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 BaB 2 O 4 -BaF 2 -BaO system resulted in the discovery of the new Ba 7 (BO 3 ) 42x F 2+3x solid solution belonging to the BaF 2 -Ba 3 (BO 3 ) 2 section. The distinguishing feature of the crystal structure of Ba 7 (BO 3 ) 42x F 2+3x phase is its extensive (BO 3 ) 32 « 3F 2 anionic isomorphic substitution, confirmed by X-ray diffraction study of Ba 7 (BO 3 ) 3.51 F 3.47 (x = 0.49) single crystals (space group P6 3 ; a = 11.18241(11) A ˚, c = 7.23720(8) A ˚). The area of homogeneity for Ba 7 (BO 3 ) 42x F 2+3x solid solution spans between Ba 7 (BO 3 ) 3.35 F 3.95 and Ba 7 (BO 3 ) 3.79 F 2.63 compositions (0.21 , x , 0.65). Also, a new orthorhombic phase with a tentative composition of Ba 5 (BO 3 ) 3 F has been identified in XRD powder patterns and indexed with cell parameters a = 7.605 A ˚, b = 14.843 A ånd c = 10.291 A ˚.

Electronic structure, magnetic and optical properties of the Ba7(BO3)4−F2+3 crystal

“…Thus, we supposed that the center with g max ¼1.959 is a vacancy of a fluorine atom in a Ba 6 octahedron which captured an electron (e 6 − ), while the center with g max ¼ 1.980 is an electron captured by a fluorine vacancy in the a [BO 3 F] 4-tetrahedron (e 4 − ). This assignment is consistent with the crystal structural feature in Ba 7 (BO 3 ) 4 À x F 2 þ 3x [8], as well as the fact that in an open system this compound emits fluorine at high temperature and this fluorine vacancies appear. Furthermore, the optical transition at 3.69 eV in the X-irradiation absorption spectrum is expected to be related to the e 6 − center, while that at 3.03 eV is related to the e 4 − center, as confirmed by the first-principles calculations (see Section 3.4).…”

“…Thus, the Ba 7 (BO 3 ) 4 À x F 2 þ 3x crystals with three x values (x ¼0.25, 0.5 and 0.75) were studied by the following models; for x¼0.5 one of the two F 4 pyramids was replaced by a BO 3 F pyramid in the unit cell, while for x¼ 0.25 and 0.75 three or one of the four F 4 pyramids were replaced by BO 3 F pyramids, respectively, in a (1 Â 1 Â 2) supercell. The chosen x value in the computer simulations are very close to the experimental range (0.21 ox o0.65) [8].…”

“…According to structural data in reference [8] Before X-irradiation there is no signal in ESR spectra of Ba 7 (BO 3 ) 4 À x F 2 þ 3x crystals at both 300 and 80 K. After X-irradiation weak signals from two paramagnetic centers with grg е ¼2.0023 (electron-type center) and with g Zg е (hole-type center) were observed at room temperature. At 80 K the ESR spectra from four different centers in irradiated crystals were clearly exhibited when the magnetic field Н is parallel to the [001] axis or [010] axis of the crystal (Fig.…”

“…The crystal was grown from the same composition as we used in Ref. [8], and the composition was determined to be Ba 7 (BO 3 ) 3.51 F 3.47 .…”

“…Recently we have extensively searched for new phases in the fluoroborate systems within ternary phase diagram, and optimized single-crystal growth conditions for the compounds with nonlinear optical properties [6,7]. Detailed investigations on the BaF 2 -Ba 3 (BO 3 ) 2 section in the BaB 2 O 4 -BaF 2 -BaO system resulted in the discovery of two new intermediate phases: Ba 5 (BO 3 ) 3 F and Ba 7 (BO 3 ) 4 À x F 2 þ 3x solid solution [8]. The crystal structures for these two phases are very different; Ba 5 (BO 3 ) 3 F crystallizes in the orthorhombic structure with centrosymmetric space group of Pnma, whereas Ba 7 (BO 3 ) 4 À x F 2 þ 3x belongs to the noncentrosymmetric space group P6 3 .…”