Investigation of Magnetic and Structural Transitions in Hybrid Molybdenum(III) 2D Halide Double Perovskites

Magnetic hybrid layered halide double perovskites are a rapidly emerging class of two-dimensional (2D) materials, particularly for their potential in magnetic and magneto-optical applications. This study reports five layered double perovskites: Dion-Jacobson (DJ) phases, (1,4-BDA) 2 M I Mo X 8 ( M I = Na +, Cu +, Ag + ), where 1,4-BDA is 1,4-butanediammonium, and Ruddlesden–Popper (RP) phases, (1-PA) 4 M I MoX 8 ( M I = Na +, Ag + ), where 1-PA is 1-propylammonium. Their optical band gaps are tunable in the 1.88–2.07 eV range, depending on the specific univalent metal cation and halide anion. Magnetic studies reveal paramagnetic behavior at room temperature and antiferromagnetic ordering at 2.6 K–4.6 K, with exchange strengths varying with composition. The DJ phases, in which the 1,4-BDA cation bridges adjacent perovskite layers, exhibit a temperature-driven phase transition attributed to conformational changes in the cation from gauche - anti - gauche to anti - anti - anti . In contrast, the RP phases show no such transition, as the flexible alkyl chain of the 1-PA cation has ample space within the van der Waals gap for thermal motion, preventing rigid structural changes. Furthermore, the DJ phases are remarkably more stable under ambient conditions than the RP phases. These results advance our understanding of structure–property relationships in transition-metal double perovskites, guiding future development of optoelectronic and magneto-optical applications.