Exploring the AgSb1−xBixI4 phase diagram: Thermochromism in layered CdCl2-type semiconductors

Abstract AgSbI4 and several AgSb1–xBixI4 compositions have been synthesized via solid state reactions. Both end members and all intermediate compositions crystallize with the CdCl2 structure type (space group ​= ​R 3 ¯ m) and a random distribution of the Ag+ and Sb3+/Bi3+ cations. Hypothetical cation ordered structures have been generated and their stabilities evaluated with density functional theory. This analysis suggests that while charge balanced layers are strongly favored, the long-range interactions that favor ordered stacking of layers are too weak to produce three-dimensional cation ordering. The colors of these materials change noticeably when heated, signaling a large thermochromic effect. For example, the band gap of AgSb0·25Bi0·75I4 changes from 1.68 ​eV ​at room temperature to 1.55 ​eV ​at 150 ​°C. This thermochromic shift in band gap is larger than comparable diamond-like semiconductors, such as CdTe. VASP calculations and molecular dynamics simulations indicate the large thermochromic effect is primarily caused by significant electron-phonon coupling.