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The magnetic susceptibility of a powdered sample of Wurster's blue perchlorate, which is a free radical, has been measured as a function of temperature between room and liquid-air temperature by a Gouy balance. The susceptibility maximum at 189°K is probably due to a transition from a paramagnetic to a diamagnetic state. The Curie-Weiss law with the paramagnetic Curie temperature of -23±5°K is applicable to the susceptibility above 220°K.
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Kōichi Okumura (1963) studied this question.
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