Raman spectroscopic investigations of the antiferromagnetic   phase of solid oxygen at low pressure (up to 1.25 GPa)

Abstract

Elementary magnon, libron and vibron excitations as well as combined two-libron excitations of solid α oxygen have been investigated by means of Raman scattering at several isobars in the pressure range up to 1.25 GPa. We deduced the band frequency, bandwidth and relative band intensity of all modes as a function of temperature and pressure. On the basis of these results we can exclude the possibility of all second-order phase transitions in the low temperature, low pressure range of oxygen stated in the literature. The disappearance of the sublattice magnetization σ could be estimated from the frequency of the higher energy magnon mode at the critical pressure which is in the vicinity of the phase transition from the antiferromagnetic ordered δ phase to the non-magnetic ε phase. The change of several spectroscopic features under increasing pressure clearly indicates that anharmonic contributions in the libron potential are altered.

Low-temperature phases of solid oxygen at pressures up to8GPadetermined by Raman and infrared spectroscopy

“…Whereas the magnon mode vanishes with increasing pressure, the two-libron excitations are seen throughout the ␣ phase and become more intense at higher pressures. This trend-already observed at low pressure 16 -continues even in the ␦ phase, where also the two-libron excitations exist. The intensity of the two-libron excitations relative to the intensity of the one-libron excitations is shown in the inset in Fig.…”

“…32 These we explained as originating with magnetic coupling of two molecules in the primitive cell performing librations. In our recent low-pressure Raman study 16 on ␣ oxygen we found that due to an enhancement of the spin coupling mechanism, the intensities of two-libron excitations increase by a factor of about 5 as one applies pressure from 0 to 1.25 GPa. Therefore, the pressure dependence of frequencies and intensities of these two-libron excitations have to be investigated in the pressure range up to 8 GPa to obtain Raman spectroscopic information about magnetism of the high-pressure ␣ phase and the ␦ phase at low temperatures.…”

“…This does not occur: the intensity of the magnon mode diminishes with increasing pressure. 15,16 At pressures higher than 3 GPa-where the ir-active vibron mode at ϳ1550 cm −1 exists-it is so weak that it is not possible to detect it with our conventional Raman system. Therefore one cannot use the magnon mode to study magnetism in ␣ and ␦ oxygen at higher pressure by Raman spectroscopy.…”

“…Recently we investigated the lowtemperature ͑T Ͻ 30 K͒ low-pressure ͑p Ͻ 1.25 GPa͒ range of the ␣ phase by means of Raman spectroscopy. 16 We could exclude the possibility of all second-order phase transitions in the ␣ phase at pressures lower than 4 GPa.…”

“…8 and 9͒ and 1.8 K ͑Ref. 15͔͒ and since we know 16 that this procedure does not produce thermodynamically stable samples, new Raman measurements on equilibrium crystals will be necessary to determine the ͑equilib-rium͒ phase diagram of oxygen. In addition to these discrepancies in optical spectra, structural investigations on solid oxygen also show an inconsistent image.…”