SINGLE-CRYSTAL STUDIES OF THE ZERO-FIELD SPLITTING AND MAGNETIC EXCHANGE INTERACTIONS IN THE MAGNETIC-SUSCEPTIBILITY CALIBRANT HGCO(NCS)4

Powder and single-crystal magnetic susceptibility data are reported for HgCo(NCS)4 in the temperature range 1.7-300 K. Confusion concerning various previously reported magnetic parameters is rectified by developing an understanding of the magnetic behavior throughout the temperature range examined. Curie-Weiss analysis of the data is valid only in the temperature interval of 40-300 K, where the best-fit parameters C = 2.305 (g = 2.218) and theta = -0.32 K allow the experimental values to be calculated to better than 0.02% when the diamagnetic correction is taken as -190 X 10(-6) emu/mol and the temperature independent paramagnetic correction is taken to be the spectroscopically determined value of +428 X 10(-6) emu/mol. Between 1.7 and 40 K, a ''best-fit' Curie-Weiss model simply does not represent the data well enough for it to be used as a parametric equation for a calibrant. The low-temperature data are best understood in terms of a model that accounts for zero-field splitting (D = +10.6 cm-1) of the 4A ground state and for antiferromagnetic exchange interactions (zJ = -0.095 cm-1) between the Co(II) ions. The exchange is XY in character because of the positive D value (S = \+/- 1/2] ground state). Since there are two possible types of NCS-Hg-SCN bridge geometries between nearest neighbors, the magnetic lattice is of mixed dimensionality and additional studies will be needed to further clarify the magnetic structure at lower temperatures.