Phase stability and magnetic behavior of hexagonal phase ofN2−O2system with kagome lattice under high pressure and low temperature

The pressure-temperature $(P\text{\ensuremath{-}}T)$ phase diagram of ${\mathrm{N}}₂\text{\ensuremath{-}}{\mathrm{O}}₂$ 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 $(Pm3n)$ with the structure of $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{O}}₂$ or $\ensuremath{\delta}\text{\ensuremath{-}}{\mathrm{N}}₂$ 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 ${\mathrm{O}}₂$.