K5Ba10(BO3)8F: A New Potassium Barium Borate Fluoride with a Perovskite-Like Structure

A new borate fluoride, K5Ba10(BO3)8F, has been discovered through spontaneous crystallization, which is the first reported compound in the potassium barium borate fluorides system. The single-crystal X-ray structural analysis shows that K5Ba10(BO3)8F crystallizes in the trigonal space group R3̅c. The parameters of the trigonal unit cell are a = 15.293(2) Å, c = 22.699(3) Å, and Z = 6. The structure of K5Ba10(BO3)8F features a perovskite-like structure, and it can be written as [K3(K/Ba)6(BO3)8]FBa6. Meanwhile, the structure of K5Ba10(BO3)8F exhibits an intricate three-dimensional (3D) network composed of interpenetrated K–B–O net and Ba–O–F net. K5Ba10(BO3)8F possesses a large experimental band gap of 6.05 eV and an ultraviolet (UV) cutoff edge of about 205 nm proved by diffuse reflection spectrum. The birefringence of K5Ba10(BO3)8F was also evaluated by the first-principles calculations. Thermal behavior and Raman spectrum of K5Ba10(BO3)8F were also reported in this work.