Magnetically driven molecular orientational ordering in plastic crystals: The case of d-camphor

Many solid crystals exhibit a structural phase transition where a subset of its ions or entire molecules become orientationally ordered. As to why such ordering occurs remains mostly unresolved. We consider the extremely weak magnetic elements arising from the reorientations of the molecules experiencing mutual resonance to play the chief role. Two new features are identified in d-camphor: (1) the magnetic susceptibility abruptly changes when crossing the order-disorder phase transition at TII-III = 239.8 K during cooling and at TIII-II = 245.2 K during warming and (2) the complex dielectric constant exhibits two successive discontinuities only 0.2 K apart near the critical temperatures when the sweeping rate is only 0.1 K/min. We discuss how the change in entropy associated with order-disorder transitions in plastic crystals represents temporal changes rather than spatial changes in the system. Our findings may be extended to study why many other crystalline solids exhibit orientational ordering and irreversibility.