~ N D JT7.
annealing does nothing to clarify the assignment, since the entire band d changes shape completely in sharp contrast to the behavior in krypton and xenon. This change on annealing of the band d in argon occurred e v e ~ ifi only a small amount of diffusion of the trapped species was permitted as judged by the absorption intensity of the monomer band. No specific assignment of the absorption bands at b or c seems possible other than attributing them to higher polymeric species. Summary This mork has demonstrated the feasibility of using the matrix isolation technique to obtain infrared spectral data for high temperature inorganic species. Certain precautions, however, must be noted. (1) X given vibration of the trapped species may have more than one absorption maximum iii a particular matrix environment. Spurious conclusions may result if this is not realized. I n the present mork this type of splitting was recognized from the different band shapes observed in the three matrices, but it is always possible that for other trapped species such differences may not occur. The splitting of an absorption band in a matrix spectrum is usually attributed to the trapped species occupying more than one type of site in the matrix, the different environments causing changes in the interaction energy between the trapped species and the matrix cage. Since the different sites which the trapped species may occupy are presumably due to crystal imperfections in the lattice it would be interesting to try compressing the matrix to remove the imperfections and to observe if the splitting disappears. (2) Large frequency shifts betmeen gas and matrix phase spectra have been observed in the present work,
F.HALL
1701.
67
and, if thermodynamic quantities are to be calculated, this limitation must be recognized. For the lithium halides investigated, frequency shifts in the matrix varying between the two extremes of $17 and -17% have been observed. Theoretical calculations of frequency shifts arc usually hampered by lack of experimental information and by uncertainties coiireriiiiig the actual environment of the trapped species in the matrix. Thus for lithium fluoride monomer Linevsky3 was able to calculate from available experimental data the matrix frequency shift due to the dipole-induced dipole interaction, assuming the lithium fluoride monomer occupied a substitutional site in a perfect rare gas crystal lattice, but was unable to make any prediction as to the magnitude of the dispersion interactioii. Finally, although frequency shifts in the vibration spectra of trapped species are an obvious disadvantage of the technique, such large shifts as observed for lithium fluoride and lithium chloride may not be too common. Most of the reported rare gas matrix frequency shifts have been of the order of a few per cent for essentially non-polar trapped species, and this might indicate that perhaps large frequency shifts are likely only if highly polar molecules are involved. Some work in this Laboratory12 in which the matrix isolation spectra of some group IIB chlorides were observed teiids to support this view-the largest frequency shift being less than 4sT,. However, further work is required to clarify the situation before any firm prediction concerning matrix frequency shifts can be made, Acknowledgment.-This research was carried out under the auspices of the U. 5. Atomic Energy Commissiorr . (12) R. JV. XIcNrtmee, Jr., Ph.D. Thesis, University of California, 1962.
THE VAPORIZATION BEHAVIOR OF BOROK NITRIDE A S D ALTiMINUM XITRIDEl B Y D. L.
HILDEXBR.4KD SICD
w.F. HALL
Research Laboratory, Aeionutronic Division of Ford Motor Co., ‘Vewport Beach, California Received October 20, 196d The dissociation pressures of crystalline boron nitride and aluminum nitride have been measured by the torsion-effusion method over the ranges 1850 to 2160°K. and 1780 to 1970°K.) respectively. lllthough the measured pressures for both substances showed a strong dependence on effusion orifice area, a simple expression relating equilibrium and observed pressures to effusion cell geometry was found to correlate the data quite satisfactorily. Equilibrium pressures evaluated by extrapolation of observed pressures to zero effective orifice area are in good agreement with dissociation pressures calculated from available thermodynamic data. The heats of formation of crystalline BN and A N a t 298°K. have been derived from the dissociation pressures as -59.8 i 0.6 and -76.1 f 2.1 kcal./mole, respectively. The magnitude of the hole-size dependence indicates that the while that for AlX is
