Phase diagrams to 40 kbar and crystallographic data for RbNO 2 and CsNO 2

CsNO ₂ I at ambient conditions is cubic, space group O_h¹- Pm3 m, with a₀ = 4.389Å, and transforms at -94°C to rhombohedral CsNO ₂ II, space group D ⁵_3d-R overline3m , with a_rh = 4.307Å, α = 87°22'. The CsNO ₂{II}/{I} transition line rises with pressure. RbNO ₂ I at ambient conditions is cubic, space group O_h⁵- Fm3 m, with a₀ = 6.934Å. It transforms at -12°C to monoclinic RbNO ₂ II with a₀ = 8.904, b₀ = 4.828, c₀ = 8.185Å, β = 115.7° at -62°C. RbNO ₂ II appears to be ordered, whereas RbNO ₂ I has a configurational entropy of Rln32. The RbNO ₂{II}/{I} transition line is terminated at 0.3 kbar with the appearance of RbNO ₂ III which is 17.3% denser than RbNO ₂ I. The RbNO ₂{II}/{III} transition pressure increases with decreasing temperature to a triple point at 1.2 kbar, -65°C where a further dense phase RbNO ₂ IV appears. The RbNO ₂{IV}/{III} phase boundary is very similar to the CsNO ₂{II}/{I} boundary. The RbNO ₂{II}/{IV} transition pressure rises slightly with decreasing pressure. The melting curve of RbNO ₂ I passes through a maximum at 2.2 kbar, 390°C, and is terminated at the RbNO ₂ III/I/liquid triple point at 5.2 kbar, 382°C. The melting curve of RbNO ₂ III rises steeply with pressure.