Antiferromagnetic Ordering and Crystal Field Behavior of Ni<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cl</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>4<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">H</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>O

Magnetic-susceptibility and heat-capacity measurements on single crystals of Ni${\mathrm{Cl}}_{2}$ \ifmmode\cdot\else\textperiodcentered\fi{} 4${\mathrm{H}}_{2}$O, which is isomorphic to Mn${\mathrm{Cl}}_{2}$ \ifmmode\cdot\else\textperiodcentered\fi{} 4${\mathrm{H}}_{2}$O and Mn${\mathrm{Br}}_{2}$ \ifmmode\cdot\else\textperiodcentered\fi{} 4${\mathrm{H}}_{2}$O, indicate an antiferromagnetic-paramagnetic transition at (2.99\ifmmode\pm\else\textpm\fi{}0.01) \ifmmode^\circ\else\textdegree\fi{}K. The large zero-field splitting of the ${\mathrm{Ni}}^{2+}$ ion in this material results in an 11 \ifmmode^\circ\else\textdegree\fi{}K doublet-singlet splitting of its ground state with the doublet lying lower. A molecular-field model has been used to obtain an exchange-parameter value of $\frac{\mathrm{zJ}}{k}=\ensuremath{-}5.25$ \ifmmode^\circ\else\textdegree\fi{}K. The effect of the crystal field anisotropy on the transition temperature has been interpreted and the results seem to indicate that the molecular-field theory gives a better approximation than do the Green's-function techniques.

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