Influence of photoexcitation on the EPR spectra of Мо$^{5+}$ in Li$_2$Zn$_2$(MoO$_4$)$_3$ : Се$^{3+}$, Cu$^{2+}$ crystals annealed in a CO$_2$ atmosphere

The influence of recovery annealing in a CO$_2$ atmosphere at 700$^\circ$C on the properties of Li$_2$Zn$_2$(MoO$_4$)$_3$ crystals doped with cerium and copper ions has been studied. The EPR investigation of Li$_2$Zn$_2$(MoO$_4$)$_3$ crystals annealed in a CO$_2$ atmosphere has revealed that the annealing leads to the formation of oxygen vacancies in positions adjacent to the oxygen octahedron of lithium, M3, and the oxygen tetrahedron of molybdenum, Mo1. In this case, the charge state of molybdenum becomes Mo$^{5+}$ and appears in the EPR spectra in the form of one magnetically nonequivalent position. The analysis of the angular dependence of the EPR spectrum of Mo$^{5+}$ made it possible to calculate the spectral parameters $g_\parallel$ = 1.862, $g_\perp$ = 1.933, $A_\parallel$ = 71.8 G, and $A_\perp$ = 34.1 G. The cross relaxation on the hyperfine structure from the molybdenum isotope $^{97}$Mo is found in the EPR spectra. The photoexcitation of Li$_2$Zn$_2$(MoO$_4$)$_3$ crystal doped with cerium ions leads to the saturation of the EPR spectrum of Mo$^{5+}$ and to the formation of the hyperfine structure from one lithium ion with a hyperfine structure constant of 14 G. For Li$_2$Zn$_2$(MoO$_4$)$_3$ crystals doped with copper ions, a very weak EPR spectrum of Mo$^{5+}$ is observed in the initial crystals. As a result of the photoexcitation, an increase in the intensity of this spectrum by an order of magnitude and manifestation of the EPR spectrum of Cu$^{2+}$ ions take place. It is assumed that such a behavior of the EPR spectra of molybdenum ions in Li$_2$Zn$_2$(MoO$_4$)$_3$ crystals doped with cerium and copper ions under photoexcitation is caused by different positions of the energy levels of cerium and copper ions with respect to the energy level of the molybdenum ion.