Pb2Bi2GaB3O11, a new congruent-melting galloborate containing two types of asymmetric cations with a moderate birefringence.

Pb2Bi2GaB3O11, a new congruent-melting galloborate, which simultaneously contains two types of lone-pair cations and a π-conjugated system in one crystal structure, has been discovered through spontaneous crystallization. Single-crystal X-ray structural analysis shows that Pb2Bi2GaB3O11 crystallizes in the monoclinic space group of C2/c. The parameters of the unit cell are a = 17.318(5) Å, b = 8.868(3) Å, c = 6.779(2) Å, β = 90.238(4)° and Z = 6. The structure of Pb2Bi2GaB3O11 features three kinds of tunnels, circled by cation polyhedra, in two of which isolated BO3 groups reside. However, the open Bi4O4 channel, as the third tunnel formed by the Bi-O polyhedra, is hollow. Pb2Bi2GaB3O11 possesses an experimental band gap of 4.43 eV and an ultraviolet (UV) cut-off edge of about 280 nm, which is proven by its diffuse reflection spectrum. Theoretical calculations have also been executed to evaluate its band structure, partial density of state (PDOS) and birefringence. The results indicate that the title compound possesses a direct band gap with a theoretical value of 3.48 eV, and even indicate that the Pb2+ and Bi3+ ions are stereochemically active and cooperate with the BO3 groups, which contributes to a moderate birefringence of about 0.07 at 589 nm for Pb2Bi2GaB3O11.