On the existence of temperature induced changes in the magnetic topology of crystals that show no first-order crystallographic phase transitions

The First-Principles Bottom-Up study of the 88 K and 273 K X-ray diffraction structures of the bis-2,3-dimethylpyridinium tetrabromocuprate molecular magnet shows that the analysis of the magnetic properties of a molecule-based magnet, that does not present any first order polymorphic transition in the range of temperature studied, depends on the X-ray structure employed. The reason is the thermal expansion anisotropy when the crystal goes from the low temperature phase to 273 K, which induces changes in the radical-radical J(AB) interactions. As a consequence, the magnetic topology of the low temperature and 273 K structures change, a fact that induces a change in the macroscopic magnetic susceptibility curve (only the 88 K structure of bis-2,3-dimethylpyridinium tetrabromocuprate reproduces well the two-leg spin ladder experimental properties of this magnet). When anisotropic thermal effects are suspected one should use low temperature structures to study the magnetic properties at low temperature, and high temperature structures for the study of the magnetic properties in that range of temperatures. (C) 2008 Elsevier Ltd. All rights reserved.