EPR and optical absorption of ${\mathrm{Fe}}^{+}$, ${\mathrm{Fe}}^{2+}$, ${\mathrm{Fe}}^{3+}$, and ${\mathrm{Fe}}^{4+}$ on tetragonal sites in CdSi${\mathrm{P}}_2$

An EPR investigation of CdSi${\mathrm{P}}_2$:Fe reveals the presence of substitutional, photosensitive ${\mathrm{Fe}}^{+}$, ${\mathrm{Fe}}^{2+}$, ${\mathrm{Fe}}^{3+}$, and ${\mathrm{Fe}}^{4+}$ impurities, each charge state being associated with a highly anisotropic EPR spectrum. The non-Kramers ions ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{4+}$ are still observable at 300 K. At 20 K all Fe centers, except ${\mathrm{Fe}}^{+}$, exhibit a well-resolved phosphorus ligand hyperfine splitting. The anisotropy of this splitting is used to demonstrate the magnetic inequivalence of the two Si sites in CdSi${\mathrm{P}}_2$. For ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{4+}$ a complete analysis of the $S=2\mathrm{}$ spin Hamiltonian is possible. Near-ir absorption bands due to ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{4+}$ are observed. A correlation with the EPR results allows the determination of the crystal-field parameters $\mathrm{Dq}$, $\mathrm{Ds}$, and $\mathrm{Dt}$. Static crystal-field theory, supplemented to include covalency effects, according to the model of Vallin and Watkins, accounts satisfactorily for all experimental results, except for the EPR data of ${\mathrm{Fe}}^{4+}$. It is suggested that this discrepancy is due to covalent effects beyond those explicitly considered.