Experimental Investigation of the Glass Transition Temperature in Amorphous Selenium under High Pressures

The effect of pressure on the glass transition temperature and the supercooled liquid region of amorphous selenium (a-Se), which was prepared through melting quenching, was investigated. The glass transition temperature $ {T}_{\text{g}} $ and crystallization temperature T_x were determined through the differential thermal analysis (DTA) during isobaric heating. The experimental results from piston-cylinder apparatus showed that both $ {T}_{\text{g}} $ and T_x increase with the increasing pressure in the pressure range of 0.1－1700 MPa. The glass transition middle temperatures $ {T}_{1/2,\text{g}} $ and extrapolated crystallization onset temperatures T_el,x were linearly fitted to pressure. The fitting results are ${T}_{1/2,\text{g}}\left(p\right)=322+0.046\,2p$ and ${T}_{\mathrm{e}\mathrm{l},x}\left(p\right)=398+0.030\,2p$, where the unit of temperature is K, and the unit of pressure is MPa. The smaller slope of T_el,x(p), compared with that of $ {T}_{1/2,\text{g}} $(p), induces the temperature range (T_el,x−$ {T}_{1/2,\text{g}} $) in the supercooled liquid region to be narrower with the increase of pressure. DTA data in the pressure range of 2000−4500 MPa was performed by using a cubic press. A slope change in T_x(p) curve is found. T_x increases with the increasing pressure within 0.1−1700 MPa, and the rate slows down when the pressure is above 2000 MPa. In the previous diamond anvil reports, a similar pressure dependence of T_x and T_g was observed, i.e., T_x and T_g both increase initially with the increasing pressure, and then become nearly constant above 1000 MPa. Since the slope changes in T_g(p) and T_x(p) curves occur nearly at the same pressure when the microstructure of a-Se changes, in view of the pressure of 2000 MPa corresponding to the inflection point of T_x(p) curve obtained in this study, it is speculated that the pressure of the inflection point of T_g(p) curve may be around 2000 MPa. The different pressures corresponding to the slope change obtained by the diamond anvil cell and the large-volume press may be related to the measurement method of T_g and T_x, as well as the pressure measurement error.