Structure and crystallization behavior of La2O3⋠3B2O3 metaborate glasses doped with Nd3+ or Eu3+ ions

The local structure and crystallization behavior of the stoichiometric La2O3⋅3B2O3 (LaMB) metaborate glass doped with Nd3+ or Eu3+ ions are studied using differential thermal analysis (DTA), X-ray diffraction (XRD), Raman scattering and 11B magic-angle spinning nuclear magnetic resonance (MAS NMR), optical absorbance and luminescence techniques. In the crystallized samples, XRD, NMR and Raman spectroscopy have detected the formation of only one crystalline phase (congruent crystallization of LaB3O6). No intermediate metastable crystalline phase has been detected before LaB3O6 crystals formation (single stage crystallization process). The observation of heat treated glass samples by scanning electron microscopy (SEM) and optical microscopy coupled with the study of the effect of varying the glass particle size on the DTA curves have both revealed that LaB3O6 crystallization only occurs by a heterogeneous nucleation mechanism (needle-shape crystals) from glass surface. The activation energy Ec of crystal growth has been determined by performing DTA experiments at different heating rates with the Kissinger (784 kJ/mol) and Ozawa (801 kJ/mol) equations than can be used for surface crystallization processes. The heterogeneous crystallization behavior and the spectroscopic results obtained in this work by comparing the LaMB glass with the LaB3O6 crystalline phase suggest the existence of significant structural differences between the amorphous and the crystalline phases contrary to what was previously reported in literature.