Development of long-range order in Hg3-δAsF6 and Hg3-δSbF6

Development of long-range order between the mercury chains of Hg3-δAsF6 and Hg3-δSbF6 compounds has been studied by neutron diffraction. The results are compared to a model of long-range order in which intrachain interactions are treated exactly and interchain interactions are approximated by a mean field. For Hg3-δSbF6 the model accurately predicts the temperature dependence of the Bragg peak intensities. For Hg3-δAsF6 the intensities are found, unexpectedly, to be proportional to the first power rather than the square of the calculated order parameter.