Conversion of the valence states of europium ions in reduced SrTiO3 based on the photoluminescence spectra

Abstract

The photoluminescence (PL) of Eu ions and their modulation with oxygen vacancy amount were investigated in SrTiO3 with site substitution (A-site vs. B-site in ABO3-type perovskite) dependence, aiming at obtaining the color tunability by the change in the valence states of Eu ions. The samples, which were synthesized in air using a solid state reaction method, exhibited strong orange-red emissions near 600 nm, which is a characteristic of Eu3+. We annealed our samples in H2 atmosphere to increase the oxygen vacancies, and we established that the orange-red emissions of the Eu ions were suppressed. However, surprisingly, new emissions of Eu2+ emerged near 400 nm, which has not been reported. More interestingly, this behavior was more robust in the B-site-doped sample than in the A-site-doped sample. In the samples with O2-ambient annealing to decrease oxygen vacancies, the PL intensities returned to the initial states for both site substitutions, indicating the reversible change in the valence states (+2/+3) of Eu ions. Our new finding of the tunable emission property of Eu ions in SrTiO3 could be explained in terms of the substitution sites and the related charge compensations with regulating oxygen vacancies.