Spectroscopic Investigation and Crystallization Study of Rare Earth Metaborate Glasses

Abstract

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

Exploration of glass domain in the SiO2-B2O3-La2O3 system

“…Previous studies on a RE-rich 7-oxides borosilicate glass, used as a model for nuclear waste glass developed to immobilize high concentration of waste, showed that the incorporation of RE 3+ ions was greatly impacted by variations of B2O3 content, which could be due to the existence of a structural relationship between RE and B in the glassy structure [3], while studies performed on other RE-rich borosilicate glasses suggest the formation of a local RE-metaborate structure incorporating preferentially RE 3+ ions [4,5,6]. To improve the understanding of RErich borosilicate glasses structure and also to enhance the immobilization properties of nuclear waste glasses, it is thus of great interest to elucidate the chemical and structural relationship that could exist between boron and RE in glasses, starting with the simplest systems bearing simultaneously these two oxides, like B2O3-RE2O3 binary system [7,8,9,10] or SiO2-B2O3-RE2O3 ternary system, and then progressively increasing the complexity of the glass composition.…”

Comparative study of the rare earth environment in rare earth metaborate glass (REB 3 O 6 , RE = La, Nd) and in sodium borate glasses

“…It is assumed to be between 3.2 and 4.5 according to [17]. [20] for the Nd 3+ absorption spectrum of crystalline Nd-metaborate). Significant differences are also observed between the spectroscopic data of this glass and of the Na-borate and Na-silicate glasses of this study that are detailed in the following.…”

“…6). However, the comparison of the data of the glassy and crystalline forms allows making an important distinction: (i) the Nd 3+ 4 I 9/2 → 2 P 1/2 absorption band is located at 428.5 nm in the crystal versus 429.8 nm in the glass [20] indicating a higher covalency in the glass (nephelauxetic effect). This is also confirmed by the 5 D 0 → 7 F 4 transition, considerably more intense in the crystal than in the glass [23], suggesting that the electronic density on the oxygens is higher in the glass, (ii) the mean Nd-O distance in the glass is significantly lower than in the crystal (respectively 2.48 Å and 2.56 Å) and (iii) the R ratio in the glass is much higher than in the crystal (respectively 2.4 and 0.4).…”

“…However, the melt of this composition can be easily quenched to form a glass: the best way is to leave the melt naturally cooling in the crucible without any mechanical perturbation after leaving the furnace. Indeed, the crystallization is remarkably heterogeneous [20,21]. This suggests significant structural differences between the RE 2 O 3 ·3B 2 O 3 glass and crystal phase, leading to an elevated nucleation enthalpy barrier.…”

“…In the crystal phase, 1 BØ 4 − unit coexists with 2 BØ 2 O − units, forming three-membered rings connected in chains [22]. The proportion of BØ 4 − units (N4) is slightly higher in the glass of same composition, although not significantly different (RE = La 3+ , N4 = 40%, 11 B MAS NMR measurement [20,21]). Marked changes are thus expected in the RE 3+ environment between the glass and crystal phases, as already noticed from Eu 3+ absorption and fluorescence spectra [23].…”