Hydrostatic Pressure Dependence of the Paramagnetic Resonance of anS-State Ion in a Noncubic Lattice:Mn2+in Calcite

The interaction of the substitutional impurity ion, ${\mathrm{Mn}}^{2+}$, with the crystalline lattice of calcite (CaC${\mathrm{O}}_3$) was studied at room temperature by means of electron paramagnetic resonance (EPR) at 9.4 kMc/sec as a function of hydrostatic pressure up to 7000 atm. Using a point-charge model, and assuming the parameter $D$ of the spin Hamiltonian is linearly dependent on the axial crystalline field, it was deduced that the local compressibility about the ${\mathrm{Mn}}^{2+}$ was equal to the pure host lattice within 30%. The cubic-field parameters $a_0$ and $a_{0}b$ of the spin Hamiltonian depend approximately on the second power of the related crystalline field. Arguments are presented suggesting the point-charge model adequately represents the crystalline field effects, even in the case of covalent bonding, for determining small relative changes in the spin-Hamiltonian parameters. No pressure dependence in the $g$ factor or linewidth was found. The fractional change in the magnitude of the hyperfine constant was about one-half the fractional change in the volume of the crystal.