A new structure type of the Na3Yb(BO3)2: Synthesis, crystal structure, thermal behavior, ionic conductivity, and spectroscopy

Abstract

New structure of Na₃Yb(BO₃)₂ has been synthesized for the first time using a solid-state reaction method. The crystal structure of the title compound was elucidated using a simulated annealing method. Samples used in powder diffraction analysis for structure determination were prepared via solid-state synthesis. To refine obtained crystal structure, the Rietveld method was applied, yielding the following parameters: triclinic symmetry (sp. gr. P $\overline{1}$.), a = 5.1661(1) Å, b = 6.6249(2) Å, c = 8.5991(2) Å, α = 92.089(1)°, β = 93.281(2)°, γ = 88.010(1)°, Z = 2, V = 293.47(1) ų, R_wp = 4.83, GOF = 4.85. The double borate Na₃Yb(BO₃)₂ congruently melted at 1119 °C exhibited a complex thermal profile, as evidenced by DSC, with four polymorphic transitions observed at 277 °C, 497 °C, 653 °C, and 694 °C. Ab initio calculated IR spectrum of Na₃Yb(BO₃)₂, exhibited a high degree of agreement with the experimentally obtained IR spectrum. The band gap of the title compound was calculated to be 4.7(2) eV using the combination of the Tauc method and DASF method. The calculated energy barrier for sodium ion migration, equal to 0.5 eV, was in a reasonable agreement with the experimentally determined activation energy of 0.75 eV. The title compound exhibited an ionic conductivity of 0.4 × 10⁻³ S/cm at 1023 K.