Experimental observation of “configuration” modes of bistable centers in CdF₂:In crystals

Abstract

New “configuration” modes, which were predicted by us for CdF₂:In crystals, have been revealed at the frequencies ν₁ ≈ 32.4 cm⁻¹ and ν₂ ≈ 96.3 cm⁻¹ for deep and shallow impurity states, respectively. The frequencies of these oscillations exactly correspond to the potential-energy curves calculated by us for shallow and deep states of In with regard to the reduced mass M = 2m ₁ m ₂/(m ₁ + 2m ₂) of the In ion (m ₁) and two F ions (2m ₂) per primitive fluorite cell. This correspondence confirms the correct choice of the height of the potential barrier between the impurity states of In in CdF₂ (0.02 eV), which was used in the calculations. The dielectric contributions of the noted modes were determined, which made it possible to calculate the concentrations of In impurity ions in the deep (N ₁) and shallow (N ₂) states. The obtained ratio N ₂/N ₁ ≈ 2 directly indicates that photoionization of deep In centers leads to the formation of a doubled number of shallow centers and that two electrons are localized in the deep state of the In ion; such behavior is characteristic of DX centers. A photoinduced increase in the real (ε′) and imaginary (ε″) parts of the dielectric constant has been found (at a frequency of 25 cm⁻¹, Δε′ ≈ 0.2 and Δε″ ≈ 0.06). These changes correspond to the changes in the dielectric contributions of the configuration modes under illumination. A photoinduced decrease in the lattice reflection of CdF₂:In, related to the impurity lattice modes, has also been revealed.