High-Pressure Synthesis of Rare-Earth Borate-Nitrate Crystals for Second Harmonic Generation

The crystals of a novel family of rare-earth borate-nitrate compounds, Ln₇(BO₃)₃(NO₃)N₃O (Ln = Pr, Nd), were grown at high-pressure in KAs flux and their crystal structure was determined. The new type of the crystalline structure consists of parallel chains of Ln₆ octahedra connected by common faces and forming the channels with the NO₃ triangular planar motifs in the center, and isolated OLn₄ tetrahedra separated from each other by N₃ triangular motifs. Each NO₃ triangle is in fact a part of rather unusual (NB₃O₁₂) block consisting of 3 distorted BO₄ tetrahedra around central nitrogen atom. Under near-infrared (NIR) (_ex = 1064 nm) excitation, both compounds revealed a strong signal of second harmonic generation (SHG) at half the excitation wavelength (_em = 532 nm), which is in agreement with their noncentrosymmetric structure. In addition, a photon up-conversion (UC) emission at _em = 880 nm was observed for microcrystals of Nd₇(BO₃)₃(NO₃)N₃O, which was assigned to the UC process occurring within the 4f electronic manifold of Nd³⁺ ions. The dual-emission (SHG/UC) properties of Nd₇(BO₃)₃(NO₃)N₃O microcrystals, concomitant with the absence of photobleaching, makes them prospective candidates for microscopic probes in biological studies.