Structure and Properties of F Centers in Ionic Crystals
Authors: Seidel H. 
Published: March 2014

The F center, an electron trapped at an anion vacancy, is known in a large variety of ionic crystals. F centers in alkali halides of the NaCl‐type have been most extensively investigated, and shall be the main object of this exposition. F centers can be produced by heating the crystals in the alkali metal vapour (“additive coloration”), by high temperature electrolysis, as radiation damage from X‐rays or bombarding particles, and by a number of photochemical reactions involving impurities. The most conspicuous property of the F center is its optical absorption, a band of Gaussian shape and roughly 2000 cm−1 width in the visible region (F = Farbe, color). The band maxima for different crystals with lattice constants a are well described by the relation vmax · a2 = const. This absorption is interpreted as the transition from an s‐type ground state to a bound p‐type excited state. A number of theoretical approaches on this basis gave good agreement with the observations. Shape and width of the band and their temperature dependence result from coupling to vibrational modes of the lattice. Weaker transitions to higher excited states are also known.

Upon excitation in the F band or higher absorptions, F centers luminesce, at low temperatures and low concentrations with quantum efficiency near one. Emission bands lie in the near infrared, with Stoks shifts of typically 10.000 cm−1. The lifetimes of the excited state are ≈︁ 10−6 sec, which would be relatively long for an allowed p‐s‐transition. At higher temperatures the luminescence is quenched and photoconductivity appears, showing that the relaxed excited state is close to the conduction band. This implies that the electron is rather delocalized in the relaxed excited state. The low transition moment to the more localized ground state may be understandable in this way. Certain features of the unrelaxed excited state, i.e. before the lattice adjusts to the new electronic state within ≈︁ 10−12 sec, can be studied by magnetooptical experiments. For this purpose a modified alkali atom model with inverted spin‐orbit‐splitting between the 2P1/2 and 2P3/2 states has turned out appropriate.

The unpaired spin of the F electron makes it accessible to spin resonance investigations. ESR spectra show only in a few cases well resolved hyperfine structure, confirming beyond any doubt the atomistic model of the center, and giving precise values of the electron density at the nearest neighbour nuclei. By electron‐nuclear double resonance (ENDOR) hyperfine interactions in many higher “shells” of surrounding ions have been resolved. Thus the ground state wavefunction of the F electron can be analysed in very detail out to several lattice constants from the center. Current theories show good agreement with the experimental data in the region close to the center (nearest and next‐nearest neighbours), but the far out tail of the wavefunction is not yet well understood. In connection with optical pumping and magnetooptic effects, ESR of the relaxed excited state has recently also been observed. So far this has not settled the question if it is a p‐state with static or dynamic Jahn‐Teller effect or an s‐state.

A large number of modified F centers have been produced, and their investigation contributed to a refined understanding of this basic defect structure. In alkali halide crystals doped with foreign alkali ions, e.g. KCl: Li, “FA centers” with one foreign alkali ion as nearest neighbour can be synthesized photochemically and oriented by bleaching with polarized light, showing then anisotropic absorption bands. In crystals with divalent cation impurities a number of different “Z centers” are formed, whose structure is not yet completely known. Dimers and trimers of nearest‐neighbour F centers, known as M centers and R centers, have also been widely investigated by almost all methods which we mentioned for the F center.