Search For New Borate Compounds And The Growth Of RbNbB₂O₆ Crystal

In this dissertation, preparation and structures of some layered Alumino-borates and some new rare-earth borate compounds were studied. The samples have been synthesized using high-temperature techniques and characterized by powder or single-crystal X-ray diffraction methods. Meanwhile, the growth and properties of RbNbB₂O₆ crystal were also studied.Following the introduction chapter (chapter 1 and 2), in the chapter 3, two compounds NaSr0.5Al₂B₂O₇ and NaCa0.5Al₂B₂O₇, have been found to crystallize in a new structure type by Rietveld refinement from X-ray powder diffraction data. Their structure belongs to hexagonal space group P63/m, with lattice parameters of a = 4.8564(1)?, c = 15.6635(4)? for NaSr0.5Al₂B₂O₇ and a = 4.8129(1) ?, c = 15.4056(4)? for NaCa0.5Al₂B₂O₇, respectively. The structure is built up by [Al₂B₂O₇] double layer and Na/Ca or Na/Sr ions alternatively stacking along the c-axis. The sites in the inter-double layer are fully occupied jointly by Na and Ca or Sr, but the intra-double layer sites are only half occupied solely by Na. A mechanism of the transition of the structure from CaAl2-+2++2+2B2O7 to present structure type by replacing only 1% Ca by Na (2%) as observed by K.S. Chang and D.A. Keszler (Mater. Res. Bull. 33(1998), 299) is also proposed. We also found that Na2Al₂B₂O₇ has a phase transition at the room temperature. That is, part of thephase will transfer to P6cP13?3/m phase slowly at room temperature, and the transition can be reversed by heating the aged sample above 220°C.In the chapter 4, we studied the rare-earth borates system in order to find new materials as nonlinear optical, self-frequency doubling, phosphorous or laser crystals. Five new compounds K₃Y3B4O12, LiY6O5(BO₃)₃, K₃LnB₂O₆ (Ln=Nd-Gd), K₃LnB₂O₆(Ln=Tb-Lu) and K₃.75La1.75(BO₃)₃ were discovered. Their structures were solved using single crystal X-ray data.K₃Y3B4O12,space group: P21/c, cell parameters: a=10.4667(16)?, b=17.361(3)?,c=13.781(2)?,β=110.548(8)°; The structure consists sheets of [Y₈B₈O₂₄]∞linked by out of sheet BO3 groups and Y ions to form a three-dimensional framework. The luminescent properties of Eu3+ and Tb3+ doped K₃Y3(BO3)4 materials have also been studied.LiY6O5B3O9, is isostructural with LiGd6O5(BO₃)₃ space group (J. P. Chaminade, et al. J. Solid State Chem., 146(1999), 189-196), space group, P21/n, cell parameters: a =8.330(3)?,b=15.444(4)?, c =8.780(3)?,β=91.85(3)°; It exhibits a three-dimensional framework of yttrium-oxygen polyhedra interconnected by common edges and corners. B and Li atoms are located in the planes and cavities formed by oxygen atoms, respectively.K₃LnB₂O₆ (Ln=Nd-Gd), K₃SmB₂O₆(representative), space group: Pnma, cell parameters: a = 9.046(3)?, b = 7.100(2)?,c = 11.186(3)?; This structure contains infinite SmBO7 chains formed by corner sharing SmO7 pentagonal dipyramid and BO3 group, and those chains are interconnected by the other BO3 groups.K₃LnB₂O₆(Ln=Tb-Lu), K₃YB₂O₆(representative), space group: Pnnm, cell parameters: a=9.3377(9)?, b=6.7701(6)?, c=5.5058(4)?; It's structure can be described as a three-dimensional framework formed by isolated YO6 octahedron jointed by BO3 triangles by sharing their apical oxygen atoms.K₃.75La1.75B3O9, space group: Pnc2 (or Pn), cell parameters: a=16.358(3)?, b=6.7750(14)?, c= 9.4300(19)?,(β=90°)。The structure consists wave-like sheets of [La6B6O18]∞linked by out of sheet BO3 groups to form a three-dimensional framework. Powder SHG test showed that the SHG intensity for K₃.75La1.75B3O9 is close to that of KDP. The UV transmission cutoff is observed at about 230nm.RbNbB₂O₆ as NLO crystal was reported by Nichillos et al, however, its nonlinear optical properties have not been fully investigated. In the chapter 5, the results encompass the growth of RbNbB₂O₆ crystal and nonlinear optical properties. Crystals up to 20×20×10mm in size have been grown from high temperature melt by top-seeded solution growth method using LiBO2-RbBO2 flux. The flux shows improved conditions including the lower growth temperature, transparent melt and lower volatility. All independent nonlinear optical coefficients dij of the crystal were measured with the Maker fringe technique. The results show that the d31=1.83pm/V, d32=1.55 pm/V, d33=0.76 pm/V.