EPR of ${\mathrm{Mn}}^{2+}$ -doped ${\mathrm{NiSO}}_4$ · ${7\mathrm{H}}_2$O and ${\mathrm{MgSO}}_4$ · ${7\mathrm{H}}_2$O: ${\mathrm{Mn}}^{2+}$ ${-\mathrm{N}\mathrm{i}}^{2+}$ exchange constant

$X$-band EPR measurements on ${\mathrm{Mn}}^{2+}$ -doped isostructural single crystals of paramagnetic ${\mathrm{NiSO}}_4$ · ${7\mathrm{H}}_2$O and diamagnetic ${\mathrm{MgSO}}_4$ · ${7\mathrm{H}}_2$O have been made at room, liquid-nitrogen, and liquid-helium temperatures. The spin-Hamiltonian parameters are evaluated from the data using a rigorous least-squares-fitting program suitable for electron-nuclear spin-coupled systems. Using the $g$ shift in the paramagnetic lattice from that in the diamagnetic lattice a value of 4.293 GHz for the ${\mathrm{Mn}}^{2+}$-${\mathrm{Ni}}^{2+}$ exchange constant in ${\mathrm{NiSO}}_4$ · ${7\mathrm{H}}_2$O is estimated. For ${\mathrm{NiSO}}_4$ · ${7\mathrm{H}}_2$O host the linewidths exhibit temperature and magnetic field dependence. The temperature dependence of the zero-field splitting parameter $b_2^0$ for both hosts is found to be linear, thus being explained as mainly due to the thermal expansion of the lattice.