J. Phys. Chem. 1983, 87, 2463-2465
2463
Spectroscopic Studies of Molecular Interactions of Oxygen-16 and Oxygen-18 in the High- Density E-Phase B. I. Swanson,' S. F. Agnew, L. H. Jones, R. L. Mills, and 0. Schlferl
Los A l e m s National Laboratory, University of Callfornle, Los Alemos, New Mexico 87545 (Received: March 24, 1983)
Infrared and Raman spectra were obtained for solid 1602 and 180z as a function of pressure, using diamond-anvil cells. When both isotopes are compressed into the t-phase near 9.9 GPa, a strong infrared band appears in the 0-0stretch region, 48 cm-' lower than the Raman-active0-0 stretch. As pressure is increased,the infrared absorption shifts to lower frequency, eventually splitting into two bands at 16.0 GPa. Two combination modes, which also shift to lower frequency with increasing pressure, are observed in the 3000-cm-' region of the IR spectra. The t-phase also exhibits a second Raman mode around 130 cm-', which shifts to higher frequency intermolecular with pressure. The results indicate that the 0-0 bond weakens at the expense of forming 02-02 interactions in the t-phase. The spectroscopic results show that the structure of the t-phase is centric with, most likely, four molecules per primitive cell.
Introduction The behavior of solid 0, at high pressure is presently of considerable interest.l" At low pressure, O2crystallizes into three structures which are (in order of increasing temperature) monoclinic a-O2 (space group C2/m), rhombohedral p-02 (Rgm), and cubic 7-0, (Pm3r1).~ At room temperature, O2solidifies at approximately 5.4 GPa1p2into ~ t r u c t u r e .Between ~ ~ ~ 9.3 and 9.9 GPa an orthothe 8-0, rhombic a'-02 (Fmmm) form, which is closely related to a-02,is p table.^ At about 9.9 GPa, there appears a new phase of O2 (designated e-O2by Nicol and c o - ~ o r k e r s lthe ~~~ structure ~) of which has not yet been determined. The e-phase of O2 is strongly dichroic and varies from dark red to light yellow, depending on crystal ~ r i e n t a t i o n . ~Visible .~ absorption spectra show that the e-phase has several strongly polarized transitions in the blue! The W a n frequency of the 0-0 stretch mode drops abruptly by about 3 cm-' upon transition from the a'- to e-phase, and continues to drop with increasing pressure up to about 12 GPa, but begins to rise again above this pressure.'^^ In this work we report for eo2strong infrared absorption in the 0-0 stretch region and several new features in the Raman spectra. We conclude that the structure of E-O2 is centric with a primitive unit cell large enough to accommodate at least three and most probably four molecules. Experimental Section Merrill-Bassett7 diamond-anvil cells with type IIa diamonds supported by either strengthened beryllium8v9or (1) Nicol, M.; Hirsch, K. R.; Holzapfel, W. B. Chem. Phys. Lett. 1979, 68,49-52. (2) Schiferl, D.; Cromer, D. T.; Mills, R. L. Acta CrystaZlogr., Sect. B 1981, 37, 1329-32. (3) d-Amour,H.; Holzapfel, W. B.; Nicol, M. J.Phys. Chem. 1981,85, 130-1.
(4) Syassen, K.; Nicol, M. 'Physics of Solids under High Pressure"; Schilling, J. S.,Shelton, R. N., Ed.; North Holland: New York, 1981;pp 33-8. (5) Schiferl, D.; Cromer, D. T.; Schwalbe, L. A.; Mills, R. L. Acta CrystaZlogr., in press. (6) Donohue, J. "The Structures of the Elements";Wiley: New York, 1974. (7) Merrill, L.; Bassett, W. A. Reu. Sci. Instrum. 1974, 45, 290-4. (8) Schiferl, D.High Temp. High Pressures 1977, 9, 71-5. (9) Keller, R.;Holzapfel, W. B. Reu. Sci. Instrum. 1977, 48, 517-23.
0022-3654/83/2087-2463$01.50/0
TABLE I: Vibrational Frequencies ( c m - ' ) Observed for e - 0 2 at 13.5 GPa 160*
0-0 stretchb combination band combination band
0-0 stretch
Infrared 1513 3059 2964 Raman 1578
libron
133
' 8 0 2
1433 ( 1 4 2 6 p 2884 ( 2 8 8 4 p 2791 (2794)' 1 4 8 5 (1488)' 1498 126 ( l 2 5 p
' Values of I8O2peak positions scaled from those of 1602 according t o (16/18)1'2.
