Cu_7.62Bi₆Se₁₂Cl₆I: Discovery of a Low Band Gap, Low Thermal Conductivity Mixed-Anion Material

The exploration of higher-dimensional chemical phase spaces and the synthesis of novel compounds can be achieved by applying a multiple-anion approach to materials discovery. The ability to combine and tune the stoichiometry of anions in a material can enable enhanced control of both the physical and electronic structures, providing a strategy for the modification of the properties of new materials being developed for a variety of applications, including solar absorbers and thermoelectrics. Here, we report the synthesis of Cu_7.62Bi₆Se₁₂Cl₆I, a quadruple-anion (Se^2-, (Se₂)^2-, Cl^-, I^-) material within the Cu-Bi-Se-Cl-I phase space. Crystal growth reactions yield black, needle-like crystals, which exhibit a highly anisotropic and complex structure containing the four distinct anion types, solved from single-crystal X-ray diffraction data. Compositional analysis confirms the complex material stoichiometry, and a low band gap of 0.94(5) eV is measured to understand the potential for solar-absorbing applications. Cu_7.62Bi₆Se₁₂Cl₆I has a low thermal conductivity of 0.25(2) W K^-1 m^-1, which is attributed to multiple structural features via analysis of experimental heat capacity data and is achieved through the diversity in bonding that is accessed through the combination of four different types of anion.