X-ray-diffraction study of the crystal structures and orientational glass state of ${\mathrm{Ar}}_{1\mathrm{-}\mathit{x}}$(${\mathrm{N}}_2$${)}_{\mathit{x}}$ solid solutions

Solid solutions of ${\mathrm{Ar}}_{1\mathrm{-}\mathit{x}}$(${\mathrm{N}}_2$ ${)}_{\mathit{x}}$ have been investigated by x-ray powder diffraction in the range 0.5<x<1.0, 8<T<60 K, and under hydrostatic pressures up to 2200 bars. The structural hcp-cubic (Pa3) phase transition is identified as a martensitic transformation with a wide transformation hysteresis. For x<0.8 and T<30 K the solid solutions show the orientational glass state, which is characterized by a superposition of inhomogeneous and homogeneous lattice strains. The inhomogeneous component is described by the random-field model. The homogeneous component corresponds to an orthorhombic distortion of the hcp lattice. Two products of the Pa3 phase have been detected: an orientationally disordered fcc state and the high-pressure γ modification. It is concluded that the Pa3 phase and its products have no direct relevance to the orientational glass state. The analogies to the glasslike state of the mixed cyanides and of ortho-para-${\mathrm{H}}_2$ are worked out.