Complex phase transitions in stable thiazyl radicals: spin-gap, antiferromagnetic ordering and double melting

We examined the magnetic and thermal properties of a stable thiazyl radical 1,3,2-benzodithiazolyl, BDTA. While the diamagnetic room-temperature phase had been known for BDTA, we found a new phase transition to a paramagnetic solid phase at 346 K and a superheating process that resulted in the double melting (melt recrystallization-melt process) above 360 K. The supercooling of the paramagnetic high-temperature phase also occurred, leading to an antiferromagnetic ordering at 11 K with gamma-type anomaly in the temperature dependence of the heat capacity. Based on these data, the complex phase diagram of BDTA has been determined. (C) 2002 Published by Elsevier Science B.V.