Synthesis and crystal structures of novel alkali rare-earth orthoborates K3RE3(BO3)4 (RE = Pr, Nd, Sm-Lu)

We report a series of novel alkali rare-earth orthoborates K3RE3(BO3)(4) (RE = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) prepared by high-temperature solid-state syntheses. Single crystals of K3Pr3(BO3)(4), K3Er3(BO3)(4), K3Tm3(BO3)(4), K3Yb3(BO3)(4) and K3Lu3(BO3)(4) are obtained by spontaneous crystallization using a flux. Complementary single-crystal and powder X-ray diffraction measurements followed by structure refinements reveal that both Pr- and Nd-containing phases crystallize in the space group P (1) over bar (Z = 4), whereas the other K3RE3(BO3)(4) members with higher atomic number (RE = Sm-Lu) crystallize in the higher symmetry space group P2(1)/c (Z = 8). Amid differences in the space groups, each member of the series keeps close structural correlations in their motif and connectivity. The crystal structures of K3RE3(BO3)(4) (RE = Sm- Lu) consist of quasi-two-dimensional [RE8(BO3)(8)] layers parallel to the ab-plane which are connected by out-of-layer RE2O12 and RE2O14 dimers as well as BO3 groups in the c-direction, forming a 3D framework. On the other hand, both K3Pr3(BO3)(4) and K3Nd3(BO3)(4) phases are comprised of the [RE4(BO3)(4)] layers, indicating the absence of 2(1) screw axis along the b-axis. The K+ cations are located between the [RE8(BO3)(8)] and [RE4(BO3)(4)] layers, occupying the interstitial voids. All samples exhibit characteristic absorption features in the UV/Vis range relevant to the rare-earth cations, and their optical band gaps are evaluated by both conventional Tauc plot and derivation of absorption spectrum fitting (DASF) methods. The geometric deviations away from the D-3h symmetry of the BO3 planar groups are verified using Fourier transformed infrared and Raman spectra, and supported by the X-ray diffraction results.

Automated summary

A series of novel alkali rare-earth orthoborates were synthesized using high-temperature solid-state methods, with single crystals obtained by spontaneous crystallization using a flux. The crystal structures of these compounds show variations in space group symmetry, but maintain close structural correlations in their motifs and connectivity. Characteristic absorption features in the UV/Vis range relevant to rare-earth cations were observed, and geometric deviations in the BO3 planar groups were verified using Fourier transformed infrared and Raman spectra.