Pengyun Chen, M. Mangir Murshed, Michael Fischer, Thomas Frederichs, and Thorsten M. Gesing

Inorg. Chem (2020), 59, 24, 18214–18224

https://doi.org/10.1021/acs.inorgchem.0c02684

We report a detailed structural, spectroscopic, and thermogravimetric investigation of a new series of mixed-alkali rare-earth orthoborates KLi₂RE(BO₃)₂ (RE = Dy, Ho, Er, Tm, Yb, and Y). Single crystals were directly prepared by a flux method as well as mechanically separated from the polycrystalline powder obtained from the conventional solid-state reactions. All KLi₂RE(BO₃)₂ members are isotypic and crystallize in the space group P2₁/n. The novel structure type is comprised of [RE₂(BO₃)₄O₄]^14– anionic clusters where the edge-sharing REO₇ pentagonal bipyramids are connected by BO₃ groups and both K⁺ and Li⁺ cations are located at the interstitial voids of the 3D network. The metric parameters and crystal structural features obtained from the single-crystal data are in excellent agreement with those refined from the powder data. The change of the lattice parameters and unit cell volumes can be explained in terms of the lanthanide contraction effect. A comparison between KLi₂RE(BO₃)₂ and other rare-earth borates with similar chemical compositions indicates that the sum of the ionic radii of the alkali-metal cations governs the symmetry of the crystals. Diffuse reflectance UV–vis spectra display the characteristic absorption behaviors of the RE³⁺ cations and the fundamental absorption edge. Both the Tauc’s and derivation of absorption spectrum fitting (DASF) methods were used to identify the magnitude and type of bandgap, respectively, which are compared with those obtained from density functional theory (DFT) calculations. The calculated phonon density of states and the vibrational frequency at the gamma point help explain the Fourier transform infrared and Raman spectra of KLi₂RE(BO₃)₂. The incongruent melting behavior and the thermal stability of each member of the KLi₂RE(BO₃)₂ series were also studied by thermogravimetric analyses.