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 K 3 RE 3 (BO 3 ) 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 K 3 Pr 3 (BO 3 ) 4 , K 3 Er 3 (BO 3 ) 4 , K 3 Tm 3 (BO 3 ) 4 , K 3 Yb 3 (BO 3 ) 4 and K 3 Lu 3 (BO 3 ) 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 ¯ ( Z = 4), whereas the other K 3 RE 3 (BO 3 ) 4 members with higher atomic number (RE = Sm–Lu) crystallize in the higher symmetry space group P 2 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 K 3 RE 3 (BO 3 ) 4 (RE = Sm–Lu) consist of quasi-two-dimensional [RE 8 (BO 3 ) 8 ] layers parallel to the ab -plane which are connected by out-of-layer RE 2 O 12 and RE 2 O 14 dimers as well as BO 3 groups in the c-direction, forming a 3D framework. On the other hand, both K 3 Pr 3 (BO 3 ) 4 and K 3 Nd 3 (BO 3 ) 4 phases are comprised of the [RE 4 (BO 3 ) 4 ] layers, indicating the absence of 2 1 screw axis along the b -axis. The K + cations are located between the [RE 8 (BO 3 ) 8 ] and [RE 4 (BO 3 ) 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 BO 3 planar groups are verified using Fourier transformed infrared and Raman spectra, and supported by the X-ray diffraction results.