The pressure-temperature $\left(P\text{−}T\right)$ phase diagram of ${\mathrm{N}}_2\text{−}{\mathrm{O}}_2$ mixture with a composition of ${\mathrm{N}}_2$-48 mol % ${\mathrm{O}}_2$ has been investigated using x-ray diffraction and the phase stability of a hexagonal phase (space group: $P6$/mmm), with the kagome lattice examined under high-pressure and low-temperature conditions. While the phase appears as a low-temperature phase of the cubic phase $\left(Pm3n\right)$ with the structure of $\gamma \text{−}{\mathrm{O}}_2$ or $\delta \text{−}{\mathrm{N}}_2$ and is stable in a wide range of pressures and temperatures, it transforms to lower symmetry monoclinic or orthorhombic phases at lower temperature, accompanied with a distortion of the kagome lattice. Based on Rietveld refinements, the monoclinic and orthorhombic phases are found to be in the $P{2}_1/a$ and Cmmm space groups, respectively. In magnetization measurements, a magnetic transition is observed with a relatively large drop of magnetization, corresponding to the cubic-to-hexagonal phase transition. This suggests that the hexagonal phase has a certain magnetic ordered state that arises from the molecular magnetic moment of ${\mathrm{O}}_2$.

• Received 14 January 2016
• Revised 23 July 2016

DOI:https://doi.org/10.1103/PhysRevB.94.064104