Cs+(18-crown-6)(2)e(-): A 1D Heisenberg antiferromagnet with unusual phasetransitions

Crystalline Cs+(18-crown-6)(2)e(-) was found to be a linear chain Heisenber g antiferromagnet (J/k(B) = -38.3 K). It undergoes a slow, irreversible tra nsition above similar to 230 K from a crystalline low temperature (LT) phas e to a disordered Curie-Weiss paramagnetic high temperature (HT) phase. A 1 00 ppm diamagnetic (more shielded shift of the Cs-133 MAS NMR peak accompan ies this transition. EPR studies confirm the transition and are in accord w ith the conversion of the known crystal structure to a disordered phase. Th is HT phase undergoes an additional first-order reversible transition upon cooling below similar to 220 K (Delta H = 0.33 kJ mol(-1) upon rewarming) a ccompanied by the formation of two peaks in the Cs-133 NMR spectrum. Variab le-temperature powder X-ray diffraction confirms the disordered or microcry stalline nature of the HT phase(s).