An X-ray investigation of the high-temperature phase of K₂SnCl₆

At room temperature the background of X-ray single-crystal photographs exhibits diffuse streaks along the line Δ from Γ to X and less sharp diffuse domains in the directions Σ. The streaks are explained by a soft rotary phonon A2(Δ) corresponding to an in-phase rotation of the octahedra in a plane. The displacements of the Cl ions due to their librations are identified in a difference-Fourier chart. The second diffuse system is explained by a transversal accoustical mode B1(Σ). The in-phase rotation of the octahedra in a plane minimizes the electrostatic energy, independently of the phase of the rotation in adjacent planes. In the model the repulsive parameter between the Cl and K ions decides whether the space group P4/mnc, as a result of an A2g(X) condensation, or the orthorhombic Pnnn, as a result of a second rotary mode condensation, is energetically favorable.