Ba 2 Cd(B 3 O 6 ) 2 : A Congruent‐Melting Compound with Isolated B 3 O 6 Units

The single crystals of Ba 2 Cd(B 3 O 6 ) 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) Å, c = 17.405(16) Å, and Z = 3. The structure is characterized by isolated B 3 O 6 units, and the Ba 2+ and Cd 2+ cations connect with B 3 O 6 rings to form three dimensional structure. The TG/DSC and XRD results reveal that Ba 2 Cd(B 3 O 6 ) 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 Ba 2 Cd(B 3 O 6 ) 2 crystal has a large birefringence (Δ n = 0.0875–0.0569 from 270 nm to 2600 nm), which demonstrates that Ba 2 Cd(B 3 O 6 ) 2 is a potential birefringence crystal.