Antiferromagnetism in CoCl2·2H2O. II. Chlorine Nuclear Magnetic Resonance and Paramagnetic Susceptibility

The chlorine NMR in Co${\mathrm{Cl}}_2$·2${\mathrm{H}}_2$O has been studied in the paramagnetic and antiferromagnetic states. Zeeman-splitting studies in the paramagnetic state (76°K) yield $h^{-1\mathrm{}}{e}^2\mathrm{qzz}{Q}^{35}=9.866\mathrm{}\pm 0.001$ Mc/sec, $\eta =0.44\mathrm{}\pm 0.01$, and magnetic field shifts ${\left(\frac{\Delta H}{H_0}\right)}^{\alpha \alpha }=0.032\mathrm{}\pm 0.002$, ${\left(\frac{\Delta H}{H_0}\right)}^{\beta \beta }=0.112\mathrm{}\pm 0.002$, ${\left(\frac{\Delta H}{H_0}\right)}^{\gamma \gamma }=0.108\mathrm{}\pm 0.002$. The principal-axis orientations of the electric-field-gradient and field-shift tensors are $Z\parallel b$, $X\parallel (a^{*}-30\mathrm{}°)$, and $\gamma \parallel b$, $\alpha \parallel (a^{*}+43\mathrm{}°)$, respectively. In the antiferromagnetic state (4.0°K) zero-field resonances occur at (${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$) 6.576, 11.534, 16.415, (${}_{}{}^{37}{}_{}{}^{}\mathrm{Cl}$) 5.702, 9.611, 13.460 (±0.002) Mc/sec. Using the paramagnetic-state asymmetry parameter, the observed frequencies yield $h^{-1\mathrm{}}{e}^2\mathrm{qzzQ}$: (${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$) 9.855, (${}_{}{}^{37}{}_{}{}^{}\mathrm{Cl}$) 7.767 Mc/sec, and effective 0°K internal magnetic fields $H_i$: (${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$) 27.558, (${}_{}{}^{37}{}_{}{}^{}\mathrm{Cl}$) 27.595 kOe. The internal field assignments correspond to a magnetic hyperfine anomaly for the two isotopes of (1.3±0.4) × ${10}^{-3\mathrm{}}$. 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 $\delta$ coincides with $b$; the other two axes (