Hans Rau

Thermodynamics of sulphur vapour

Density measurements of sulphur vapour up to saturation in the temperature range between 823 and 1273 K were performed. From the results, together with literature data, a set of equations was derived which allows the partial pressures of the different molecular species to be calculated as a function of total pressure and temperature. Real gas corrections are included, so that these equations can be used up to 1273 K and the saturation pressure of 144 atm. A FORTRAN program for calculation of the vapour density and the partial pressures of S2, S3,…, S8 from the total pressure and the temperature is available from the first author. The standard enthalpy of formation of S2(g) was found to be Δ H ° ( S 2 , g , 2 9 8 . 1 5 K ) = ( 3 1 2 0 0 ± 5 0 ) c a l t h m o l - 1 .

Defect equilibria in cubic high temperature copper sulfide (digenite)

Abstract Within the homogeneity region of the cubic high temperature copper sulfide the variation of sulfur content with sulfur pressure was measured under isothermal conditions. From the results were deduced the equilibria involving atomic disorder. Probably the most important atomic imperfections giving rise to deviations from stoichiometry are the effectively neutral copper vacancy (generated by replacing two Cu + ions by one Cu 2+ ion), the effectively negatively charged copper vacancy (represented by one Cu + ion missing), and an association of these two imperfections. If high temperature Digenite is to be denoted by a chemical formula, then Cu 7 S 4 is preferable to Cu 2 S or Cu 9 S 5 .

High temperature saturated vapour pressure of sulphur and the estimation of its critical quantities

Vapour pressure measurements on sulphur were performed up to a temperature of 1273 K (144.5 atm) with the aid of a special all-silica Bourdon gauge. Densities of the coexisting vapour and liquid were measured to 1273 K (vapour) and 1193 K (liquid). From the results the critical quantities of S were estimated to be: critical temperature, To = 1313 K; critical pressure, pc = 179.7 atm; critical volume, Vc = 158 cm3 mol−1; critical density, pc = 0.563 g cm−3; mean number of atoms per molecule at the critical point, vo = 2.78. The vapour pressure of sulphur between 590 K and the critical point is given by the formula: log ⁡ 10 ( p s a t / atm ) = 6 0 . 9 1 0 6 - 2 4 9 7 1 ( K / T ) + 1 . 0 8 1 7 × 1 0 7 ( K / T ) 2 − 2.2060 × 10 9 ( K / T ) 3 − 14.4102 log ⁡ 10 ( T / K ) .

Vapour composition and critical constants of selenium

Abstract Density measurements on selenium liquid and vapour were performed up to about 40 atm and 1373 K. From the results and literature data a set of equilibrium-law equations was derived which allows the equilibrium partial pressures of Se 2 , Se 3 ,…, Se 8 to be calculated from the total pressure and the temperature. Real gas corrections are included. A computer program in Fortran is available from the author. Thermodynamic data for the different molecular species are given. The critical constants of selenium are estimated.

Energetics of defect formation and interaction in pyrrhotite Fe1−xS and its homogeneity range

Abstract Sulphur fugacities in equilibrium with Fe 1− x S (pyrrhotite) were measured at different temperatures in the range 820–1374 K by direct (silica gauges) and indirect (H 2 S/H 2 ) methods. The results give direct evidence of the boundaries of the homogeneity range and can be interpreted in terms of iron vacancy formation and interaction and of an additional defect, e.g. iron on sulphur sites. The homogeneity range of Fe 1− x S is calculated as a function of temperature from the energetic parameters of the defects and the sulphur fugacities limiting the existence region of this phase. Fe 1− x S exists with positive values of x except at the eutectic with iron (1261 K) where x is 0±0.0001.

Vapour composition and van der Waals constants of arsenic

Abstract Standard enthalpies of formation and van der Waals constants of As4(g) and As2(g) are derived from vapour pressure and density measurements up to 77 atm and about 1400 K. The following results were obtained: ΔHfo(As4, g, 298.15 K) = (37.34 ± 0.2) kcalth mol−1; ΔHfo(As2, g, 298.15 K) = (45.54 ± 0.5) kcalth mol−1; a = 7.12 × 107atm cm6 mol−2; b = 349.1 cm3 mol−1.

Defect energetics and range of homogeneity of α-MnS

Abstract Sulphur pressure measurements between 10 −12 and 20 atm as a function of temperature (873–1364 K) and composition were performed on α-MnS samples. From the results and literature data on optical and electrical properties a quantitative defect structure model is derived. In the range investigated the deviation from stoiehiometry is caused by the formation of twice negatively charged manganese vacancies and, at high sulphur pressures, effectively neutral interstitial sulphur atoms (perhaps present as S″ 2 on S″ sites). At low sulphur fugacities thermal generation of “holes” and “electrons” is important. These are both localized on manganese sites as Mn 3+ and Mn 1+ ions, respectively. The boundary of the homogeneity range of α-MnS towards elemental sulphur is calculated from the model.

Homogeneity range of cubic high temperature cuprous sulfide (digenite)

Abstract Within the homogeneity range of the cubic cuprous sulfide Cu 2−δ S the sulphur fugacity was measured as a function of composition and temperature using direct (silica gauges) and indirect (H 2 S/H 2 ) methods. The experiments were performed between 789 and 1321 K and in the whole field of compositions from copper-rich to the sulphur-rich boundary of the homogeneity range. The experimental data are interpreted by a combination of the thermodynamics of mixed phases and the point defect theory. The results show that Cu 2−δ S is essentially a mixture of Cu 2 S and CuS. Other defects than CuS dissolved in Cu 2 S, e.g. copper interstitials, are present in small amounts at high temperatures and low sulphur fugacities. These results are in close agreement with data from the literature, derived from metallurgical, DTA, thermobalance and electrical conductivity measurements.

Range of homogeneity and defect interaction in high temperature nickel sulfide Ni1−xs

Abstract Sulphur pressures were measured directly and indirectly (H2S/H2) inside and adjacent to the homogeneity range of Ni1−xS. The results give direct evidence of the boundaries of the homogeneity range and can be interpreted in terms of metal vacancy formation and interaction. Interstitial metal atoms could not be detected (in contrast to Fe1−xS). Ni1−xS melts congruently at 1265 K under a total sulphur pressure of about 24 atm at a composition of 51.96 ±0.1 atomic per cent S.

The chromium-sulphur system between 873 K and 1364 K

Abstract The chromium-sulphur system was studied by direct and indirect (H 2 S/H 2 ) sulphur pressure measurements as a function of the composition in the range 50–60 at.% S and 873–1364 K. The boundary towards metallic Cr and three small two-phase regions were detected, separating four high temperature phases from each other. Phase relations are discussed in the light of results reported by different authors. CrS seems to be Cr deficient while Cr 2 S 3 is deficient in sulphur. It is suggested that Cr 3 S 4 exhibits a change in modification at about 1170 K.