The crystal structure at room temperature and the heat-capacity and magnetic susceptibilities at low temperatures of single crystals of [${\left(\mathrm{C}{\mathrm{H}}_3\right)}_3$NH]Co${\mathrm{Cl}}_3$ · 2${\mathrm{H}}_2$O are reported. The orthorhombic crystals belong to the space group $\mathrm{Pnma}$ with $a=16.671\mathrm{}\left(3\right)\mathrm{}$ Å, $b=7.273\mathrm{}\left(1\right)\mathrm{}$ Å $c=8.113\mathrm{}\left(2\right)\mathrm{}$ Å, and $Z=4\mathrm{}$. The structure consists of chains of edge-sharing trans- [Co${\mathrm{Cl}}_4$${\left(\mathrm{O}{\mathrm{H}}_2\right)}_2$] octahedra running parallel to the $b$ axis. At 4.135 °K, [${\left(\mathrm{C}{\mathrm{H}}_3\right)}_3$NH]Co${\mathrm{Cl}}_3$ · 2${\mathrm{H}}_2$O undergoes a second-order phase transition. The heat-capacity data have been analyzed using Onsager's solution for the two-dimensional Ising model. The lattice contribution was taken into account by using the lattice of the isostructural [${\left(\mathrm{C}{\mathrm{H}}_3\right)}_3$NH]Cu${\mathrm{Cl}}_3$ · 2${\mathrm{H}}_2$O and a corresponding states procedure. The three principal-axis single-crystal susceptibilities display an unusual amount of anisotropy. Two of the data sets (one parallel and one perpendicular to the chemical chain) have been fit to an Ising linear-chain model. A small molecular-field correction significantly improved one of those fits. The $a$-axis susceptibility displays an unusually sharp rise, with decreasing temperature, very close to $T_N$ along with the suggestion of a net moment persisting below the transition. Thus, a Dzyaloshinsky-Moriya antisymmetric exchange interaction was assumed to be operative, since it is symmetry alowed in this space group, and Moriya's molecular-field-susceptibility calculation was used to fit this data set. All four of the data sets are quite reasonably described, the specific heat and one of the susceptibility sets within experimental error, by the following set of parameters: rectangular lattice exchange parameters of $\frac{J}{k}=(7.7\mathrm{}\pm 0.2)$ °K and $\frac{J^{\prime }}{k}=(0.09\mathrm{}\pm 0.01)$ °K; spectroscopic splitting parameters of $g_a=2.95\mathrm{}\pm 0.05$, $g_b=3.90\mathrm{}\pm 0.01$, and $g_c=6.54\mathrm{}\pm 0.01$; and a Dzyaloshinsky-Moriya antisymmetric exchange parameter of $\left|\frac{D}{k}\right|=(4.9\mathrm{}\pm 0.2)$ °K. Finally, a consistent spin structure is proposed in which the intrachain interaction is ferromagnetic with the compound ordering antiferromagnetically below 4.135°K such that there is a small net moment in the $a$ direction.

• Received 24 October 1972

DOI:https://doi.org/10.1103/PhysRevB.8.2185