Studies on some ternary oxyborates of the Na2O-Me2O3-B2O3 (Me=rare earth or aluminum) systems: Synthesis, structure and crystal growth

Sodium rare-earth oxyborates Na₂RE₂O(BO₃)₂ (RE=Y, Nd, Er) were prepared for the first time in the present study. They were found to be isostructural with phases of the same composition containing Sm, Eu or Gd and reported by Corbel et al. J. Solid State Chem.144 (1999) 35-44]. It was shown that the yttrium and erbium compounds could be synthesized at 900-1000 °C by a solid-state reaction between oxides in an equimolecular ratio. With both oxyborates melting led to decomposition into a mixture of Y(Er)BO₃, Y₂(Er₂)O₃ and Na₂B₄O₇. Just the opposite was observed during thermal treatment of the oxide mixture containing Nd₂O₃, from which a practically pure phase of Na₂Nd₂O(BO₃)₂ was only obtained after melting. The attempts to synthesize the oxyborate Na₂La₂O(BO₃)₂ showed it to be unstable, this leading to the formation of a mixture containing, in addition to Na₂La₂O(BO₃)₂, also other already known stable phases of the system Na₂O-La₂O₃-B₂O₃ along with an unknown ternary oxide phase. This phase was found to represent a new oxyborate of sodium and lanthanum with the formula Na₃La₉O₃(BO₃)₈, whose single crystals were obtained by flux growth. It was established that synthesis of a polycrystalline material with the same composition was also possible using solid-state interaction between Na₂CO₃, La₂O₃ and H₃BO₃ at 1000-1100 °C. X-ray diffraction experiments on single crystals were used to solve the structure of Na₃La₉O₃(BO₃)₈. The unit cell was found to be hexagonal, space group P62m (No. 189) with Z=1. The compound can be regarded as the forefather of a second group of oxyborates representing a new family of isostructural compounds, Na₃RE₉O₃(BO₃)₈. Such phases were obtained with RE=Nd, Sm and Eu whereas with RE=Y and Gd, the synthesis experiments failed.The concentration and temperature regions of crystallization of the double-oxyborate Na₂Al₂O(BO₃)₂ in the ternary system Na₂O-Al₂O₃-B₂O₃ were determined. This compound was shown to melt incongruently at 970±3 °C, which made high-temperature solution growth most appropriate for obtaining its single crystals with NaBO₂ as the best solvent. On the basis of the data obtained, a composition of the initial solution was proposed, the validity of the choice being demonstrated by the growth of Na₂Al₂O(BO₃)₂ single crystals on a seed using the top-seeded solution growth (TSSG) technique and slow cooling of the solution.