Lost horses on the frontier: K2BiCl5 and K3Bi2Br9

Two missing members, K3Bi2Cl9 and K3Bi2Br9 , within the series of Bi-containing defect perovskite halides A(3)Bi(2)X(9) were sought. K3Bi2Cl9 could not be obtained, but the previously identified but uncharacterized compound K 2 BiCl5 was prepared. K3Bi2Br9 was successfully prepared. K2BiCl5 and K3Bi2Br9 were synthesized by reactions of binary halides at 450-500 degrees C and their structures were determined by single-crystal X-ray diffraction methods. K2BiCl5 adopts the K2PrCl5-type structure in orthorhombic space group Pnma with Z = 4 and cell parameters of a = 12.5461(7) angstrom, b = 8.7630(5) angstrom, and c = 7.9865(5) angstrom. This structure features one-dimensional columns of edge-sharing BiCl7 capped octahedra. K3Bi2Br9 adopts the Tl3Bi2Br9-type structure in monoclinic space group P2(1)/c with Z = 4 and cell parameters of a = 18.718(3) angstrom, b = 8.1148(13) angstrom, c = 13.154(2) angstrom, and beta = 91.329(2)degrees. In this defect perovskite variant, Bi atoms occupy two-thirds of the octahedral sites formed by close-packed arrays of the K and Br atoms, and the BiBr6 octahedra are connected by corner-sharing within slabs. Among defect perovskite halides A(3)Bi(2)X(9) containing corner-sharing octahedra, K3Bi2Br9 exhibits the greatest degree of distortion from the ideal cubic perovskite structure observed so far. The band gaps, as measured from optical diffuse reflectance spectra, are 2.8 eV for K2BiCl5 and 2.2 eV for K3Bi2Br9. Electronic structure calculations indicate an indirect gap for K2BiCl5 and a direct gap with rather flat bands for K3Bi2Br9.

Automated summary

Two missing members, K3Bi2Cl9 and K3Bi2Br9, within the series of Bi-containing defect perovskite halides A(3)Bi(2)X(9) were sought. K3Bi2Cl9 could not be obtained, but the previously identified but uncharacterized compound K2BiCl5 was prepared. K3Bi2Br9 was successfully prepared with its structure determined by single-crystal X-ray diffraction methods.