1578
P
Harry B. Skinner and Alan W. Searcy
= the critical exponent describing the shape of the coexistence curve of a single-component system
W A and W ~ = A
PA
the mass fraction of A and the mass fraction of A a t the plait point
= number density of component A
= relative molar mass of component A and bA = the slope and intercept of the straight line relation of the mass fraction of component A and the mass density L = Avogadro's number H = magnetic field K = thekelvin, unit of thermodynamic temperature
MA mA
Mass Spectrometric Studies of Gaseous Oxides of Rhenium Harry
B. Skinner and Alan W. Searcy*
Inorganic Materials Research Division, Lawrence Berkeley Laboratory and Department of Materials Science and Engineering, College of Engineering, University of California, Berkeley, California (Received November 2, 7972) Publication costs assisted by the U. S. Atomic Energy Commission
Effusion cell mass spectrometry has been employed to study vapor species of the rhenium-oxygen system. The equilibrium partial pressure of Re207(g) above a mixture of Re207(s) and Reo~(s) from 327 to 463°K is given by log P(Re207)= -(7.44 f 0.08)10*/T (12.35 0.21) where the pressure is in atmospheres. The enthalpy of sublimation at 430°K is 34.0 f 2 kcal/mol. Weight loss measurements show the probable com~ositionof the dirhenium heptoxide phase in equilibrium with ReOds) at 400°K to be Re2Os.9s+o.o2. The reaction of rhenium with zinc oxide at 1047 at 1267°K produces RezOdg), ReOdg), and possibly Re206(g). Reaction of rhenium with magnesium oxide from 1770 to 2143°K produces principally ReOa(g). The enthalpy of formation of Re03(g) is -67.0 4 kcal/mol at 298°K and that of RezOdg) is 2 -212 f 20 kcallmol a t 298°K. Re205(g), ReOz(g), and ReO(g) are not present a t measurable concentrations under any of the conditions of study. The heats of formation of ReO(g) and ReO2(g) are more positive then than -10 and +48 kcal/mol, respectively.
+
*
*
Introduction The vapor pressure for the reaction ReaOds) -* Re207(g) was measured by Ogawa,l by Smith, Line, and Be11,2 and more recently by Glemser, Muller, and Stocke.3 A mass spectrometer investigation of vapors from the solids Reo3 and Re02 was first made by Semenov and Ovchinnikov,* who reported that both ReO3(g) and Re207(g) are important species in vaporization of both phases. However, Battles, Gundersen, and EdwardsS showed that Reo&) and ReOZ(s) do not vaporize congruently as reported by Semenov and Ovchinnikov4' and that Re207(g) was the only major vapor species over the solid-phase mixtures ReO3-Re02 and ReO2-Re. Norman, Winchell, and Staley6 were able to study the reaction of rhenium metal with oxygen to form ReOj(g) but obtained conflicting measurements of its stability. This paper reports mass spectrometer investigations of vaporization of Re207-Re03 solid mixtures, of Re-ZnO solid mixtures, and of Re-MgO solid mixtures which were undertaken to obtain, if possible, more precise data for ReOs(g) and to determine if additional gaseous rhenium oxides could be identified and studied. In the course of the work it was discovered that solid Re207 does not vaporize congruently in vacuo but has a low partial pressure of oxygen. The Journal of Physical Chemistry, Vol. 77, No. 12, 1973
Dirhenium Heptoxide Vaporization
Experimental Section. This study was carried out with a 60" sector, 1-ft radius, magnetic deflection mass spectrometer built by Nuclide Analysis Associates. The dirhenium heptoxide obtained from Alfa Inorganics Co. had Mg as the only impurity detectable by spectroscopic analysis. The sample was resistively heated by nichrome wire wound closely around an anodized aluminum effusion cell. The interior of the cell was gold plated and a gold lid and liner were installed to prevent any contact of the rhenium heptoxide sample with the aluminum. The sample color changed after prolonged heating, which suggested that some reaction other than simple congruent sublimation was occurring. X-Ray diffraction measurements demonstrated that a residue of ReOd s) reE. Ogawa, Bull. Chem. SOC.Jap., 7, 265 (1932). W. T. Smith, L. E . Line, J r . , and W . A. Bell, J. Amer. Chem. Soc., 74,4964 (1952). 0. Glemser. A. Muller, and U. Stocke, Z. Anorg. Allg. Chem., 333, 25 (1964). G . A. Semenov and K. V. Ovchinnikov, J. Gen. Chem. USSR, 3 5 , 1517 (1965). J . E. Battles, G . E. Gundersen, and R. K. Edwards, J . Phys. Chem., 72,3963 (1968). J. H. Norman. P. Winchell, and H. G. Stal.ey, G . A. 8472, OCB Work No. 3111A,T. 0. No. 3110 (67) (1968).
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Mass Spectrometric Studies of Gaseous Oxides of Rhenium TABLE I: Relative Intensities of Ions Formed from the Vapor, from Re207(s), and from the Re 4- ZnO and Re 4- MgO Reactions at 70 eV and Their Appearance Potentials Intensity and standard deviation
Appearance potentials, eV
Ion
Re207(S)
Re -4- ZnO
Re +- MgO
Re207+ Re20ef
1000 91 f 2 17.4 h 0.4 276 f 11 154 f 9
1000 131 12 23 2 720 f 260 218 52
