Crystal Coulomb energies. V. The phase transition in Wurster’s blue perchlorate

In an extension of earlier work on Wurster’s blue perchlorate, it is shown that Madelung energy calculations by Ewald’s method account fairly well for the small observed enthalpy of transition at 186 °K; the Madelung energy differences are probably due only to observed rearrangements in the perchlorate geometry. The Madelung energy does not, however, favor the large extent of alternation in cation–cation distances determined in the low-temperature crystal structure; this finding may shed some light on the mechanism of transition between the high temperature Curie–Weiss paramagnet and the low-temperature spin-paired diamagnet for this pseudo-one-dimensional organic crystal.