Structural origin of high-density Gd₂O₃-MoO₃-B₂O₃ glass and low-density β'-Gd₂(MoO₄)₃ crystal: a study conducted...

In this study, the short- and medium-range ordering of 21.25Gd₂O₃-63.75MoO₃-15B₂O₃ glass (GM15B glass) was studied at temperatures ranging from 297 K to above the crystallization temperature. This was achieved using extended x-ray absorption fine structure and high-energy x-ray diffraction analyses to elucidate the structural origin of the higher glass density (d) as compared to that of precipitated crystals, i.e. GM15B glass with d  =  4.762 g cm^-3 and β'-Gd₂(MoO₄)₃ crystal with d  =  4.555 g cm^-3, and the self-powdering phenomenon in GM15B glass. The bond lengths in the short-range order, i.e. Gd-O, Mo-O, and Gd-Mo in GM15B, were extremely similar to those in the β-phase; Gd-Gd and Mo-Mo bonds in Gd-O-Gd and Mo-O-Mo, respectively, were not found. The distance between the Gd-Gd pairs in Gd-O-Mo-O-Gd in GM15B glass, r(Gd-Gd)  =  6.19 Å, was considerably smaller than that in β-Gd₂(MoO₄)₃ crystals (6.71 Å) in the crystallized glass at 808 K. This indicates that the glass had a more packed structure than did the precipitated crystals. The probable origin of the self-powdering phenomenon in GM15B glass was the large expansion of the structure during crystallization, which is due to the formation of an open structure with long Gd-Gd pairs and varying bond angles.