Cs+(18-crown-6)2e-: A 1D Heisenberg Antiferromagnet with Unusual Phase Transitions

Abstract

Crystalline Cs+(18-crown-6)2e- was found to be a linear chain Heisenberg antiferromagnet (J/kB = −38.3 K). It undergoes a slow, irreversible transition above ∼230 K from a crystalline low temperature (LT) phase to a disordered Curie−Weiss paramagnetic high temperature (HT) phase. A 100 ppm diamagnetic (more shielded) shift of the 133Cs MAS NMR peak accompanies this transition. EPR studies confirm the transition and are in accord with the conversion of the known crystal structure to a disordered phase. This HT phase undergoes an additional first-order reversible transition upon cooling below ∼220 K (ΔH = 0.33 kJ mol-1 upon rewarming) accompanied by the formation of two peaks in the 133Cs NMR spectrum. Variable-temperature powder X-ray diffraction confirms the disordered or microcrystalline nature of the HT phase(s).