Detection of first-order liquid/liquid phase transitions in yttrium oxide-aluminium oxide melts

We combine small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS) with aerodynamic levitation techniques to study in situ phase transitions in the liquid state under contactless conditions. At very high temperatures, yttria-alumina melts show a first-order transition, previously inferred from phase separation in quenched glasses. We show how the transition coincides with a narrow and reversible maximum in SAXS indicative of liquid unmixing on the nanoscale, combined with an abrupt realignment in WAXS features related to reversible shifts in polyhedral packing on the atomic scale. We also observed a rotary action in the suspended supercooled drop driven by repetitive transitions (a polyamorphic rotor) from which the reversible changes in molar volume (1.2 ± 0.2 cubic centimeters) and entropy (19 ± 4 joules mole–1 kelvin–1) can be estimated

Temperature-Induced Structural Modifications Between Alkali Borate Glasses and Melts.

High-temperature neutron diffraction and Raman spectra have been obtained on M2O–2B2O3 (M=Li, Na, K) glasses and melts. Both techniques indicate a coordination change of boron atoms: the tetrahedral boron sites present in the glasses are converted into triangular boron sites. These changes of the borate network yield modifications of the alkali environment, as assessed for Li using the isotopic substitution technique. We observe that Li atoms are in a charge-compensating position in the glass and in a modifying position in the liquid. These structural modifications have important implications toward understanding the physical properties of borate melts

Spectroscopic Investigation and Crystallization Study of Rare Earth Metaborate Glasses

International audienceBesides their interesting optical properties, Rare Earths (RE) can be found abundantly as fission products at the end of the nuclearfuel cycle. After reprocessing, they are stored by dissolution in a borosilicate glassy matrix. RE are also used to simulateactinides when studying nuclear waste immobilization. It is very important to understand the environment of RE in such complexglasses. Moreover, composition changes can strongly affect RE solubility in the glass melt and RE-rich phases (such as REsilicateapatite) can crystallize during cooling. As boron oxide seems to play an important role on the solubility of RE bypreventing the crystallization of RE-rich phases, we focused our study on a system containing only B2O3 and RE2O3 (with RE=La or Nd), and more precisely on the RE-metaborate composition RE2O3.3B2O3. As this glass, which has a congruentcrystallization, is thought to have a structure similar to the corresponding crystalline phase REB3O6, we intended to go further inthe understanding of its structure using 11B MAS NMR, EXAFS at the Nd-LIII edge and optical absorption. The crystallizationbehavior of the glasses thermally treated was also studied using DTA, XRD and optical absorption