Thermochemical Properties of the Osmium Oxides

摘要

Osmium thin films, used as optical coatings in spectrometers because of theirhigh reflectivity in the vacuum ultraviolet, are found to disappear quickly on space-borne instruments. It is surmised that the disappearance is related to chemistry initiated by the collisions of high-velocity O atoms with surfaces. Possible causes for the mass loss include the following: the formation of volatile oxides of osmium; the distribution of the excess reaction energy to the material lattice, resulting in subsequent vaporization of reaction products or unreacted material; or chipping of surface materials, particularly thin films. To determine which process causes the mass loss and develop a predictive understanding of the interaction between the high-velocity atmospheric oxygen atoms and the surfaces of materials, it is necessary (among other things) to determine the thermo-chemical properties of gaseous oxides of the materials. For osmium, OsO4(g) is a well-known gaseous oxidizing agent which is used for many applications, including as a biological fixative. The high vapor pressure of OsO4(g) at room temperature has facilitated its study, and infrared, ultraviolet, and photoelectron spectra have been obtained. The commonly accepted heat of formation of OsO4(g) was determined from the heats of formation and vaporization of OsO4(s). Far fewer data are available for the other osmium oxides.