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, Ln(7)(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 Ln(6) octahedra connected by common faces and forming the channels with the NO3 triangular planar motifs in the center, and isolated OLn(4) tetrahedra separated from each other by N-3 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) (lambda(em) = 1064 nm excitation, both compounds revealed a strong signal of second harmonic generation (SHG) at half the excitation wavelength (lambda(em) = 532 nm), which is in agreement with their noncentrosymmetric structure. In addition, a photon up-conversion (UC) emission at lambda(em) = 880 nm was observed for microcrystals of Nd-7(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 Nd-7(BO3)(3)(NO3)N3O microcrystals, concomitant with the absence of photobleaching, makes them prospective candidates for microscopic probes in biological studies.