Physics and Chemistry of Te and HgTe-Based Ternary Melts

With all of the experimental and theoretical results presented in the earlier chapters of this book, the review and analysis in this chapter are focused on the physics and chemistry of the melts of Te and two HgTe-based ternaries, i.e., the Hg-Cd-Te and Hg-Zn-Te systems. Using the data of thermal and electrical conductivities of the solids and melts of Te, HgTe, Hg0.9Cd0.1Te and Hg0.8Cd0.2Te, the derived Lorenz number indicates a transition from semiconductor to metal during melting. From the results and analyses of various thermophysical properties, the Te melt has consistently shown structural transformations. The implication of a two-fold coordination semiconducting domain transforms to the three-fold coordination metallic domain in the range of 500–600 ℃ was evident from the data of density measurements, neutron scattering, electrical conductivity, the free activation energy of the viscus flow and the viscosity-density correlation theory. From the Gibbs energy of dissociation for the molecular species, namely, HgTe, CdTe and ZnTe, in the binary systems, the structural transition of the liquid phase can be interpreted as: when the temperature increases, the atomic species gradually starts to dominant the liquid phase and eventually, at rather high temperatures for the cases of CdTe and ZnTe, approaches to become a normal homogeneous phase of atomic species. The neutron scattering measurements on the three binary systems show that, in the case of CdTe and ZnTe, the coordination number remains to be 4, same as the solid, but it increases to 6.3 for the HgTe melt. The heavier HgTe compound undergoes a major structural change from semiconducting to metallic during the melting that the bond distances and coordination numbers increase. An Ab Initio molecular dynamics simulation confirmed the cases of CdTe and ZnTe. In the HgCdTe system, the density for HgTe and Hg0.9Cd0.1Te melts go through a maximum at temperatures just above the melting point, similar to the case of Te melt. From the viscosity-density correlation theory, the data of the HgTe and Hg0.9Cd0.1Te melts imply a structural transition at approximately, 805 ℃ and 793 ℃, respectively. On the studies of the relaxation behavior, after rapid cooling from 857 ℃ to 800 ℃, the measured viscosity of the Hg0.8Cd0.2Te melt went through a maximum at 2 h, then started to decline approximately 60% over a period of 50 h to reach its equilibrium value. On the HgZnTe system, a maximum in the melt density was not observed for the Hg0.90Zn0.10Te and Hg0.84Zn0.16Te melts and the density of Hg0.90Zn0.10Te sample showed a hysteresis loop with a higher measured value during cooling than heating. The measured viscosity of the Hg0.84Zn0.16Te melts start increasing just above the liquidus temperature, go through a maximum at 810 ℃ and decreases to a saturated value at 850 ℃. On the relaxation studies on the viscosity of Hg0.84Zn0.16Te melt, two sets of data were obtained as functions of time after the ampoules were cooled from 850 ℃ and stabilizing at temperatures of 810 ℃ 790 ℃. It took one day and 5 days, respectively, for the 810 ℃ and 790 ℃ stabilization, to reach the steady state. Similar observation has been reported on the viscosity measurements in the Al-rich Al-Y melts and it was claimed to be the results of competition of two simultaneous effects. Lastly, the effects of these phenomena in the HgTe-based ternary melts on the process of crystal growth from melt are discussed.