An investigation by electron paramagnetic resonance of the Jahn-Teller effect in alkali-halides doped with transition group impurities nd_ configurations in six-fold coordination

The EPR spectra of x-irradiated alkali-chloride single crystals doped with transition group impurities have been observed near 77°K at x-band frequencies. The optical spectra have been taken at room temperature. Samples of NaCl:Ag, NaCl:Cu, KC1:Ag, and KC1:Cu obtained from both Harshaw Chemical Company and lsomet were studied. Modifications to and operation of the existing back reflection homodyne EPR spectrometer of 1011AH spins sensitivity is described. An nd9 configuration such as Cu2+ or Ag2+ in the sixfold coordinated site of the alkali-chloride lattice has a doubly degenerate vibronic ground state as a result of lattice vibrational coupling to the electronic state. An effective Hamiltonian for this doublet vibronic ground state of the divalent impurities is formulated including the strain, Zeeman, and hyperfine interactions. An extension of the doublet formalism to include the effects of a nearby excited singlet vibronic state is discussed. No EPR spectra were observed in as-grown samples. After annealing at 500°C and x-irradiation at 77°K, two distinct types of spectra were observed at 77°K. One, observed only in KC1, is a pair of hyperfine lines separated by several hun-dred gauss, exhibiting ligand hyperfine structure, and attrib-utable to Ag°. The other, observed in both NaC1 and KC1, is a highly anisotropic pattern of several lines varying in intensity - the well-known Vk center resonance. Annealing to room temperature for the few minutes necessary for the Vk center fluorescence to die out and subsequent quenching to 77°K did not noticeably affect the neutral spectrum. However, the Vk center spectrum had almost completely vanished and in its place appeared a new axially symmetric spectrum from either a Cu2+ or a Ag2+ center. The divalent silver spectrum exhibited 13 hyperfine and ligand hyperfine lines in the perpendicular component and 2 hyperfine lines in the parallel component along a <100> axis in NaC1 and KC1. Along the same axis in NaC1 only, the EPR spectrum of the divalent copper displayed a broad resonance with some barely resolvable structure in the perpendicular component and a four line hyperfine pattern in the parallel component. The axial spectrum was not observed in KC1:Cu subsequent to x-irradiation and Vk-center anneal. This failure is attributed to an insufficient concentration of the monovalent copper impurity ions in substitional sites. All features of the observed axial spectra are explained within experimental error assuming the paramagnetic complex to be in the 2Eg ground state of the divalent transition group impurity ion in cubic symmetry. A static Jahn-Teller effect resulting from strong vibronic coupling can explain all of these observations.