Spin-lattice relaxation of the Vk-center in LiF

The angular and temperature dependences of the spin-lattice relaxation (SLR) rate of V_k-centers in LiF doped with Mg or Ag have been investigated. In the temperature interval 4.2–100 K the results can be fitted by the formula $\text{τ}{}^{\mathrm{?1}}\text{= A(θ)T + Be}{}^{\mathrm{<mtext>?\Delta </mtext><mtext>T</mtext>}}$ with A(0°) = 0.11 sec^?1K^?1, A(90°) = 1.3 sec^?1K^?1, B = 3 × 10⁵sec^?1 and Δ = (175 ± 15)K.A mechanism for the SLR is considered, assuming the modulation of the hyperfine interaction by phononinduced transitions between the ground and excited states of the resonant molecular vibrations of the V_k-center. This mechanism is found to explain the value, the temperature dependence and the isotropy of τ^?1 in the interval T = 20–100 K.The one-phonon SLR mechanisms of the V_k-center in the T < 10 K region are discussed.