XAFS characterization of the structural site of Yb in synthetic pyrope andgrossular garnets

The incorporation and site preference of minor amounts (about 1 wt%) of Yb3 + in synthetic pyrope (Mg3Al2Si3O12) and grossular (Ca3Al2Si3O12) garnet we re studied by X-ray Absorption Fine-Structure (XAFS) Spectroscopy. The meas urements, performed in the temperature range 77-343 K at both Yb L-I- and L -III-edges, demonstrate that Yb3+ enters the garnet structure and is locate d in the dodecahedral site in both samples. The coordination environment of Yb3+ in the two samples was compared to that of the X-site cation in end-m ember synthetic pyrope and grossular and in Yb3Al5O12 as determined by sing le-crystal X-ray diffraction. The local geometry around Yb3+ is different f rom that of Mg and Ca in the bulk of the garnet, and also from that of Yb3 in Yb3Al5O12. The XAFS results indicate that, (1) structural relaxation occurs around Yb3 + in the garnet structure; (2) the host garnet matrix exerts a major struct ural control on the incorporation of Yb3+ and (3) minor amounts of Yb3+ in garnet are located in structural sites and not in ill-defined defects.