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

A series of oxides with high rare earth contents, RE₃TeBO₉ (RE = La, Pr, Nd, Sm-Dy), were synthesized, and they all crystallize in the noncentrosymmetric space group P6₃ resembling that of a previously reported Bi₃TeBO₉ (J. Am. Chem. Soc. 2016, 138, 14190-14193), which showed a very strong second harmonic generation (SHG) signal believed to come from all three structural units [BO₃], [TeO₆], and [BiO₆]. Surprisingly, compared with the isostructural compound Bi₃TeBO₉, both theoretical calculations and powder SHG tests show that RE₃TeBO₉ has an abnormally small SHG effect (0.2 × KDP). Based on the group theory analysis, the possible reasons are attributed to the misalignments and lack of distortions of the [BO₃], [TeO₆], and [REO₈] groups in the unit cell. The 2₁ symmetric operation (a subset operation of the 6₃-screw axis) of the [TeO₆], [BO₃], and [REO₈] groups leads to complete cancellation of their contributions to the xy directions, while the absence of lone pair electrons in RE³⁺ and Te⁶⁺ results in small distortion in the c axis, hence small contributions to the SHG coefficients d involving the z axis. Diffuse reflectance spectra of RE₃TeBO₉ (RE = La, Pr, Nd, Sm-Dy) show typical absorption of different RE³⁺, and the similarity of their infrared spectra is due to the same crystal structure and same structural units of the [TeO₆] and [BO₃] groups. The strong fluorescence intensities of Eu₃TeBO₉ and Tb₃TeBO₉ show the characteristic emissions of Eu³⁺ and Tb³⁺, which may serve as luminescence materials with proper doping.