Electron-Paramagnetic-Resonance and Optical Spectra ofCr3+in SnO2Single Crystals

EPR and optical spectra have been obtained for ${\mathrm{Cr}}^{3+}$-doped Sn${\mathrm{O}}_2$ crystals both as grown and after subsequent heat treatment. Two distinct EPR and optical spectra have been observed. ${\mathrm{}\left(\mathrm{EPR}\right)\mathrm{}}_{\mathrm{I}}$ and ${\mathrm{}\left(\mathrm{Opt}\right)\mathrm{}}_{\mathrm{I}}$ are characteristic of the as-grown crystal. The ${\mathrm{}\left(\mathrm{EPR}\right)\mathrm{}}_{\mathrm{I}}$ is shown to be due to ${\mathrm{Cr}}^{3+}$ in substitutional sites with spin Hamiltonian parameters $D=-0.6840\mathrm{}\pm 0.0002$ ${\mathrm{cm}}^{-1\mathrm{}}$, $E=0.1852\mathrm{}\pm 0.0002$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $g_{\mathrm{av}}=1.975\mathrm{}$. The lines are superhyperfine-broadened with superhyperfine structure (shfs) of two- and four-tin nuclei approximately equal to 35 and 7 G, respectively. EPR spectra of ${\mathrm{Fe}}^{3+}$ and ${\mathrm{V}}^{4+}$ show some similarities in shfs. The ${\mathrm{}\left(\mathrm{Opt}\right)\mathrm{}}_{\mathrm{I}}$ spectrum consists of intense bands centered at 18 250 ${\mathrm{cm}}^{-1\mathrm{}}$ and 19 400 ${\mathrm{cm}}^{-1\mathrm{}}$ ($\alpha \sim 100$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\Delta \nu \sim 3000$ ${\mathrm{cm}}^{-1\mathrm{}}$) with light polarized perpendicular and parallel to the $c$ axis, respectively. After heating in a reducing atmosphere, ${\mathrm{}\left(\mathrm{Opt}\right)\mathrm{}}_{\mathrm{I}}$ disappears completely. New weak bands ${\mathrm{}\left(\mathrm{Opt}\right)\mathrm{}}_{\mathrm{II}}$ appear at slightly different energies of 21 400 ${\mathrm{cm}}^{-1\mathrm{}}$ and 19 700 ${\mathrm{cm}}^{-1\mathrm{}}$ ($\alpha \sim 0.6$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\Delta \nu \sim 800$ ${\mathrm{cm}}^{-1\mathrm{}}$) with light...