Structure and Motions of the HA (Li+)-Type BrCl- and ICl- Centers in Doped KCl
Abstract
The HA(Li+)-type BrCl- center is an interstitial center, which can be produced by ionizing irradiation and suitable thermal and optical treatment, in KCl crystals strongly doped (~1 wt%) with Li+ and Br- ions. As the name implies, its structure is very similar to that of the HA(Li+) center in KCl: Li+. The electron paramagnetic resonance (EPR) analysis at 77 K shows that is consists of a BrCl--molecule ion, occupying a single negative-ion site, next to a substitutional Li+ ion. The BrCl- internuclear axis makes a 25° angle with <100> in a {110} plane, and the molecular bond is bent by about 8°. The center is formed when a mobile, interstitial, chlorine atom is stabilized by a pair of substitutional Li+ and Br- impurity ions which are nearest neighbors of one another. The HA(Li+)-type BrCl- possesses a thermally activated (E≅0.093 eV) pyramidal motion of C4v symmetry around a given <100>, and motionally averaged EPR spectra are observed at 200 K. In contrast to the HA(Li+) center, it does not appear to possess a restricted interstitial motion of C3v symmetry around <111>. It is concluded that the Cl side of the BrCl- molecule is closer to the Li+ ion, and that the bending of the BrCl- bond is toward the Li+ ion. This conclusion permits a more refined description of the HA(Li+) center model: The Cl-2 bond there too is bent toward the Li+ ion. Results for the HA(Li+)-type ICl- center in KCl: (Li+, I-) are also given. Its properties are very similar to those of the HA(Li+)-type BrCl- center.
- Publication:
-
Physical Review B
- Pub Date:
- August 1972
- DOI:
- 10.1103/PhysRevB.6.1562
- Bibcode:
- 1972PhRvB...6.1562S