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Effect of deuteration on the neel temperature of CoCl2.6H20 Sahri, Darshan Singh

Abstract

The nuclear magnetic resonance technique has been used to study the effect of deuteration on the Neel temperature of CoCl₂•6H₂0 single crystals. The Neel temperature varies with a period of 180° as the external magnetic field is rotated about the b-axis⊥ the a-c plane. On deuteration, the Neel temperature rises for all orientations, the more deuterium being introduced the higher the transition point. The maximum increase of approximately 6% in Neel temperature is obtained with a 92% concentration of deuterium. In addition, the anisotropy in TN decreases from approximately 0.08°K for 0% deuteration to slightly less than 0.05°K for 92% deuteration. Further, the orientation-averaged Neel temperature seems to vary linearly with the cube root of relative concentration of deuterium. In the vicinity of H₀ = 4,000 gauss, the Neel temperature increases with an increase in the external magnetic field [formula omitted] being ⊥ c-axis. This unusual behaviour is common to deuterated as well as non-deuterated samples and has not yet been explained. Following Haseda's conjecture, a semi-empirical attempt is made to establish a connection among the change in Neel temperature upon deuteration, the super-exchange parameter and the potential of a proton in a hydrogen bond. The picture presented is that the Neel temperature rises on deuteration because of a change in the average of the super-exchange parameter over the ground vibrational state of the hydrogen atom. An x-ray analysis shows that at room temperature the CoCl₂•6H₂0 and CoC1₂•6D₂0 crystals have the same symmetry and their cell dimensions do not differ by more than 0.2%. The infra-red spectrum of CoC1₂•6D₂0 has been used to determine the value of electrostatic field gradient at the deuteron sites. This value is consistent with the observed quadrupole splitting of the n.m.r. spectrum of the deuterons. The n.m.r. lines belonging to deuterons in the water molecules not forming a square configuration around cobalt ions have been identified.

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