${\mathrm{Cr}}^{+}$ and ${\mathrm{Cr}}^{3+}$ defects in ${\mathrm{CaF}}_2$ and ${\mathrm{SrF}}_2$

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±0.001 for ${\mathrm{CaF}}_2$ and ${\mathrm{SrF}}_2$, $A_{\parallel }^F$=39±2 MHz, $A_{\perp }^F$=24±2 MHz for ${\mathrm{CaF}}_2$, and $A_{\parallel }^F$=30±2 MHz, $A_{\perp }^F$=18±2 MHz for ${\mathrm{SrF}}_2$. The hyperfine interaction with the ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}$ isotope is resolved giving an hyperfine constant of 66±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±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 $A_{\parallel }^F$=31±2 MHz in ${\mathrm{CaF}}_2$, 30±2 MHz in ${\mathrm{SrF}}_2$, and $A_{\perp }^F$<3 MHz in both cases. The hyperfine splitting with ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}$ corresponds to $A_{\parallel }$=45±2 MHz and $A_{\perp }$=53±2 MHz for ${\mathrm{CaF}}_2$ and $A_{\parallel }$=47±2 MHz and $A_{\perp }$=54±2 MHz for ${\mathrm{SrF}}_2$.