Rb₃InCl₆: A Monoclinic Double Perovskite Derivative with Bright Sb³⁺-Activated Photoluminescence

Here, we present the synthesis and crystal structure of Rb₃InCl₆ prepared from air stable reagents via a two-step process that proceeds through the intermediate Rb₂InCl₅·H₂O. Rb₃InCl₆ crystallizes with the Rb₃YCl₆ structure type (C2/c), which can be derived from the double perovskite structure by noncooperative tilting of isolated [InCl₆]^3- octahedra. Despite this lowering of symmetry, the optical properties are similar to the cubic double perovskite Cs₂NaInCl₆. Partial substitution of In³⁺ with Sb³⁺ in Rb₃InCl₆ results in intense cyan-green photoluminescence originating from localized 5s² to 5s¹5p¹ electronic transitions of [SbCl₆]^3- polyatomic anions. In comparison with the cubic double perovskite phosphor Cs₂NaInCl₆:Sb³⁺, the octahedral tilting distortion increases the electronic isolation of the In/Sb-centered octahedra thus facilitating electron and hole localization on Sb³⁺ sites, leading to bright photoluminescence. The distorted crystal structure also leads to a larger Stokes shift (1.29 eV) and a corresponding red shift of the emission peak (λ_max = 522 nm) compared to the more symmetric Cs₂NaInCl₆:Sb³⁺ (Stokes shift ≈ 0.94 eV, λ_max = 445 nm).