NiSn${\mathrm{Cl}}_6$·6${\mathrm{H}}_2$O forms rhombohedral (trigonal) crystals containing one axially distorted octahedral ${\left[\mathrm{N}\mathrm{i}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{++}$ complex per unit cell. Measurements of (a) the magnetic susceptibility, (b) isothermal magnetization, and (c) magnetic heat capacity between ∼ 0.35 and 4.2 K can be described by the spin Hamiltonian $\mathcal{H}=D{S}_z^2+g{\mu }_B\overrightarrow{\mathrm{S}}·\overrightarrow{\mathrm{H}}$ for $S=1\mathrm{}$ with $\frac{D}{k}=+0.64\mathrm{}$ K and $g=2.25\mathrm{}$. The ground-state triplet is thus split in zero field into a lower singlet and an upper doublet. There is also evidence of antiferromagnetic coupling among ${\mathrm{Ni}}^{++}$ ions which is weak in comparison with this splitting. A system of this type should exhibit unusual properties for $T<\mathrm{}\frac{D}{k}$. Exploratory magnetocaloric observations with $H\parallel \mathrm{trigonal}\mathrm{axis}$ reveal cooling by adiabatic magnetization for $H\le \frac{D}{g{\mu }_B}$. They also suggest the establishment of spin order at sufficiently low temperatures for $H\sim \frac{D}{g{\mu }_B}$.

• Received 24 July 1972

DOI:https://doi.org/10.1103/PhysRevB.6.3488