Gibbs Energy of Formation and Partial Pressure of Hg over Te‐Saturated HgTe ( c )  Between 385 and 724 K

Abstract

The partial pressure of Hg over Te-saturated HgTe(c) has been determined between 385 K, where it is 1.0 • 10 -8 atm, and 724 K, where it is 3 • 10 -2 atm, by measuring the optical absorbance at 253.7 nm of the vapor phase. The data below the 686 K eutectic with Te(c) are used to calculate the standard Gibbs energy of formation and to obtain hH~ = -43,279 -+ 1,146 J/mol and hS~ = -20.510 _+ 2.452 J/mol degree K. Published data for the partial pressures of rig and Te2 between 770 and 943 K are reanalyzed to give the standard Gibbs energy, and hH~ = -43,802 _+ 2,761, and AS~ = -20.560 -+ 3.13. The two sets of values agree within the 95% confidence level uncertainties given. However, the standard enthalpy of formation at 298 K differs significantly from the accepted value.The partial pressures of Hg and Te2, PHg and P2, respectively, have been measured over both Hg-saturated and Te-saturated HgTe(c) between 773 and 943 K (1), and over Te-saturated HgTe(c) in the same temperature range (2). In the former study, the standard Gibbs energy of formation from. Hg(g) and Te2(g) was calculated, assuming an ideal vapor phase. In the latter, the Gibbs energy of formation from Hg(1) and Te(1) was calculated, assuming Hg(g) was non-ideal. In his critical compilation, Mills (3) used the data in Ref.(1) to obtain All~ = -39.5 kJ/mol. Low-temperature heat capacity measurements on Hg, Te, and HgTe give hS~ = -16.90 -+-2.09 J/mol-degree. On the other hand, the Gibbs energy of formation determined from EMF measurements between 310 and 430 K (4) gave hH~ = -33.890 -+ 1.67 kJ/mol and AS~ = -19.246 -+ 2.92 J/moldegree. Mills (3) used a third law analysis with this data to obtain hH~ = -31.80 k J/mole, which he selected as the best value.In order to determine AH~ more closely, we have measured P~ over Te-saturated HgTe below the 686 K HgTe-Te eutectic (5) so that, the phase coexisting with HgTe being essentially pure Te, one can calculate the standard Gibbs energy of formation of HgTe(c) from PHg and the vapor pressure of Hg at the same temperature. In addition, we make use of a recent (6) third law analysis of the crystalliquid-vapor equilibrium for tellurium to recalculate the Gibbs energies from the high-temperature data (1, 2) and to obtain AH~ and hS~ for comparison with the values obtained from the low-temperature Gibbs energies reported here. We find that the high-temperature data give hH~ = -43.802 _+ 2.76 kJ/mol and hS~ = -20.560 _+ 3.13 J/mol degree, where the uncertainties are at the 95% confidence level. Our low-temperature data gives values agreeing within these uncertainties. Thus AH~ does not agree well with the value from the EMF measurements (4), whereas hS~ does agree within experimental error with the results from both the EMF measurements and the lowtemperature heat capacities.The technique used to determine PHg as lOW as 10 -8 atm is to measure the optical alSsorbance at 253.7 nm of the vapor phase using a Hg hollow cathode lamp as a light source, as in atomic absorption. The sharp line spectra of hollow cathode lamps...

Thermodynamic description of the Hg–Te binary system

“…In next, Fig. 3 is shown calculated partial pressure of mercury along liquidus line compared with data obtained by Brebrick and Strauss [6], Goldfinger and Jeunehomme [9] and Sha et al [7]. Generally speaking, the calculation agrees with experiment.…”

“…Fig. 7 shows calculated 3-phase equilibrium partial pressure of Hg compared to Goldfinger and Jeunehomme [9], Sha et al [7] and Brebrick and Strauss [6] data. The calculation shows good agreement with experimental data.…”

“…The thermodynamic properties of the liquid can be derived from ternary equilibria between gas, liquid and solid phases based on vapor pressure measurement. The pressure of the vapor was examined by Brebrick and Strauss [6], Sha et al [7] and Su et al [8] who used optical absorption method. The information is in agreement one other.…”

“…The standard enthalpy of formation at 273.15 K was obtained by Ratajczak and Terpilowski [18] who used electromotive force experiment and reported by Mills [19]. The dissociation pressure method was used by Sha et al [7], Su et al [8], Nasar and Shamsuddin [20] and Brebrick et al [21]. Enthalpy of formation of HgTe at 298 K was measured by solution calorimetry by Rugg et al [22].…”