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 image. 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.