2272
J. Phys. Chem. 1883, 87, 2272-2273
Phase Diagram of Oxygen up to 13 GPa and 500 K Takehlko Yagi,+ K. R. Hlrsch, and Wllfrled B. Holzapfel’ Experimentalphysik,Universlt-GH Paderborn. 4790 Paderborn, FRG (Received: December 20, 1982; In Final Form: April 20, 1983)
The phase diagram of oxygen has been determined to pressures of 13 GPa and at temperatures between 300 and 500 K. The melting temperatures are higher than previously reported and the a’ phase is found to extend to higher temperatures than expected from previous work. These differences are attributed to laser heating in the previous work.
Introduction As model substances for molecular solids, solidified gases under high pressure have received again much attention from experimental and theoretical points of view due to the recent developments in the diamond anvil high-pressure technique. Raman studies on oxygen at room temperature and pressures up to 18 GPa revealed the existence of at least three solid phases in this pressure range.’ Single-crystal X-ray studies at room temperature and pressures between 5.5 and 6.8 GPa showed2v3that the first high-pressure structure is 0-0, with the space group R3m, previously known only from normal-pressure low-temperature studies. At 9.6 GPa and 297 K, the structure of a’-02was also s01ved.~ The space group Fmmm and the atom position parameters indicated a close relation to the normal-pressure low-temperature phase a-O2 which has the space group C2/m. The reduction of symmetry from a’- to a-O2has been related to a distortion which may be driven by the antiferromagnetic ordering of ( Y - O ~The .~ data support the tentative phase diagram2that had been given shortly after the discovery of the t-O2 phase. The fact that these initial studies did not determine yet the phase boundary between 0-and E-O2and that several attempts to grow single crystals of t-02at elevated pressures and temperatures had failed stimulated the present study of the phase boundaries of O2 at higher pressures (up to 13 GPa) and elevated temperatures (up to 500 K). Experimental Section A diamond anvil cell and a realtime luminescence spectrometer system were used to determine the phase diagram. The oxygen sample was prepared by immersing the diamond anvil cell6in liquid oxygen. The filling procedure is the same as that previously described.’ The spectrometer system consista of a fmed self-focusinggrating and a linear diode array connected with a microprocessor which evaluates the pressure from the ruby fluorescence spectra at realtime. The details of this system are described elsewhereS6 The temperature was measured by a thermocouple placed on one diamond very close to the sample chamber. The P-T conditions in the sample chamber were measured every second while varying the temperature slowly. After several temperature cycles at constant load, the volume of the sample chamber became stable and stayed almost constant when the temperature was varied. When the sample chamber is filled with one single phase of oxygen, the pressure increases only slightly with temperature due to the thermal pressure of the specimen. When the P-T condition of the sample comes to a phase boundary, P and T follow the phase boundary until the ‘On leave from the Institute for Solid State Physics, The University of Tokyo, Tokyo, Japan.
phase transition is completed. After completion of the phase transition, the P-T curves show again a steeper slope which is then determined by the thermal pressure of the other phase. Thus, these nearly isochoric P-T curves are characterized by clear kinks at the onset and completion of the phase transitions (Figure 1) and by an extended range which represents directly a part of the phase boundary. If one starts at room temperature at different pressures, one can thus determine the phase boundaries in a wide range of pressure and temperature. This technique worked best for the melting curve, where only a few points were checked visually. The 0-a’phase boundary was clearly resolved by visual observations of color changes and the appearance of fine grained sample structure. The distinction of d and t was more difficult due to more subtle color changes. Heating at pressures above 12 GPa led directly from t to phase. The intermediate d phase could be distinguished clearly only below 11 GPa by a sudden appearance of needle shaped, dark orange crystals when 0phase samples were either cooled or compressed. These visual observations were supported by video recordings, which were replayed after the experiments to confirm the direct visual impressions. The temperature of the sample was controlled by two small electrical heaters, one for each diamond. These heaters use coaxial heating wires soldered to small copper blocks holding the diamonds. The resistance of these heaters was matched giving the same power to both diamonds to minimize temperature gradients on the sample. The temperature gradients between the sample and the thermocouple on the one diamond were determined in a calibration with a second thermocouple but without sample between the diamonds. These temperature gradients limit the accuracy of the present temperature determination to f 2 K around 400 K and f4 K around 500 K. At room temperature the present spectrometer system allows for a pressure determination with a precision of f0.025 GPa.6 At higher temperatures, this precision is decreased first by the deterioration of the ruby luminescence spectra and second by the uncertainty of the temperature measurement which effects also the evaluation of the pressure. The total uncertainty in the present pressure determination amounts therefore to f O . l GPa below 400 K and to h0.2 GPa around 500 K. (1)M. Nicol, K.R. Hirsch, and W. B. Holzapfel, Chem. Phys. Lett., 68,49 (1979). (2)H. d’Amour, W.B. Holzapfel, and M. Nicol, J . Phys. Chem., 85, 130 (1981). (3) D. Schiferl, D. T. Cromer, and R. L. Mills, Acta Crystallogr., Sect. B , 37, 1329 (1981). (4)D. Schiferl, D. T. Cromer, L. A. Schwalbe, and R. L. Mills, Acta Crystallogr., Sect. B , in press. (5) K. R. Hirsch and W. B. Holzapfel, Reu. Sci. Instrum., 52,52(1981). (6) K. R. Hirsch and W. B. Holzapfel, J . Phys. E, 16, 412 (1983).
