Extended Occupational and Positional Disorder in Pavonite Homologous Copper Bismuth Chalcogenide Halogenides

Abstract

Abstract Melting reactions of CuBr, BiBr 3 and Bi 2 Q 3 ( Q = S or Se) resulted in black needles of quaternary chalcogenide bromides. Cu 1.5 Bi 2.64 S 3.42 Br 2.58 has a melting point of 708(5) K and crystallizes in the monoclinic space group C 2/ m with a = 1315.1(3) pm, b = 410.28(5) pm, c = 937.2(2) pm, β = 90.33(2)° at T = 293(2) K. The isostructural selenide Cu 1.57 Bi 2.37 Se 2.68 Br 3.32 , which melts at 724(5) K, has the lattice parameters a = 1353.9(3) pm, b = 417.54(5) pm, c = 960.4(2) pm, β = 91.13(2)°. Black needles of the selenide iodides Cu 1.57 Bi 4.69 Se 7.64 I 0.36 and Cu 2.31 Bi 5 Se 8.31 I 0.69 were obtained by chemical vapor transport of CuI and Bi 2 Se 3 . They also crystallize in space group type C 2/ m with a = 1373.9(4) pm, b = 417.4(1) pm, c = 1464.6(4) pm, β = 115.57(2)°, and a = 1369.1(3) pm, b = 416.6(1) pm, c = 1526.4(3) pm, β = 99.81(3)°, respectively. All four compounds belong to pavonite homologous series. Their structures are built up of two alternating types of layered modules parallel (001), denoted A and B . Composition and shape of module A are almost the same in all structures. They consist of paired rods $\rm ^1_\infty[BiQ_{3/3}X_{2/2}Z_{2/2}]$ = Br, I) running along [010] and copper(I) cations that are distributed over various voids between the rods. Modules of type B are composed of [ MZ 6 ] octahedra ( M = Bi/Cu), which are arranged in NaCl‐type fragments of thickness N = 1, 3 or 4. The Joint Probability Density Function (JPDF) reveals a continuous pathway for copper ion transport along [010] in the case of the sulfide bromide. The pseudo‐potential for hopping of copper ions was calculated as 47 meV only. The electrical and thermal conductivities as well as the Seebeck coefficient were determined to evaluate the thermoelectric properties.