Structural origin of high-density Gd2O3–MoO3–B2O3 glass and low-density β′-Gd2(MoO4)3 crystal: a study conducted using high-energy x-ray diffraction and EXAFS at high temperatures

Abstract

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.