0022-3654/83/2087-2272$01.50/0Q 1983 American Chemical Society
The Journal of Physical Chemistty, Vol. 87, No. 13, 1983
Letters
430
t
Pressure / GPa Figure 1. P-T curve for oxygen at nearly constant volume. Each point represents one P - T measurement at a rate of one per second. Changes in slope can be noticed at the onset and completion of melting. The dashed line represents the melting curve. 600
1
i
i
i
/'
i ,
Pressure / GPa
Flgure 2. Phase diagram of oxygen. The solid line represents the present data for the melting curve, the broken lines data for p-a' and &e Wansltions, and broken l i e data for the a'-€Wansition, respectively. The light dash-dotted lines reproduce the earlier data.' Error bars represent the estimated uncertainties of the present study.
Results Figure 2 shows the present results for the phase diagram of oxygen with estimated error bars on the phase lines together with the phase boundaries that have been reported previously2for temperatures below 300 K. A comparison of these results indicates that the present melting data are about 20 K higher at 6 GPa and the slope of the melting curve is significantly steeper. Furthermore, the a' phase extends to higher temperatures than previously reportedS2 The lower temperature values in the previous study2 resulted most likely from heating of the sample by the intense laser light that was not taken into account in the
2273
Raman measurements as noticed already Schiferl et al.3 Since this laser heating depends on the absorbance of the sample which increases under pressure' one can understand also the increase in the discrepancies with pressure. It should be noticed that this heating is only local and cannot be detected by the thermocouple on the diamond; however, it can be measured when the laser is focused onto the ruby chips. Therefore, the laser power for the ruby luminescence measurement was also reduced in the present experiments until no heating effects were noticeable which was typically at a level of 10 mW. The steeper slope of the liquid 0phase boundary with respect to the previously determined phase boundary agrees well with the present understanding' that the liquid-7-0 triple point is close to room temperature and the previous curve corresponds to the 7-0 phase boundary and not to melting. An extrapolation of this previous curve to the low-temperature 7-0 phase boundary data: however, leads to thermodynamic discrepancies since the 7-0 phase line should be steeper than the /3 liquid phase line at the triple point. From this point of view, further experimental work is needed for the low-temperature region of this phase diagram to solve the present uncertainties in the temperature range below 300 K. In addition, one can notice that quasi-isochoric P-T curves like the one in Figure 1 include further information about phase transitions. If this P-T curve would really represent an isochore, one could derive from the change of pressure AP, and temperature ATm along the isochroic setion of the melting curve between the two changes of slope and from the slope dP/ dTI$ of the isochor in the solid 0 phase by simple thermodynamic considerations, neglecting higher-order corrections in AP,, for the volume change on melting at constant pressure
where VP and BT@ represent the volume and the bulk modulus of the solid /3 phase just before melting. With reasonable estimates for V@(7.2 GPa, 385 K) = V@(6.5 GPa, 300 K) = 14 cm3/mol and BT@= 25 GPa from the data in ref 2 and (dP/dTI$)/(AP,/AT,) = 0.5 and AP, = 0.67 GPa both from Figure 1, one obtains AV, = 0.14 cm3/mol and, with AT, = 35 K from Figure 1, A S = AV, AP,/AT, = 2.27 J / ( K mol) = 0.32R which corresponds to about one-half of the value expected for normal melting at high pressure^.^ These considerations indicate that the P-T curve in Figure 1 represents no perfect isochor; however, it comes close to isochoric variations. With minor improvements, the present technique can result in nearly isochoric variations in smaller temperature ranges, and these improvements are being studied at the present time. Acknowledgment. We thank Dr. M. F. Nicol for stimulating discussions, and we are grateful for the support of T.Y. by an Alexander von Humboldt Fellowship. Registry No. Oxygen, 1182-44-1. (7) K. Syessen and M. Nicol, "Physics of Solids under High Pressure", J. S. Sobilling and R. N. Shelton, Ed., North Holland Publishing Co., 1981,p 33; M.Nicol, private communication. (8)J. W.Stewart, J. Phys. Chem. Solids, 12, 122 (1959). (9)S. M.Stishov, Sou. Phys. Usp., It, 816 (1969).
