Furthermore, to observe the change in radiation intensity and thereby validate the direct recombination process, we investigated the EL temperature dependence in the range from 80 to 293 K in the case of applied positive bias. The measurement revealed a few-fold increase in luminescence intensity with decreasing temperature; however, the shape of the spectrum was preserved. Because both the increase in the number of emitted photons and the sharpening of the peak shapes were observed with decreasing temperature, it could be deduced that the major carriers were frozen in impurities—acceptors (i.e., in Er).11 However, we could not determine whether the crystal field effect affects the splitting of energy levels, as 80 K was not low enough to reveal this effect. The lattice parameters and the elastic constants depend on temperature and magnetic field, the crystal-field excitation energies are modified, and these excitations are coupled to the phonons. The lowering of the symmetry by an applied external field introduces nonzero strains, proportional to the field, which change the symmetry of the lattice. The degeneracy of the ground state thus occurs, for example, because of hybridization of localized -electrons or the conduction electrons. The insufficiently low temperature thus causes damping of this splitting effect.