The epsilon phase of solid oxygen: The importance of the magnetic order

The epsilon phase of solid oxygen is a unique molecular crystal with C2/m space group symmetry being composed of the (O₂)₄ clusters. It has been a challenge for decades to elucidate the anomalous pressure dependence of the lattice parameters at 10-20 GPa. In previous calculations [1, 2], the antiferromagnetic structure with 8-atom primitive unit cell of the C2/m has been assumed as the most stable structure (called AFM2). We show that the problems are remained due to the possible mismatch between the C2/m space group and the magnetic orders. To treat this problem, we use an extended conventional unit cell (16 atom/cell) instead of 8-atom primitive cell with spin-polarized SCAN-rVV10 functionals. We found that the anti-ferromagnetic structure with 16-atom unit cell (called AFM1), is more stable than the AFM2 in pressure range from 10 GPa to 20 GPa. In the AFM1 order, there are four oxygen molecules in the computational cell where the constituent O atoms have the same value for the magnetic moment, while there four oxygen molecules having non-equivalent magnetic moments. The former ones are arranged making the (anti-)ferromagnetic order in the b- (a-)direction, while the latter ones are arranged making the (anti-)ferromagnetic order in the a- (b-)direction. This AFM1 arrangement is inconsistent with the magnetic space group corresponding to the C2/m space group. This result also suggests that there is breaking of the time-reversal symmetry, prompting future experimental investigation. The observed lattice parameters [3] and neutron powder diffraction patterns [4, 5] are compared with the calculations considering of the AFM1 structure.

References:

[1] Y. Crespo et al., PNAS 111 (29), 10427-10432 (2014).

[2] A. Ramírez-Solís et al., Phys. Chem. Chem. Phys. 19, 2826-2833 (2017).

[3] H. Fujihisa et al., Phys. Rev. Lett. 97, 085503 (2006).

[4] I. N. Goncharenko Phys. Rev. Lett. 94, 205701 (2005).

[5] S. Klotz J. Low. Temp. Phys. 192, 1–18 (2018).