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 $$\overline{1}$$ (Z = 4), whereas the other K3RE3(BO3)4 members with higher atomic number (RE = Sm–Lu) crystallize in the higher symmetry space group P21/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 21 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 D3h symmetry of the BO3 planar groups are verified using Fourier transformed infrared and Raman spectra, and supported by the X-ray diffraction results.