Ba2Cd(B3O6)2: A Congruent-Melting Compound with Isolated B3O6 Units

The single crystals of Ba2Cd(B3O6)2 were grown by the spontaneous crystallization method for the first time. They crystallize in the centrosymmetric trigonal space group R$\bar{3}$ with a = 7.143(3) angstrom, c = 17.405(16) angstrom, and Z = 3. The structure is characterized by isolated B3O6 units, and the Ba2+ and Cd2+ cations connect with B3O6 rings to form three dimensional structure. The TG/DSC and XRD results reveal that Ba2Cd(B3O6)2 melts congruently. First-principles electronic structure calculation performed with the density functional theory (DFT) method shows that the calculated bandgaps are 4.66 eV, which is in good agreement with the UV/Vis/NIR experimental value 4.59 eV. The calculation shows that the Ba2Cd(B3O6)2 crystal has a large birefringence (n = 0.08750.0569 from 270 nm to 2600 nm), which demonstrates that Ba2Cd(B3O6)2 is a potential birefringence crystal.