Cr+ and Cr3+ defects in CaF2 and SrF2.

Abstract

We study x-irradiated chromium-doped ${\mathrm{CaF}}_{2}$ and ${\mathrm{SrF}}_{2}$ using EPR. A cubic EPR signal which shows a superhyperfine (SHF) interaction with eight fluorine nuclei is associated with ${\mathrm{Cr}}^{+}$ ions in a substitutional cation site. The spin Hamiltonian parameters are g=2.001\ifmmode\pm\else\textpm\fi{}0.001 for ${\mathrm{CaF}}_{2}$ and ${\mathrm{SrF}}_{2}$, ${A}_{\ensuremath{\parallel}}^{F}$=39\ifmmode\pm\else\textpm\fi{}2 MHz, ${A}_{\ensuremath{\perp}}^{F}$=24\ifmmode\pm\else\textpm\fi{}2 MHz for ${\mathrm{CaF}}_{2}$, and ${A}_{\ensuremath{\parallel}}^{F}$=30\ifmmode\pm\else\textpm\fi{}2 MHz, ${A}_{\ensuremath{\perp}}^{F}$=18\ifmmode\pm\else\textpm\fi{}2 MHz for ${\mathrm{SrF}}_{2}$. The hyperfine interaction with the $^{53}\mathrm{Cr}$ isotope is resolved giving an hyperfine constant of 66\ifmmode\pm\else\textpm\fi{}2 MHz in both matrices. Another signal with a trigonal symmetry is associated with ${\mathrm{Cr}}^{3+}$ ions in a cation substitutional position whose environment has been distorted by a static Jahn-Teller effect. The g value is isotropic and equal to 1.965\ifmmode\pm\else\textpm\fi{}0.005 for both ${\mathrm{CaF}}_{2}$ and ${\mathrm{SrF}}_{2}$ lattices. A SHF interaction with six fluorine nuclei is resolved. The SHF constants are ${A}_{\ensuremath{\parallel}}^{F}$=31\ifmmode\pm\else\textpm\fi{}2 MHz in ${\mathrm{CaF}}_{2}$, 30\ifmmode\pm\else\textpm\fi{}2 MHz in ${\mathrm{SrF}}_{2}$, and ${A}_{\ensuremath{\perp}}^{F}$3 MHz in both cases. The hyperfine splitting with $^{53}\mathrm{Cr}$ corresponds to ${A}_{\ensuremath{\parallel}}$=45\ifmmode\pm\else\textpm\fi{}2 MHz and ${A}_{\ensuremath{\perp}}$=53\ifmmode\pm\else\textpm\fi{}2 MHz for ${\mathrm{CaF}}_{2}$ and ${A}_{\ensuremath{\parallel}}$=47\ifmmode\pm\else\textpm\fi{}2 MHz and ${A}_{\ensuremath{\perp}}$=54\ifmmode\pm\else\textpm\fi{}2 MHz for ${\mathrm{SrF}}_{2}$.