Influence of Lattice Interactions on the Jahn–Teller Distortion of the [Cu(H₂O)₆]²⁺ Ion: Dependence of the Crystal Structure of K_2<i>x</i>Rb_{2–2<i>x</i>}[Cu(H₂O)₆](SeO₄)₂ upon the K/Rb Ratio

The temperature dependence of the structures of a wide range of mixed-cation Tutton’s salts of general formula K_2<i>x</i>Rb_{2–2<i>x</i>}[Cu­(H₂O)₆]­(SeO₄)₂ has been determined over the temperature range 90 to 320 K. Crystals with a high proportion of potassium adopt a different structure (form <b>B</b>) from those with a low ratio (form <b>A</b>). In both forms, the [Cu­(H₂O)₆]²⁺ ion has an orthorhombically distorted tetragonally elongated coordination geometry, but the long and intermediate bonds occur with a different pair of water molecules in form <b>A</b> compared with form <b>B</b>. The alkali metal is surrounded by seven close oxygen atoms in form <b>B</b> but eight oxygen atoms in form <b>A</b>, and this difference in coordination number is associated with the change in the Cu–O bond distances via the hydrogen-bonding network. For crystals with between 32 and ∼41% potassium, a relatively sharp change from form <b>B</b> to <b>A</b> occurs on cooling, and the temperature at which this occurs increases approximately linearly as the proportion of potassium falls. For the whole range of mixed crystals, the bond lengths have been determined as a function of temperature. The data have been interpreted as a thermal equilibrium of the two structural forms of the [Cu­(H₂O)₆]²⁺ ion that develops gradually as the temperature increases, with this becoming more pronounced as the proportions of the two cations become more similar. The temperature dependence of the bond lengths in this thermal equilibrium has been analyzed using a model in which the Jahn–Teller potential surface of the [Cu­(H₂O)₆]²⁺ ion is perturbed by lattice strain interactions. The magnitude and sign of the orthorhombic component of this strain interaction depends upon the proportion of potassium to rubidium ions in the structure