Phase formation in ternary systems of sodium, rare-earth (scandium, indium, and ytterbium), and boron oxides

The study was aimed at examining phase equilibria in Na 2 O–R 2 O 3 –B 2 O 3 systems (R = In, Sc, Yb). Interactions in these systems were analyzed using the solid-state reaction method. Phase formation in the ternary systems of sodium, rare-earth (scandium, indium, and ytterbium), and boron oxides was studied for the first time in the subsolidus region within the temperature range of 700–750°С. Preliminary triangulation of the analyzed systems was conducted. At 750°С, the isothermal cross sections of Na 2 O–R 2 O 3 –B 2 O 3 (R = In, Sc, Yb) systems are represented by eight, thirteen, and eleven triangles of coexisting phases, respectively. The study confirmed the formation of three known double sodium-scandium borates in the Na 2 O–Sc 2 O 3 –B 2 O 3 system: Na 3 Sc(BO 3 ) 2 , Na 3 Sc 2 (BO 3 ) 3 , and NaScB 2 O 5 . Also, a double sodium-scandium borate of approximate composition Na 3 ScB 8 O 15 was found to form, which was indexed as monoclinic (approximate space symmetry group Cc). The unit cell parameters were refined using the Le Bail method: a = 12.625(6); b = 7.725(2); c = 10.409(3), β = 53.19(2)°. The Na 2 O–Yb 2 O 3 –B 2 O 3 system contains three new double borates: Na 3 Yb(BO 3 ) 2 , Na 3 YbB 8 O 15 , and borate with a composition close to the stoichiometric ratio Na 2 O•Yb 2 O 3 •B 2 O 3 . The formation of the following compounds was first experimentally shown: Na 3 YbB 8 O 15 isostructural to Na 3 GdB 8 O 15 and isoformula Na 3 Yb(BO 3 ) 2 , which is not isostructural to Na 3 Y(BO 3 ) 2 . The triple oxide system with indium was found to be eutectic, i.e., without the formation of double sodium-indium borates.