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−xF2+3x solid solution belonging to the BaF2–Ba3(BO3)2 section. The distinguishing feature of the crystal structure of Ba7(BO3)4−xF2+3x phase is its extensive (BO3)3− ↔ 3F− anionic isomorphic substitution, confirmed by X-ray diffraction study of Ba7(BO3)3.51F3.47 (x = 0.49) single crystals (space group P63; a = 11.18241(11) A, c = 7.23720(8) A). The area of homogeneity for Ba7(BO3)4−xF2+3x solid solution spans between Ba7(BO3)3.35F3.95 and Ba7(BO3)3.79F2.63 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 A, b = 14.843 A and c = 10.291 A.