Subsolidus phase equilibrium of Na2O–Sm2O3–B2O3 system has been investigated mainly by solid-state reaction and powder X-ray diffraction method. There are nine definite three-phase regions and three ternary compounds determined under present experimental conditions. A novel compound Na3SmB2O6 was found and confirmed in this system, along with its two homogeneous compounds Na3REB2O6 (RE = Pr, Eu) synthesized for the first time. The indexing results showed that all three compounds crystallize in the monoclinic space group P21/c (No.14) with the same structure type as both Na3NdB2O6 and Na3GdB2O6. The lattice parameters (a, b, and c) of new borates Na3REB2O6 (RE = Pr, Sm, Eu) decrease linearly with a decreasing radius of RE ion, which obeys the Lanthanide-contraction rule. The existence of a trigonal BO3 group in the Na3REB2O6 (RE = Pr, Sm) compounds was confirmed by analysis of their infrared absorption spectra. Thermal stabilities of the three new borates have been investigated.
