Phase formation in the BaB2O4–BaF2–BaO system and new non-centrosymmetric solid-solution series Ba7(BO3)4−xF2+3x

Abstract

Detailed study of the BaB₂O₄–BaF₂–BaO system resulted in the discovery of the new Ba₇(BO₃)_4−xF_2+3x solid solution belonging to the BaF₂–Ba₃(BO₃)₂ section. The distinguishing feature of the crystal structure of Ba₇(BO₃)_4−xF_2+3x phase is its extensive (BO₃)³⁻ ↔ 3F⁻ anionic isomorphic substitution, confirmed by X-ray diffraction study of Ba₇(BO₃)_3.51F_3.47 (x = 0.49) single crystals (space group P6₃; a = 11.18241(11) Å, c = 7.23720(8) Å). The area of homogeneity for Ba₇(BO₃)_4−xF_2+3x solid solution spans between Ba₇(BO₃)_3.35F_3.95 and Ba₇(BO₃)_3.79F_2.63 compositions (0.21 < x < 0.65). Also, a new orthorhombic phase with a tentative composition of Ba₅(BO₃)₃F has been identified in XRD powder patterns and indexed with cell parameters a = 7.605 Å, b = 14.843 Å and c = 10.291 Å.