Crystal Structure and Ionic Conductivity of Three Polymorphic Phases of Rubidium Trifluoromethyl Sulfonate, RbSO₃CF₃

The crystal structures of three polymorphic phases of rubidium trifluoromethyl sulfonate (RbSO₃CF₃, rubidium ‘triflate') were solved from X-ray powder diffraction data. At room temperature, rubidium triflate crystallizes in the monoclinic space group Cm with lattice parameters of a = 19.9611(5) Å, b = 23.4913(7) Å, c = 5.1514(2) Å, β = 102.758(2)°; Z = 16. At T = 321 K, a first-order phase transition occurs toward a monoclinic phase in space group P2₁ with lattice parameters at T = 344 K of a = 10.3434(5) Å, b = 5.8283(3) Å, c = 5.1982(3) Å, β = 104.278(6)°; Z = 2). At T = 461 K, another phase transition, this time of second order, occurs toward an orthorhombic phase in space group Cmcm with lattice parameters at T = 510 K of a = 5.3069(2) Å, b = 20.2423(10) Å, c = 5.9479(2) Å; Z = 4. As a common feature within all three crystal structures of rubidium triflate, the triflate anions are arranged in double layers with the lipophilic CF₃ groups facing each other. The rubidium ions are located between the SO₃ groups. The general packing is similar to the packing in cesium triflate. Rubidium triflate can be classified as a solid electrolyte with a specific ionic conductivity of σ = 9.89 × 10^-9 S/cm at T = 384 K and σ = 3.84 × 10^-6 S/cm at T = 481 K.