Electron paramagnetic resonance of Cr(CN)63−in NaCl: Evidence for motion of the associated cation vacancies

The electron-paramagnetic resonance spectra of the Cr(CN${)}_6^{3\mathrm{-}}$ complex in NaCl show evidence for the association of cation vacancies with the complex. The Cr(CN${)}_6^{3\mathrm{-}}$ complex goes into the lattice substitutionally, replacing ${\mathrm{NaCl}}_6^{5\mathrm{-}}$, and simultaneously, two cation vacancies are created. Two different centers (one with $D_{2h}$ symmetry and one with $C_s$ symmetry) were formed in as-grown crystals at room temperature. They were also observed in KCl as previously reported. However, the present study has provided strong evidence for the motion of the associated cation vacancies in the neighborhood of a substitutional trivalent ion in an alkali-metal halide. It was observed that half of the Cr(CN${)}_6^{3\mathrm{-}}$ ions constituting the center with $C_s$ symmetry were converted into a new center, also with $C_s$ symmetry, at low temperature. This new center begins to emerge as the temperature decreases to ∼265 K. It is found that below 215 K the two centers with $C_s$ symmetry have an equal population, each amounting to (∼20±5)% of the total concentration. The formation of this new center occurs when a vacancy jumps from the next-nearest cation site to the nearest cation site. All possible configurations for a trivalent impurity ion in an alkali-metal halide associated with two cation vacancies are discussed for cases where the vacancies occupy the nearest and the next-nearest cation sites. It is shown that for a trivalent substitutional impurity ion, a vacancy jump, in most cases, corresponds to an interconversion between different configurations. Finally, we point out that the configuration, which has been reported for all previously investigated trivalent impurity ions or complexes in alkali-metal halides, is not present in either KCl:Cr(CN${)}_6^{3\mathrm{-}}$ or NaCl:Cr(CN${)}_6^{3\mathrm{-}}$. This absence raises questions about the correctness of those interpretations.