Nuclear Magnetic Resonance in Antiferromagnetic ${\mathrm{Rb}}_2$Mn${\mathrm{Cl}}_4$·2${\mathrm{H}}_2$O and ${\mathrm{Cs}}_2$Mn${\mathrm{Cl}}_4$·2${\mathrm{H}}_2$O

The NMR spectra of H, Cl, Rb, or Cs nuclei in antiferromagnetic ${\mathrm{Rb}}_2$Mn${\mathrm{Cl}}_4$·2${\mathrm{H}}_2$O and ${\mathrm{Cs}}_2$Mn${\mathrm{Cl}}_4$·2${\mathrm{H}}_2$O were used to study magnetic ordering in these compounds. The proton resonance data show that the magnetic space group is $P_s\overline{1}$ and that the magnetic unit call is quite different from the chemical unit cell. For the I > ½ nuclei, we have used only the minimum amount of information borrowed from the paramagnetic state to find the direction and magnitude of the internal fields. To do this we have relied on the experimental evaluation of traces and derivatives of traces of the Hamiltonian. The transferred hyperfine field at the two Cl sites in the coordination octhahedron differs by 10% at 1.1 °K, and the Mn environment is octahedral only in the zeroth approximation. There appears to be no great difference in the amount of unpaired spin transferred to the Cl sites in the two compounds. Appreciable hyperfine fields are found at the Rb and Cs sites. The hyperfine field at the Cs site is larger than that at the Rb site, and it is suggested that this may have somewhat inhibited the decrease of $T_N$ arising from increased Mn-Mn distances when Rb is replaced with Cs.