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| Fig. 5 presents the room temperature photoluminescence spectra for both of the as-grown and the annealed wafer with excitation at 282 nm. A strong blue emission band centered at 435 nm can be observed for the two wafers. In the previous works, two peaks located at 420 and 485 nm were observed in the crystal fibers grown by laser-heated pedestal growth method [17], but it is a pity that they were not be explained. In the polycrystalline samples, an emission peak centered at 450 nm was reported, which agrees with our results [6, 24-27]. According to these literatures, the corner-shared XXX groups are efficient luminescent centers for the blue emission, which may be ascribed to self-trapped exciton recombination. This luminescence effect depends on the XXX bonding, because the conduction band is composed of XX orbitals and the valence band is composed of XX orbitals between the corner-sharing XXX octahedral. In other words, this luminescence was originated from the absorbing groups of the niobate octahedral group [XX]7-. So the luminescence intensity was also influenced by the oxygen vacancies in the XXX octahedral, which should be similar as the broadening of the Raman band at 905 cm-1 (Fig. 3). Having been annealed in the oxygen atmosphere, the oxygen vacancies in the crystal were eliminated. So the oxidation state of Nb ions was in the same XXX and the Nb and Mg sublattices were of higher degree of order, which results in a more perfect structure of XXX octahedral. Therefore, the annealed wafer exhibits greater intensity than the as-grown sample, as shown in Fig.5. The improvement in photoluminescence performance results from the improved crystallinity leading to higher oscillating strengths because the oxygen vacancies were eliminated [28]. |
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