Peak positions estimated from midpoint between the saturated absorption peak edges.
hardened BeCu backings were used to study O2 in the pressure range 5-16.5 GPa at room temperature. Separate samples of isotopically pure 1602,1802containing 6% l60l8O(hereafter called "1802"),and 1602 containing 10% lSO2were loaded with the indium-dam technique.lOJ1 Pressures were measured by the ruby fluorescence method,12J3assuming the R1 line shift to be 0.1322 GPa/cm-l. UV-visible absorption spectra, obtained with a PerkinElmer Model 330 spectrometer and Harrick beam-condensing unit, were used to help identify the solid phases. Infrared spectra were obtained with a Nicolet 7000-Series Fourier-transform spectrometer with a cooled mercury-cadmium-telluride detector. Typically, 1000 scans R spectra over with 1-cm-' resolution were averaged. The I the approximate ranges 1300-1350 and 1600-2750 cm-' were obscured by absorption due to the diamonds. Raman spectra were obtained in a back-scattering geometry by using various lines from Spectra Physics Model 171 argon and krypton lasers. The Raman scattered light was collected with a SPEX Model 1403 double monochromator equipped with a Nicolet data system and a Princeton Applied Research photon-counting system. For the Raman measurements, resolution was 2 cm-' for the (10) Liebenberg, D.H. Phys. Lett. A 1979, 73, 74-7. (11) Mills, R. L.; Liebenberg,D. H.; Bronson, J. C.; Schmidt, L. C. Reu. Sci. Instrum. 1980, 51, 891-5. (12) Forman, R.A.;Piermarini, G. J.; Bamett, J. D.; Block, S. Science 1972, 176, 284-5. (13) Bamett, J. D.;Block, S.; Piermarini, G . J. Reu. Sci. Instrum. 1973, 44, 1-9.
0 1983 American Chemical Society
2404
Letters
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983
-
i
9.6 GPa
4
9.9
jirl
', ,
7
'j
1 10.2
5
i
ILIOC
1LI55c
:5CO
lS5C
:ECC
70
FREQUENCY (cm-')
0-0 stretching modes and 3 cm-' for the phonon modes.
Results The spectral features for both oxygen isotopes are summarized in Table I. Although no infrared absorption is and la02 observed in either p-0, or a'-02,when both lS02 transform to the tphase near 9.9 GPa very intense infrared absorption bands are observed in their respective 0-0 stretch regions. The absorption at several pressures is shown for 1602 in Figure 1. The new absorption peak first appears about 48 cm-l lower than the Raman-active stretching mode reported by Nicol and co-w~rkers.l*~ This peak becomes more intense and shifts to lower frequencies with increasing pressure. An analogous infrared absorption peak also appears for lS02 approximately 80 cm-' lower than the lS02band. At pressures above 15.0 GPa the infrared absorption observed for lS02 splits into two equally intense, well-resolved bands. As pressure is increased in the 6-phase, two weak infrared absorption bands, separated by about 100 cm-', appear in the 3000-cm-' region. With increasing pressure these bands become much more intense, shift to lower frequencies, and increase their relative separation. A t 13.5 GPa these two modes are of comparable intensity. The infrared and Raman frequencies for both lS02 and lS02 at 13.5 GPa are compared in Table I. The Raman peaks are also shown in Figure 2. The €-phaseof l e 0 2 ha9 a single peak in the 0-0 stretch region, consistent with earlier ~ t u d i e s . lSurprisingly, ~~ 1802 exhibits two Ramanactive 0-0 stretching modes in the e-phase. The higher frequency mode first appears as a weak peak at 1505 cm-' (10.2 GPa) and, with increasing pressure, grows in intensity and shifts to lower frequency. The lower frequency IeOz peak shows slight changes with increaeing preeeure similar to that of the single 0-0stretch peak in 1e02.1*4 A t 13.5
1950
1500
1550
1600
FREQUENCY (cm-l)
Flgure 1. Pressure dependence of the infrared-active 0-0 stretch for
"02.Spectra are plotted on the same scale and are displaced for clarity.
120
Flgure 2. Comparison of Raman spectra at 13.5 GPa in the phonon
and 0-0 stretch regions for "0,and "0,.
GPa the two peaks observed for la02 are separated by 12.5 cm-' and are of comparable intensity. A very weak Raman peak corresponding to the stretching mode of the l6Ol8O impurity appears at 1518 cm-'. Raman spectra in the phonon region of the c-phase at 13.5 GPa show a peak a t 133 cm-l for 1602 and 126 cm-' for 1802,as shown in Figure 2 and Table I. The frequency of this phonon mode increases with pressure. We see no evidence for phonon modes in the lower pressure 0-or a'-phases at room temperature. Librational modes, however, have been observed at 50 cm-l in P-O2and at 44 and 78 cm-l in a-O2 at low temperature^.'^ Although the Raman-active 0-0 stretch mode of 1602 has been previously studied,ls4none of the other features described here has been reported.
