The Na2O–SrO–B2O3 diagram in the B-rich part and the crystal structure of NaSrB5O9

The subsolidus phase relations in the B-rich part of the ternary system, Na2O–SrO–B2O3, are investigated by the powder X-ray diffraction method. Four ternary compounds: NaSrBO3, NaSr4B3O9, Na3SrB5O10 and NaSrB5O9 were found in it, the two lasts are new. NaSrB5O9 crystallizes in the monoclinic space group P21/c, with the lattice parameters a=6.4963(1) Å, b=13.9703(2) Å, c=8.0515(1) Å, β=106.900(1)°. Na3SrB5O9 is also monoclinic, space group C2, a=7.290(1) Å, b=13.442(2) Å, c=9.792(1) Å, β=109.60(1). NaSrB5O9 is isostructural with another pentaborate NaCaB5O9, and its structure was refined by Rietveld method based on the structural model of NaCaB5O9. The fundamental building units are [B5O9]3− anionic groups, forming complex thick anionic sheets, extending parallel to the ac plane. The Na and Sr atoms are all eight-coordinated with O atoms, forming trigonal dodecahedra. The [NaO8] polyhedra are distributed between the B–O sheets, while the [SrO8] polyhedra located in the sheets and connect with each other by edges to form infinite chains along the c-axis.