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

Abstract

We report studies of vibron, libron, and combined two-libron excitations of solid oxygen in the pressure range up to 8 GPa using Raman scattering and infrared absorption at low temperatures. We took care to ensure that our samples have been close to thermodynamic equilibrium, as confirmed by additional measurements. On the basis of these results we present a revised phase diagram of thermodynamically stable oxygen. There the ␣ phase exists up to temperatures of the ␤ phase and up to a pressure of about 6 GPa, where a clear first-order transition to the ␦ phase occurs. Our spectroscopically determined phase diagram is in accordance with the structurally determined one by Gorelli et al. ͓Phys. Rev. B 65, 172106 ͑2002͔͒. Inconsistencies with other reported phase diagrams can be attributed to the fact that those samples were not at equilibrium. Furthermore, we provide an indirect proof, using Raman spectroscopy, of the antiferromagnetic order of ␦-O 2 . The present Raman and infrared investigations together with spectroscopic, x-ray, and neutron data provide a consistent picture of oxygen in the p-T range investigated here.

Magnetism in Solid Oxygen Studied by High-Pressure Neutron Diffraction

“…The - transition is definitely first-order at low temperatures, for reasons given further below. Above 100 K the situation is still not clear, but evidence of a small jump in the vibron frequency [19] suggests a very weak first-order nature. Further compression to 8-10 GPa leads to the monoclinic  phase (space group C2/m, Zmol=4) which has a reddish appearance when observed in a diamond anvil cell.…”

“…The low-energy spectrum of -O2 shows a sharp line at 80-90 cm -1 (width approx. 2 cm -1 ) which was so far interpreted as due to a minority  phase [19,43], based on its proximity to a strong line which occurs in -O2 at higher pressures. This appears however highly unlikely given the fact that it can be observed down to below 3 GPa, i.e.…”

“…Such an anvil profile forms -together with the gasket -a spheroidal sample chamber which produces quasi-hydrostatic pressure conditions as demonstrated in numerous measurements on other molecular solids [33]. This aspect is quite important since solid O2 is known to be highly sensitive to non-hydrostatic compression [15,18,19]. The use of nullscattering TiZr gaskets avoids Bragg reflections from the immediate sample environment.…”

Shift of close-packed basal planes as an order parameter of transitions between antiferromagnetic phases in solid oxygen: II. Temperature/pressure dependence of sound velocities and lattice parameters

“…Fig. 8 shows hypotetical pressure dependence of the modula product 22 c t c l calculated on the basis of equations (12)- (16) with characteristic values of phenomenologic parameters (see Table I). It is clear that this dependence should be nonmonotonic, with noticeable decrease of modulus while approaching to αβ and αδ-phase boundaries.…”

“…t dependence of interplane distance), softening of shear modulus and corresponding velocities in the vicinity of T αβ . We assume that with certain stipulation the developed model may be applied to interpretation of IR spectra15,16 which pressure dependence looks similar to pressure dependence of the order parameter. In particular, we predict non-monotonic pressure dependence of elastic modula in α-phase and critical behaviour in αδ-transition point.…”