[${\mathrm{H}}^{\mathrm{-}}$${\mathrm{Ca}}^{+}$${]}^0$ defect in thermochemically reduced CaO: A static and dynamical EPR study

The [${\mathrm{H}}^{\mathrm{-}}$${\mathrm{Ca}}^{+}$${]}^0$ defect has been produced in thermochemically reduced CaO crystals and studied by electron paramagnetic resonance (EPR). These defects act as low-temperature traps for electrons released during the photoconversion of F→${\mathrm{F}}^{+}$ centers. The defects are stable up to 80 K. Below 20 K the spectrum corresponds to the static situation, described by tetragonal symmetry with ${\mathrm{g}}_{\mathrm{\parallel }}$=2.0003(2) and ${\mathrm{g}}_{\mathrm{\perp }}$=1.9996(2), ${\mathrm{A}}_{\mathrm{\parallel }}$=41.2±0.2 MHz and ${\mathrm{A}}_{\mathrm{\perp }}$=21.4±0.2 MHz. The nuclear Zeeman term and the hyperfine interaction are of similar strength, and the spectrum shows a strong “forbidden” lines pattern. Above 20 K electron hopping occurs among the six equivalent orientations. Using the line-shape method based on the stochastic Liouville equation, we have studied the evolution of the EPR spectrum with temperature. The motion is thermally activated with a small activation energy ≈0.007 eV. A polaronlike behavior is proposed for the defect.