# Partial pressures of diatomic tellurium and mercury and thermodynamic properties of mercury zinc telluride solid solutions

#### Abstract

Mercury zinc telluride, (Hg$\sb{1-x}$Zn$\sb{x})\sb{1-y}$Te$\sb{y}$, has been considered as an alternative material to (Hg$\sb{1-x}$Cd$\sb{x}$)$\sb{1-y}$Te$\sb{y}$ for infrared opto-electronic devices due to its superior structural properties. Considerable research effort has been expended on the HgTe-ZnTe solid solutions. However, the experimental thermodynamic and phase diagram data for this ternary system are limited. The experimental technique consisted of measuring the optical density of the vapor phase coexisting with a sample of the solid solution in a sealed optical cell of known volume and temperature. In high temperature measurements between 500 and 800$\sp\circ$C with a high pressure Xenon lamp, the partial pressures of mercury $P\sb{\rm Hg}$ and of diatomic tellurium $P\sb2$ have been determined for $x$ = 0.10, 0.20, 0.30, 0.50, and 0.70. The optical density was measured between 250 and 750 nm. It was possible to place narrow limits on the solidus temperature for some of the $x$ values and to derive the chemical potential of HgTe in the solid solutions. In low temperature measurements between 100 and 230$\sp\circ$C with a Hg hollow-cathode lamp, the partial pressures of mercury $P\sb{\rm Hg}$ has been determined for the Te- saturated solid solutions for $x$ = 0.0, 0.10, 0.20, 0.30, 0.50, and 0.70. The optical density was measured at 253.7 nm. For Te-saturated HgTe(c), the data below the 686 K eutectic temperature with Te(c) were used to calculate the standard Gibbs energy of formation and to obtain $\Delta H\sbsp{\rm f,298}{\circ}$ and $\Delta S\sbsp{\rm f,298}{\circ}$. Results agree with previous single beam measurements and high temperature optical density measurements within the 95% confident level uncertainties. The standard enthalpy of formation at 298 K differs significantly from the value published by others. For the Te-saturated solid solutions, the partial pressures indicate a miscibility gap between $x$ = 0.10 and 0.50 for the entire 100-230$\sp\circ$C range. X-ray diffraction patterns do not show the two sets of lines required for a miscibility gap. However, the diffraction lines are significantly broader than the instrumental half-width. It appears that the attainment of equilibrium, and the formation of a miscibility gap was restricted to a surface layer thin relative to the X-ray penetration depth of about 12 $\mu$m.

#### Recommended Citation

Kuo-Tong Chen, "Partial pressures of diatomic tellurium and mercury and thermodynamic properties of mercury zinc telluride solid solutions" (January 1, 1991). Dissertations (1962 - 2010) Access via Proquest Digital Dissertations. Paper AAI9226214.
http://epublications.marquette.edu/dissertations/AAI9226214

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