Large Difference in Nonlinear Optical Activity of Rare Earth Ion Substitution of Bi3+ in A3TeBO9 (A = Bi, La, Pr, Nd, Sm-Dy)

The lone pair electrons inBi(3+) lead to a hugedifference in second harmonic generation (SHG) response between Bi3TeBO9 and its isomorphic compound RE3TeBO9. A series of oxides with high rare earth contents, RE3TeBO9 (RE = La, Pr, Nd, Sm-Dy), were synthesized,andthey all crystallize in the noncentrosymmetric space group P6(3) resembling that of a previously reportedBi(3)TeBO(9) (J. Am. Chem. Soc. 2016, 138, 14190-14193), whichshowed a very strong second harmonic generation (SHG) signal believedto come from all three structural units [BO3], [TeO6], and [BiO6]. Surprisingly, compared with theisostructural compound Bi3TeBO9, both theoreticalcalculations and powder SHG tests show that RE3TeBO9 has an abnormally small SHG effect (0.2 x KDP). Basedon the group theory analysis, the possible reasons are attributedto the misalignments and lack of distortions of the [BO3], [TeO6], and [REO8] groups in the unit cell.The 2(1) symmetric operation (a subset operation of the 6(3)-screw axis) of the [TeO6], [BO3], and[REO8] groups leads to complete cancellation of their contributionsto the xy directions, while the absence of lone pairelectrons in RE3+ and Te6+ results in smalldistortion in the c axis, hence small contributionsto the SHG coefficients d(ijk) involvingthe z axis. Diffuse reflectance spectra of RE3TeBO9 (RE = La, Pr, Nd, Sm-Dy) show typical absorptionof different RE3+, and the similarity of their infraredspectra is due to the same crystal structure and same structural unitsof the [TeO6] and [BO3] groups. The strong fluorescenceintensities of Eu3TeBO9 and Tb3TeBO9 show the characteristic emissions of Eu3+ andTb(3+), which may serve as luminescence materials with properdoping.