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Influence of Lattice Interactions on the Jahn–Teller Distortion of the [Cu(H2O)6]2+ Ion: Dependence of the Crystal Structure of K2xRb2–2x[Cu(H2O)6](SeO4)2 upon the K/Rb Ratio
journal contribution
posted on 2013-09-16, 00:00 authored by Charles J. Simmons, Horst Stratemeier, Michael
A. Hitchman, Mark J. RileyThe temperature dependence
of the structures of a wide range of
mixed-cation Tutton’s salts of general formula K2xRb2–2x[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) from
those with a low ratio (form A). 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 A compared with form B. The alkali metal is surrounded
by seven close oxygen atoms in form B but eight oxygen
atoms in form A, 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 to A 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.