Structure of theHandHAcenters in RbCaF3

Abstract

Electron spin resonance (ESR) has been used to characterize the $H$ and ${H}_{A}$ centers in the tetragonal phase of RbCa${\mathrm{F}}_{3}$. The $H$ centers are produced by electron irradiation at 77 K. They consist of a radiation-induced interstitial fluorine atom bonded to a regular lattice fluorine ion, with the resultant $\mathrm{F}_{2}^{\ensuremath{-}}$ molecule centered on a single fluorine lattice site. The ESR spectrum of the $H$ centers is dominated by equivalent hyperfine interactions (${A}_{\ensuremath{\parallel}}=920.6 \mathrm{G}$) with the two fluorine nuclei in the primary $\mathrm{F}_{2}^{\ensuremath{-}}$ molecule; no additional hyperfine interactions with adjacent fluorines are resolved. Preferential alignment of the $H$-center molecules occurs along the [001] tetragonal axis of each domain. Near 125 K, the $H$ centers thermally anneal and a new spectrum, assigned to ${H}_{A}$ centers, appears. An ${H}_{A}$ center is formed when a migrating $H$ center becomes stabilized adjacent to an unidentified impurity substituting for a ${\mathrm{Rb}}^{+}$ ion. Thermal annealing of the ${H}_{A}$ centers occurs near 170 K. The hyperfine interactions with the two fluorine nuclei in the ${H}_{A}$ center are unequal (${A}_{1z}=1020.9 \mathrm{G}$ and ${A}_{2z}=782.2 \mathrm{G}$) and the ${H}_{A}$ center's molecular axis deviates approximately 11.9\ifmmode^\circ\else\textdegree\fi{} from the [001] tetragonal direction while remaining in the (100) or (010) planes.