Studies on some ternary oxyborates of the Na 2O– Me2O 3–B 2O 3 ( Me=rare earth or aluminum) systems: Synthesis, structure and crystal growth

Abstract

Sodium rare-earth oxyborates Na 2 RE 2O(BO 3) 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)BO 3, Y 2(Er 2)O 3 and Na 2B 4O 7. Just the opposite was observed during thermal treatment of the oxide mixture containing Nd 2O 3, from which a practically pure phase of Na 2Nd 2O(BO 3) 2 was only obtained after melting. The attempts to synthesize the oxyborate Na 2La 2O(BO 3) 2 showed it to be unstable, this leading to the formation of a mixture containing, in addition to Na 2La 2O(BO 3) 2, also other already known stable phases of the system Na 2O–La 2O 3–B 2O 3 along with an unknown ternary oxide phase. This phase was found to represent a new oxyborate of sodium and lanthanum with the formula Na 3La 9O 3(BO 3) 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 Na 2CO 3, La 2O 3 and H 3BO 3 at 1000–1100 °C. X-ray diffraction experiments on single crystals were used to solve the structure of Na 3La 9O 3(BO 3) 8. The unit cell was found to be hexagonal, space group P62 m (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, Na 3 RE 9O 3(BO 3) 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 Na 2Al 2O(BO 3) 2 in the ternary system Na 2O–Al 2O 3–B 2O 3 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 NaBO 2 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 Na 2Al 2O(BO 3) 2 single crystals on a seed using the top-seeded solution growth (TSSG) technique and slow cooling of the solution.