Distribution of Eu and Er in Ca9RE(PO4)7 phosphors analyzed by anomalous X-ray scattering

The structures of Ca₉Eu(PO₄)₇ and Ca₉Er(PO₄)₇ phosphors were analyzed using single-crystal X-ray diffraction. Most Eu ions reside at the medium-sized M1 and M2 sites, with a small amount occupying the M5 site. In contrast, the smaller Er ions favor the smallest M5 site. This study also demonstrates the use of anomalous X-ray scattering measurements at the Er L_III and Er L_III-edges. The energy dependencies observed in the scattering factors for Ca₉Eu(PO₄)₇ and Ca₉Er(PO₄)₇ confirmed the preferred distribution of Eu and Er. Because this anomalous X-ray scattering method leverages the unique anomalous dispersion terms of each element, this advanced technique, coupled with a synchrotron radiation source, is suggested to be a powerful method for distinguishing near-neighbor rare-earth elements in solid-state phosphors.