Competing Structural Phases in Lead-Free Zero-Dimensional Guanidinium Bismuth Iodide Perovskite

Abstract

Lead-free halogenobismuthates(III) have been attracting attention as nontoxic, stable alternatives to lead halide perovskites, yet their temperature-driven phase behavior is often poorly understood. Here, we present a comprehensive study of the zero-dimensional perovskite-like semiconductor [C(NH2)3]3Bi2I9 (GA3Bi2I9), combining single-crystal X-ray diffraction with calorimetric, dielectric, and light absorption measurements. Five temperature-induced phases have been identified: the high-temperature hexagonal phase I (P63/mmc), the room-temperature orthorhombic phase II (Cmcm), and three low-temperature monoclinic phases III, IV, and V (all of the space group P1121/m) that coexist over an unusually broad temperature range. Throughout, the crystal retains its layered framework of Bi2I93– bioctahedra and guanidinium cations linked by N–H···I hydrogen bonds, while the transitions are driven by cation ordering, reorientations, and displacements, and the concomitant switching of the hydrogen-bonding scheme. Particularly at low temperatures, the subtle modifications in hydrogen bonding and rearrangements of the ions lead to the coexistence and competition of phases whose thermodynamic stability is comparable. Phase transitions in GA3Bi2I9 are highly sensitive to thermal history and internal stress, underscoring the subtlety of structural modifications. The crystal shows negative linear thermal expansion in most phases, distinct dielectric anomalies at the transitions, and a linear narrowing of the optical gap with increasing temperature from 2.36 eV at 80 K to 2.11 eV at 460 K, changing the crystal color from pale orange to almost black.