AlBBaF2O3, hexagonal, P6 (No. 174), a = 4.8879(6) Å, c = 9.403(1) Å, V = 194.5 Å 3 , Z = 2, Rgt(F) = 0.024, wR obs (F 2 ) = 0.036, T = 296 K. Source of materialCrystals of the title compound were grown by the spontaneous nucleation method from NaF flux. The starting materials BaF 2, Al 2O3, B2O3 and NaF were mixed in the appropriate ratio, then heated in a platinum crucible by using a vertical cylindrical electric furnace until they were completely melted . The temperature was decreased at a rate of 1 K/h from the saturation temperature. After 2 days, the solution was cooled to room temperature at a rate of 30 K/h. Small crystals for X-ray determination were obtained by mechanic fragmentation. Experimental detailsIn a preliminary study, the structure was refined in the space group P6 3 (No. 173). But, the R-values (R1 = 0.061 and wR2 = 0.083) were significantly larger that those in the present refinement. Moreover, the refinement resulted in unusual B-O distances. Also, a refinement in space group P3 failed. Finally, it was stablished that the correct space group is P6. A new alumino-borate BaAlBO 3F2 has been investigated in [7], using Rietveld refinement of powder X-ray data and suggested to be isostructural with BaGaBO 3 F 2 . According to this report, the BaAlBO 3F2 compound crystallizes in the hexagonal system with space group P6 3/m, Z = 2, and a = 4.882(1) Å, c = 9.398(1) Å, Rwp = 0.222, c2 = 2.59. The structure of BaAlBO 3 F 2 was studied in detail by X-ray analysis on single crystal in our group and we found that BaAlBO 3F2 has a non-centrosymmetric structure. The structure is composed of (001) layers of corner-sharing AlO 3F2 symmetric trigonal bipyramids and BO3 triangles. Ba cations are housed in the two dimensional (Al 3B3O6F6) frameworks as shown in both figures. The layers are repeating alternatively (···ABAB···). Co-planar BO 3 groups are normal to the c axis, but the turn of the BO 3 groups has a small divergence in neighboring layers and the B-O bond lengths are different 1.369(4) Å and 1.376(4) Å. In the AlO 3F2 symmetric trigonal bipyramids, all Al atoms and O atoms lie in the same plane normal to the c axis. The Al-F bond lengths are 1.917(4) Å and 1.751(6) Å. The structure has some similarities with that of the nonlinear optical crystal KBe 2BO3F2 (KBBF) [8], which reveals the shortest vacuum ultraviolet second harmonic generation output (184.7 nm) [9]. KBBF crystal consisted of corner-sharing BO 3 group where all B and O atoms lie in the same plane normal to the c axis and tetrahedral BeO 3 F group, Be atoms are alternately above and below the plane at a distance of 0.5792(4) Å. Discussion
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