Magnetochemistry of chromium(III): Linear-chain–to–simple-cubic crossover magnetic interactions in several halopentammines

The magnetic susceptibility of polycrystalline samples of [Cr(${\mathrm{NH}}_3$${)}_5$Cl]${\mathrm{Cl}}_2$, [Cr(${\mathrm{NH}}_3$${)}_5$Br]${\mathrm{Br}}_2$, and [Cr(${\mathrm{NH}}_3$${)}_5$F](${\mathrm{NO}}_3$${)}_2$ has been measured over the temperature interval from 300 mK to 4.2 K. The chlorine compound orders antiferromagnetically at 590 mK and the bromine compound at 780 mK. The compound [Cr(${\mathrm{NH}}_3$${)}_5$F](${\mathrm{NO}}_3$${)}_2$ behaves like a model Heisenberg linear chain, with S=(3/2 and exchange constant J/$k_B$=-0.082 K. The first two compounds present one-dimensional features but with important three-dimensional crossover effects. From lattice-dimensionality crossover calculations and analysis of the susceptibility data we find $J_z$/$k_B$=-0.35 K, R=-0.2, with R=$J_{\mathrm{xy}}$/$J_z$, for the chlorine compound and $J_z$/$k_B$=-0.33 K, R=-0.46 for the bromine compound. In every case the Landé factor is g=2. A comparative analysis of the three compounds is made.