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Raman scattering was used to characterize the crystals. Room temperature Raman spectra for the as-grown and annealed wafer are shown in Fig. 3 a and Fig. 3 b, respectively. The spectra presented the typical bands corresponding to the normal vibration modes of XXX [22]. The maximum phonon energy of this crystal is 905 cm-1, which can be indexed to Nb-O stretching band. Linewidths (FWHM) for the line at 905 cm-1 of the as-grown and annealed wafers were fitted to be 8.2 and 6.8 cm-1, as shown in the insets of Fig. 3 a and Fig. 3 b, respectively. In the as-grown crystal, the dark brown color could be attributed to the partial reduction of Nb5+ ions, which are mainly source from the XXX octahedron [17, 23]. It resulted in the change of the X-X stretching vibration, so that the band at 905 cm-1 broadened. The broadening of the Raman band at 905 cm-1 are mainly caused by the oxygen vacancies. After annealing, the oxidation state of X ions was in the same X+5 and X-X stretching vibration was more uniform, which resulted in a higher degree of order on the Nb and Mg sublattices [17]. Therefore, the color of the annealed crystal faded to light brownish and the linewideth at 905 cm-1 was narrowed. It indicates that the oxygen vacancies of the as-grown crystal can be eliminated by the annealing process in the oxygen atmosphere.

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ll921(½ð±Ò+20, ·­ÒëEPI+1): 2011-03-02 08:42:36
Raman scattering was used to characterize the crystals. £¨at£©Room temperature Raman spectra for the as-grown and annealed wafer £¨were£©shown in Fig. 3 a and Fig. 3 b, respectively. The spectra presented the typical bands corresponding to the normal vibration modes of XXX [22]. The maximum phonon energy of this crystal is 905 cm-1, which can be indexed to Nb-O stretching band. Linewidths (FWHM) for the line at 905 cm-1 of the as-grown and annealed wafers were fitted to be 8.2 and 6.8 cm-1, as shown in the insets of Fig. 3 a and Fig. 3 b, respectively. In the as-grown crystal, the dark brown color could be attributed to the partial reduction of Nb5+ ions, which were mainly source from the XXX octahedron [17, 23]. It resulted in the change of the X-X stretching vibration, so that the band at 905 cm-1 broadened. The broadening of the Raman band at 905 cm-1 are mainly (due to) the oxygen vacancies. After annealing, the oxidation state of X ions was in the same X+5 and X-X stretching vibration was more uniform, which resulted in a higher degree of order on the Nb and Mg sublattices [17]. Therefore, the color of the annealed crystal faded to light brownish and the linewideth at 905 cm-1 was narrowed. It (indicateed) that the oxygen vacancies of the as-grown crystal (could) be eliminated by the annealing process in the oxygen atmosphere.
2Â¥2011-02-27 08:24:25
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