Influence of photoexcitation on the EPR spectra of Mo5+ in Li2Zn2(MoO4)3: Ce3+,Cu2+ crystals annealed in a CO2 atmosphere

The influence of recovery annealing in a CO₂ atmosphere at 700°C on the properties of Li₂Zn₂(MoO₄)₃ crystals doped with cerium and copper ions has been studied. The EPR investigation of Li₂Zn₂(MoO₄)₃ crystals annealed in a CO₂ 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 Mo⁵⁺ 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 Mo⁵⁺ made it possible to calculate the spectral parameters g _∥ = 1.862, g _⊥ = 1.933, A _∥ = 71.8 G, and A _⊥ = 34.1 G. The cross relaxation on the hyperfine structure from the molybdenum isotope ⁹⁷Mo is found in the EPR spectra. The photoexcitation of Li₂Zn₂(MoO₄)₃ crystal doped with cerium ions leads to the saturation of the EPR spectrum of Mo⁵⁺ and to the formation of the hyperfine structure from one lithium ion with a hyperfine structure constant of 14 G. For Li₂Zn₂(MoO₄)₃ crystals doped with copper ions, a very weak EPR spectrum of Mo⁵⁺ 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 Cu²⁺ ions take place. It is assumed that such a behavior of the EPR spectra of molybdenum ions in Li₂Zn₂(MoO₄)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.