically inaccessible to previous measurements.