Discussion The most surprising result of this work is the observation in t-O2 of strong infrared-active fundamental and combination modes involving the 0-0 stretch. There are several reasons why these infrared features are not explained by the collision-induced IR model16 that has been applied successfully to the infrared absorption in solid H2 After the oxygen sample is fully converted to the c-phase, the fundamental absorption is saturated for a sample thickness of no more than 40 pm. From the infrared peak for impurity 1601a0 molecules in l8O2,we estimate the absorbance of this saturated fundamental to be about 160/mm. In contrast, the absorbance in solid H2is 0.8/ mm, a factor of 200 less. Furthermore, for H2, the ob~
~~~
~
(14)Chill, J. E.; Leroi, 0. E. J. Cheem. Phys. 1868,61, 97-104. (15) Van Kranmdonk, J, Phyrico 1918,24, 347-82. (16)Prikhot'ko, A. F.; Plkur, Yu. 0.;Shanrkii,L.I. JETPLett. 1980, 32, 287-80. Prikhot'ko, A. F.; Oitrovrkii, V. 8,; Pikui, Yu. 0.;Shnnikii, L. I. Sou. J. Low Temp, Phys. 1980,6, 518-22.
Letters
servation of the overtones requires a path length of about 120 mm as compared to 40 pm here. Finally, if they were only collision induced, the infrared absorption peaks in the ephase should be clearly observable in the p- and a-'phases as well. Instead, they appear abruptly with the transformation to the e-phase. We attribute the IR bands near 3000 cm-' to combination modes involving infrared, Raman, and possibly inactive 0-0 stretches. The frequencies, however, are not straightforward sums of the component fundamentals. Frequencies of the combination modes are substantially lower than any binary combination of the Raman and the two IR 0-0 stretching modes. This is indicative of large anharmonicity and/or the involvement of an inactive or unobserved 0-0 stretching mode of low frequency. From Table I the correspondence between the respective modes of 1 6 0 2 and lSO2 is apparent, and within the experimental uncertainty the isotope shifts are in agreement with the calculated values. The observation of two Raman peaks in the lSO2sample is, at first glance, perplexing. On the basis of the isotopic shifts, we assign the lower frequency component a t 1485 cm-' to the expected 1sO-180 stretch. We believe that the second Raman-active 1sO-180 stretch at 1498 cm-' (13.5 GPa) is induced by the presence of the ls0-l80impurity. We note that a sample of 1 6 0 2 doped with 10% 1802 and no 160's0 exhibits only one l60-l6O Raman-active stretch. The present work allows us to draw several inferences about the crystal structure of eo2.First, the mutual exclusion of infrared and Raman modes strongly suggests that the structure is centric. Second, we can estimate the number of molecules per unit cell from the number of independent 0-0 stretch frequencies and combination modes. The observation of one Raman and two IR active fundamental 0-0 stretches for lSO2 at high pressures shows that the primitive unit cell must contain at least three O2 molecules. However, we believe that there are four molecules per primitive unit cell on the basis of packing arguments. It is not possible to pack three O2 molecules into a centric primitive cell and maintain the simple order observed in the contiguous a-,p-, and a'-phases. Since the precursor a'-phase contains only one molecule per primitive unit cell, the a'-e-phase transformation most likely involves distortions along two zoneboundary phonons, thereby quadrupling the unit cell.
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983
2465
The observation of strong infrared absorptions in the c-phase can only be attributed to the presence of strong attractive 02-02intermolecular interactions in the ground state. The observation that the 0-0 infrared mode appears at a significantly lower frequency than the Raman mode and that it softens as the pressure is increased indicates that the 0-0 intramolecular interaction is weakening in the t-phase. In effect, charge density is being transferred from the 0-0 bond to the 02-O2 intermolecular region in the e-phase, producing out-of-phase stretching modes with considerable transition dipole moments. Finally, the fact that the phonon mode at about 133 cm-' can be observed at room temperature is consistent interactions. This contrasts with p-0, with strong 02-02 in which the librons observable at low temperature become too broad at room temperature to be seen. In earlier studies of the electronic absorption spectra of low-temperature a-02,16several features were observed that seem related to the new infrared and Raman features of e - 0 2 seen in the present study. A 70-cm-' progression built on the two electronic origins, 20891 (AZ) and 26 308 (ZZ) cm-', was attributed to the previously observed libron mode.16 A 4-6-cm-' splitting in the origins of these two electronic bands was also noted and attributed to higher order (i.e., greater than 02-02pair) interactions. Thus, the conclusion that pairwise (and to a lesser extent, higher order) interactions are important for the excited states of low-temperature a-O2 was reached. Interestingly, the observed 0-0 stretch that is built upon the 20 891-cm-' electronic origin is 1443 cm-'. This vibration is required by symmetry to be ungerade (zone boundary mode) and is, therefore, analogous to the infrared-active modes that appear in the c-phase. Acknowledgment. B. B. McInteer and Maxwell Goldblatt of Los Alamos kindly prepared the lSO2gas sample. This study was sponsored by the Los Alamos National Laboratory Center for Materials Science, the Fundamental Research Program on Explosives, and by the Laboratory's Institutional Supporting Research Program. The work was performed under the auspices of the US.Department of Energy and was supported in part by the Division of Materials Sciences of the Office of Basic Energy Sciences and by the Division of Military Applications. Registry No. 02, 7782-44-7; 32767-18-3.
