Paramagnetic Susceptibilities and Temperature-Dependent Excitation Energies in Linear Organic Crystals

Abstract

The pseudospin solutions of the regular and the alternating Heisenberg antiferromagnet are shown to explain quantitatively the temperature dependence of the absolute paramagnetic susceptibilities of organic molecular crystals containing linear arrays of free radicals. The pseudospin solutions also reproduce, for a rigid lattice, the temperature dependence of the singlet—triplet energy gap in triplet exciton crystals. This effect, previously interpreted as evidence for exciton—phonon interactions, is again shown to be a many‐body effect, which is found even in the absence of exciton—phonon coupling. Information about the structures of these organic crystals is obtained. It is shown that triethylammonium—(TCNQ)2 corresponds to a weakly alternating Heisenberg antiferromagnet rather than, as previously suggested, to a very strongly alternating antiferromagnet. The results for the absolute paramagnetic susceptibility of Wurster's blue perchlorate confirm the previously proposed dimerization below its phase transition at ∼190°K.