Antiferromagnetism in CoCl2.2H2O. II. Chlorine Nuclear Magnetic Resonance and Paramagnetic Susceptibility
The chlorine NMR in CoCl2.2H2O has been studied in the paramagnetic and antiferromagnetic states. Zeeman-splitting studies in the paramagnetic state (76°K) yield h-1e2qzzQ35=9.866+/-0.001 Mc/sec, η=0.44+/-0.01, and magnetic field shifts (∆HH0)αα=0.032+/-0.002, (∆HH0)ββ=0.112+/-0.002, (∆HH0)γγ=0.108+/-0.002. The principal-axis orientations of the electric-field-gradient and field-shift tensors are Z∥b, X∥(a*-30°), and γ∥b, α∥(a*+43°), respectively. In the antiferromagnetic state (4.0°K) zero-field resonances occur at (35Cl) 6.576, 11.534, 16.415, (37Cl) 5.702, 9.611, 13.460 (+/-0.002) Mc/sec. Using the paramagnetic-state asymmetry parameter, the observed frequencies yield h-1e2qzzQ: (35Cl) 9.855, (37Cl) 7.767 Mc/sec, and effective 0°K internal magnetic fields Hi: (35Cl) 27.558, (37Cl) 27.595 kOe. The internal field assignments correspond to a magnetic hyperfine anomaly for the two isotopes of (1.3+/-0.4) × 10-3. The principal directions and principal values of the magnetic-susceptibility tensor have been determined in the paramagnetic state (20-120°K). The susceptibility is characterized by extreme rhombic anisotropy. The major axis δ coincides with b; the other two axes (ξ,ζ) nearly coincide with the Co-Cl bond directions. The measured principal values are analyzed on the basis of a two-parameter crystal field model which includes the effect of spin-orbit coupling. The chlorine magnetic hyperfine coupling constants in the paramagnetic state are evaluated by combining the field-shift measurements with spin expectation values obtained from the crystal field model. The results for 35Cl are Aξξ=5.8(+/-0.3)×10-4, Aζζ=6.85(+/-0.15)×10-4, Aδδ=4.09(+/-0.08)×10-4 cm-1. A comparison of the paramagnetic and antiferromagnetic state results suggests that Cl-Cl interactions may contribute significantly to the transferred hyperfine field at the chlorine nucleus. The electric field gradient is compared with predictions of a point-charge model.