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 P1¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{1}$$\end{document} (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.

中文翻译：

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新型碱金属原硼酸盐 K3RE3(BO3)4 (RE = Pr, Nd, Sm–Lu) 的合成和晶体结构

我们报道了一系列通过高温固相制备的新型碱金属原硼酸盐 K3RE3(BO3)4 (RE = Pr、Nd、Sm、Eu、Gd、Tb、Dy、Ho、Er、Tm、Yb 和 Lu)。状态综合。K3Pr3(BO3)4、K3Er3(BO3)4、K3Tm3(BO3)4、K3Yb3(BO3)4和K3Lu3(BO3)4的单晶是通过使用助熔剂自发结晶获得的。互补的单晶和粉末 X 射线衍射测量以及随后的结构改进表明，含 Pr 和 Nd 的相在空间群 P1¯\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} 中结晶\usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{1}$$\结束{文档}（Z = 4），而其他具有较高原子序数（RE = Sm-Lu）的 K3RE3(BO3)4 成员在较高对称性空间群 P21/c (Z = 8) 中结晶。在空间群的差异中，该系列的每个成员在其主题和连接性上都保持着密切的结构相关性。K3RE3(BO3)4 (RE = Sm–Lu) 的晶体结构由平行于 ab 面的准二维 [RE8(BO3)8] 层组成，这些层通过层外 RE2O12 和 RE2O14 二聚体连接以及 c 方向的 BO3 基团，形成 3D 框架。另一方面，K3Pr3(BO3)4 和 K3Nd3(BO3)4 相均由 [RE4(BO3)4] 层组成，表明沿 b 轴没有 21 个螺旋轴。K+ 阳离子位于 [RE8(BO3)8] 和 [RE4(BO3)4] 层之间，占据间隙空隙。所有样品都表现出与稀土阳离子相关的 UV/Vis 范围内的特征吸收特征，并且它们的光学带隙通过传统的 Tauc 图和吸收光谱拟合推导 (DASF) 方法进行评估。使用傅立叶变换红外光谱和拉曼光谱验证了远离 BO3 平面群 D3h 对称性的几何偏差，并得到 X 射线衍射结果的支持。