Tailorable Magnetic Exchange and Optical Absorption in 1-D Double Perovskite-like Halides (CH3NH3)2NaMoCl(6–x)Brx

Incorporation of open-shell transition-metal ions in halide double perovskite, A₂M^IM^IIIX₆ (where A is a monocation, M^I is a monovalent metal, M^III is a trivalent metal, and X is a halide), can lead to the simultaneous emergence of interesting magnetic and optoelectronic properties for various applications. Mixing different halides at the perovskite X-site adds a new direction in tuning the stability and the optical properties. However, their synthesis is often difficult due to complicated compositional evolution and phase segregation. Here, by incorporating a Mo³⁺ ion (4d³; t_2g³e_g⁰) as the magnetic metal in the M^III site and mixing Cl/Br in the X-site, we report four new one-dimensional hybrid double perovskite-like halides: (MA)₂NaMoCl_(6–x)Br_x (MA = CH₃NH₃⁺; x = 0.0, 1.17, 2.71 and 4.10). These compounds retain alternate ordering of face-sharing [NaCl_(6–x)Br_x] and [MoCl_(6–x)Br_x] polyhedra in infinite one-dimensional chains with the MA cation in the interchain space. We investigate the effect of mixing Cl and Br on optical absorption and magnetic exchange properties using experimental and theoretical methods. Their band gaps are tunable in the 1.98–2.13 eV range, which red shift on increasing the Br content x. The magnetic properties are stimulating. The unpaired electrons of the Mo³⁺ ion are ordered antiferromagnetically with those of the nearest neighbor Mo³⁺ ions at transition temperatures in the ∼5.2–6.8 K range. The Curie–Weiss temperature and the transition temperature increase with increasing Br content x due to an enhanced superexchange effect as a consequence of an increase in the covalency of metal–halide bonds. This work presents facile preparation of phase pure hybrid double perovskite-like mixed halides. Their ability to precisely control the chemical composition, optical properties, and magnetic behavior makes them viable candidates as multifunctional materials.