Isotope Effect on the Néel Temperature of CoCl2·6H2O

Abstract

Using the nuclear-magnetic-resonance technique, it has been discovered that an increase of approximately 6% in the Néel temperature $T_N$ of Co${\mathrm{Cl}}_2$·6${\mathrm{H}}_2$O is obtained when 92% of the protons are replaced by deuterons. The anisotropy of $T_N$ decreases from 0.08°K for 0% deuteration to slightly less than 0.05°K for 92% deuteration. Following a conjecture by Haseda, a semiempirical attempt is made to establish a connection among $T_N$, the exchange energy, and the potential energy of a proton in a hydrogen bond. The picture presented is that $T_N$ rises on deuteration because of a change in the average of the exchange energy over the vibrational state of the hydrogen atom.