Properties of Divalent Cobalt and Nickel as Impurities in Silver Chloride Single Crystals

An experimental study of the optical spectra of single AgCl: Co and AgCl: Ni crystals and the EPR (electron paramagnetic resonance) spectra of oriented AgCl: Co crystals is described. Six optical absorption peaks due to Co and five due to Ni diffused into AgCl were found at 77°K in the 3500 to 26 000 Å range. The optical data are interpreted using the Tanabe and Sugano crystal field theory as due to divalent Co and Ni in an octahedral field. The crystal field theory parameters are $Dq=680\mathrm{}\pm 30$ ${\mathrm{cm}}^{-1\mathrm{}}$, $B=790\mathrm{}\pm 20$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $C=3320\mathrm{}\pm 70$ ${\mathrm{cm}}^{-1\mathrm{}}$ for ${\mathrm{Co}}^{++}$ and $Dq=670\mathrm{}\pm 5$ ${\mathrm{cm}}^{-1\mathrm{}}$, $B=805\mathrm{}\pm 7$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $C=3130\mathrm{}\pm 50$ ${\mathrm{cm}}^{-1\mathrm{}}$ for ${\mathrm{Ni}}^{++}$. The EPR spectra of AgCl: Co samples oriented by the back-reflection x-ray technique were obtained at 77 and 2.1°K where the Co hyperfine structure was resolved. The EPR data are interpreted as due to divalent cobalt in two different centers both with octahedral symmetry distorted axially in a [100] direction. The spin Hamiltonian parameters for the two centers are $g_{\mathrm{II}}=5.38\mathrm{}\pm 0.03$, $g_{\perp }=3.87\mathrm{}\pm 0.02$, $A=(233\mathrm{}\pm 3)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $B=(89\mathrm{}\pm 2)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ for the most prominent EPR lines and $g_{\mathrm{II}}=3.48\mathrm{}\pm 0.02$, $g_{\perp }=4.70\mathrm{}\pm 0.05$, $A=(55\mathrm{}\pm 10)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $B=(170\mathrm{}\pm 20)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ for the weak lines. Thermal and optical bleaching experiments demonstrated the stability of the divalent state for Co and Ni in AgCl. Attempts to produce intrinsic centers by quenching AgCl from a ${\mathrm{Cl}}_2$ anneal or by irradiation with blue light at 2.1 and 77°K did not produce measurable EPR or optical spectra.