Dilatometric and positron annihilation lifetime spectroscopic studies on amorphous and polycrystalline selenium

Macroscopic expansion and microscopic positron annihilation behaviors on the three forms of selenium (Se): amorphous (a-Se), crystalline (c-Se) and partially crystalline (pc-Se) have been investigated over wide temperature ranges simultaneously by means of length dilatometry or positron annihilation lifetime spectroscopy, respectively. No positronium formation is detected in two morphological forms (a-Se) and (pc-Se). Dilatometric and PALS responses on the a-Se and pc-Se samples exhibit behavior characterized by the presence of more or less pronounced bend effects at the characteristic transition temperatures T _g(a-Se) and T _x (pc-Se) in contrast to the crystalline form without any bend effect. The positron lifetimes for a-Se are systematically lower than those for pc-Se. PALS experiments show that the annihilation of positron is sensitive to the glass transition similarly to ortho -positronium. The thermal expansion coefficients of the macroscopic length and approximately, of the macroscopic volume and that of the positron lifetime in both a-Se and pc-Se have been determined and compared. In a-Se, the respective thermal expansion coefficients of positron lifetime in both the physical states are approximately two-fold higher than the estimated volumetric ones of amorphous Se and in pc-Se even about five-fold larger in both the temperature regions below and above the transition temperature T _x comparable with the glass transition of a-Se. All these findings lead to the physical picture of pc-Se sample as non-perfect crystalline matrix having the disordered regions between microcrystals similar to a-Se but with distinct electron density and expansion characteristics.