1032
R. F. BREBRICK
Tellurium Vapor Pressure and Optical Density at 370-615"
by R. F. Brebrick Lincoln Laboratory,l Massachusetts Institute of Technology, Lexington, Massachusetts
0.91 73
(Received September 13, 1967)
The optical density of the vapor at 755" originating from a Te reservoir between 370 and 615" has been measured between 2100 and 1990 Aiand near 4357 d. The results strongly indicate that Beer's law is satisfied so that vapor pressures can be obtained. It is found that log P varies linearly with 1/T down to the triple point of 450". This is in disagreement with the apparently most precise measurements on liquid tellurium. Between 450 and 370" log P(atm) = -8.001 x 10S/T 7.540.
+
Introduction I n general, the vapor pressure of tellurium is probably known with good accuracy between 200 and 890°.2 However, there are few measurements between 400 and 512" and none between 427 and 484". I n the former temperature interval, the vapor pressure lies between 4X and lov3atm and for liquid tellurium shows a marked deviation from the nearly linear dependence of log P upon 1/T observed at higher temperatures according to one study.3 Our determinations of the partial pressures of Tez(g) over a number of crystalIine tellurides4-' from the optical density of the coexisting vapor have depended upon the published vapor pressure of liquid tellurium. Here we report the results of optical densit? measurements between 2100 and 1990 d and at 4357 -4of the vapor over pure tellurium. The optical density near 2000 .%is about ten times larger than that at 4357 8, where our previous measurements were made, and accurate measurements are easily atm. This made at Ten pressures as low as covers the range where log P vs. 1/T is claimed to be nonlinear for liquid tellurium. Moreover, if the absorption can be shown to follow Beer's law, the usual precedure can be reversed; i.e., the optical density measurements can be used to determine the relative vapor pressure of tellurium. Although we cannot prove absolutely the validity of Beer's law for our optical density measurements, we conclude it is probably valid with a high degree of certainty. Accepting this conclusion, we then find that the reported low temperature curvature3 of log P us. 1/T for liquid tellurium is not real. Using the published vapor pressure at the one temperature of 508", our results yield the vapor pressure between 370 and 615". Experimental Section I n essence, the method was to measure the optical density (D E log Io/I) of the vapor contained in a cylindrical silica optical cell at a fixed temperature of 755" using a double-beam spectrophotometer. The vapor originated from pure Te contained in a side arm whose temperature was independently controlled and T h e Journal of Physical Chemistry
measured at temperatures from room temperature to about 615". Chunk tellurium, from American Smelting and Refining Co., N. J., and 99.99 at. % pure by their emission spectrographic analysis, was kept molten under vacuum for 15 min in a previously outgassed silica tube and then cooled. The surface layers were discarded and about 1 g was put into a silica optical cell previously outgassed at 1040" and 3 X lo-' torr for 16 hr. The cell and tellurium were then outgassed at 220" for 3 hr and the cell was sealed off below torr. The cylindrical, fused-silica optical cells were 22 mm 0.d. with flat parallel windows and a 21.6 cm long side arm at right angles to the cylindrical axis. One optical cell had an inside optical path length of 22.1 mm; the other had a length of 98.3 mm. The end of the side arm away from the cell was the seal-off point and during measurements contained the condensed tellurium phase. A schematic of the furnaces used to keep the optical cell proper at 7.55" during the measurements, while the temperature of the tellurium reservoir was held at room temperature or above, has been published.4& The temperature of the optical cell proper was constant to within It 2" during the measurement. The gradient across the optical cell was 20". A close-fitting cylindrical silver liner, 10 cm long, with walls 6.4 mm thick, was put around the end of the side arm or tellurium reservoir to minimize the temperature gradient. Along the 2.5-cm length containing the tellurium the temperature gradient was 0.5" or less during the measurements. At the middle position of the tellurium reservoir the temperature was measured (1) Operated with support from the U. S. Air Force. (2) An. N. Nesmeyanov, "Vapour Pressures of the Elements," translated by J. I. Carasso, Academic Press, New York, N. Y., 1963. (3) R. E. Macho1 and E. F. Westrum, Jr., J . Am.. Chem. Soc., 80, 2950 (1958). (4) (a) R. F. Brebrick and A . J. Strauss, J . Chem. f h y s . , 40, 3230 (1964); (b) ibid., 41, 197 (1964). (5) R.F. Brebrick, ibid., 41, 1140 (1964). (6) R.F. Brebrick and A. J. Strauss, J . Phys. Chem. Solids, 25, 1441 (1964). (7) R. F.Brebrick and A. J. Strauss, ibid., 26, 989 (1965).
TELLURIUM V.APOR PRESSURE AND OPTICAL DENSITYAT 370-615" to a precision of =tO.l" using a calibrated Pt-Pt-l3% Rh thermocouple with an ice junction and a Leeds and Northrup K-3 potentiometer. The emf from this thermocouple was recorded and generally varied by less than *0.2" during the 4 min needed to measure the optical density between 2100 and 1990 8. The optical call proper with surrounding furnace was placed in the sample beam of a Cary Model 14H double-beam spectrophotometer with the side arm tilting downward about 15" from the horizontal. With the center of the optical cell proper a t 755" and the reservoir at room temperature, the zero of optical density between 2100 and 1990 8 was recorded. The zero run was repeated with a reservoir temperature of about 300". The temperature was then raised and the optical densities measured after the reservoir temperature and optical density a t 2007 8 had reached a steady value. This generally took 20 to 40 min. The reservoir temperature was then raised about 5" and the whole procedure was repeated. Optical densities in the visible part of the spectrum were measure? a t 6000,5500, and 5000 8 and between 4400 and 4300 A in a similar fashion but in separate runs. The ultraviolet spectra were obtained using the hydrogen light source of the Cary 14H, a scanning speed of 0.5 A/sec, a 12.7-cm/min chart speed giving 2.36 8/cm, and a l-sec period for the recorder pen. The spectral band pass a t 2100 and 2000 was 1.28 and 1.68 b, respectively. The visible spectra were obtained using a tungsten light source, a scanning speed of 0.63 8/sec, a 12.7-cm/min chart speed, and a l-see pen period. A t 4357 8 the spectral band pass was 1.7 8. The spectra were generally obtained with increasing reservoir temperatures. On several occasions the furnaces sat a t temperature overnight and runs were continued the next day.
1033 OPTICAL DENSITY
2056
2059 2003
4361
2025
4381
