Mercury pressure over HgTe and HgCdTe in a closed isothermal system

Abstract

The solid‐vapor equilibrium kinetics over HgTe and the mercury‐vapor pressure evolution over HgTe in the presence of CdTe are studied by means of mass‐loss measurements. For the first case, a theory is given relating the mercury pressure in an isothermal closed system to the mercury vacancy and interstitial concentrations in solid HgTe, the sample‐volume–to–free‐volume ratio as well as the equivalent pressure of mercury added in excess. In the simplest case—no mercury in excess, no interstitials—it is found that the mercury pressure first increases as the square root of time, before reaching an equilibrium value. At all times the pressure is proportional to the square root of the sample–to–free‐space volume ratio. Experiments, conducted at 550 °C, confirm this behavior as well as the relation with the ’’excess mercury pressure’’. The diffusion coefficient of mercury vacancies in HgTe is estimated to be 8×10⁴ μ²/h. For HgTe in the presence of CdTe, equilibrium cannot be reached because of material transport from HgTe to CdTe. The mercury pressure is determined by two main phenomena: outdiffusion from HgTe, as studied above, and diffusion into CdTe and the solid solutions formed on it by the material transport. For a given time, the experimental variations of the mercury pressure with the different parameters are formally similar to the preceding one (HgTe case). The analysis of the pressure evolution with time shows that the mercury pressure passes through a maximum which is experimentally verified.