KCu0.9Bi2.7S5: New Semiconducting Quaternary Bismuth Sulfide

Abstract

Bismuth chalcogenides are quite interesting due to their potential as useful thermoelectric materials. Recently, many new ternary and quaternary compounds have been discovered that possess promising thermoelectric property. Furthermore, fundamental interest in these compounds is growing due to their structural diversity they exhibit. For example, the compounds of a general formula of ABi3Q5 (A=K, Rb, Cs; Q=S, Se, Te) are known to have six different structure types. The polymorphism is made possible by subtle changes in the edge-sharing modes between the infinite BiQ6 (Q=S, Se) octahedral chains found in the frameworks. Various edge-sharing modes between the octahedral chains can form many types of building units, resulting in a large number of new bismuth chalcogenide frameworks. This is because each BiQ6 octahedron in one chain has maximum 10 edges available for edge-sharing with others in the neighboring chains. If all the 10 edges are shared with ten other BiQ6 octahedra in six neighboring chains, then NaCl-type structure is formed. If fewer edges of an octahedron are to be shared, there are many possible ways of edge-sharing modes to be imagined. For example, (1 × 3) building bar units, formed by sharing edges of BiQ6 octahedra between three chains, are found in α-RbBi3Se5. (2 × 3) building bar units, formed by the two (1 × 3) building bar units, are found in α-RbBi3Se5, γ-RbBi3S5, γ-RbBi3Se5, CsBi3S5, K2Bi6.33S10, and K2Bi8S13. Furthermore, (1 × 2) building bar units in Bi2S3, (2 × 2) units in αRbBi3S5, (1 × 4) units in CsBi3Se5, and even (1 × 8) units in KBi3S5 are found. This suggests that proper utilization of various types of building bar units in well designed reactions will lead to many new compounds with open frameworks or low-dimensional characteristics. Recently, quaternary A/Bi/M/Q (M=Zn, Cd, Mn, Cu; Q=S, Se) systems have been explored and many new compounds of Cs2Bi2MS5 (M=Zn, Cd, Mn), K3Bi5Cu2S10, RbBi2.66CuSe5, ABi2CuS4 (A=K, Cs), and A3Bi5Cu2S10 (A=Rb, Cs) are prepared. They contain infinite linear chains of BiQ6 octahedra as well as MQ4 tetrahedra. Once again, (1 × 3) octahedral building bar units in RbBi2.66CuSe5 and (1 × 2) building bar units in Cs2Bi2MS5 (M=Zn, Cd, Mn), K3Bi5Cu2S10, ABi2CuS4 (A=K, Cs), and A3Bi5Cu2S10 (A=Rb, Cs) are found. Furthermore, the presence of additional MQ4 tetrahedral infinite chains complicates the connectivity between them. The tetrahedral metal ion plays an important role as a binder in connecting the octahedral BiQ6 building bar units enriching the structural diversity of bismuth chalcogenides. Here, we report on the synthesis, structural, and optical characterization of the new quaternary bismuth chalcogenide compound of KCu0.9Bi2.7S5. The structure of KCu0.9Bi2.7S5 is isostructural with that of RbBi2.66CuSe5. It has a three-dimensional framework with tunnels running parallel to a-axis as shown in Figure 1. Its unique three-dimensional structure is built from the linear infinite BiS6 octahedral chains and CuS4 tetrahedral chains. The linear infinite BiS6 octahedral chain is formed by edgesharing along the crystallographic a-axis. The linear CuS4 tetrahedral chain is formed via vertex-sharing of tetrahedrons along the crystallographic a-axis. The (1 × 3) octahedral building bar unit (see Figure 1) is formed by sharing edges of each BiS6 octahedron in the central chain with 4 neighboring octahedra in two adjacent chains. These (1 × 3) building bar units are then sharing edges of their terminal octahedra with the neighboring ones to form 2-D stepwise layers as shown in Figure 2. These layers are interconnected with each other by sharing S(1) vertex of terminal octahedra in (1 × 3) building bar units to form a 3-D framework with tunnels. Cu ion is sitting in a tetrahedron of