Hybrid Bismuth(III)-Halide Double Perovskite-Derived Ferroelastic (Pip)₂[KBiBr₆] with Excitonic and Bi(III) Luminescence due to Electronic Confinement along Inorganic Pillars

Organic–inorganic halide double perovskites with the A2MM′X6 composition are considered a more stable and environmentally friendly alternative to lead-based APbX3 compounds (3D HOIPs). Herein, we report the synthesis, crystal structure, physicochemical characterization, and the results of density functional theory (DFT) calculations for a double-perovskite-related (piperidinium)2[KBiBr6]. Inorganic structure is built of 1D inorganic pillars composed of Bi(III)Br6 octahedra face shared with trigonal KBr6 antiprisms. It undergoes a room-temperature order–disorder phase transition at Tc = 300/303 K (cooling/heating), associated with molecular dynamics and formation of stable hydrogen-bond interactions. Dielectric relaxation in the vicinity of Tc indicates gradual ordering of piperidinium. The C2/m to P1− symmetry reduction leads to the formation of a switchable ferroelastic domain structure. The material shows purplish-blue photoluminescence from high-energy excitons and Bi(III) emission due to the electronic confinement along the inorganic pillars. DFT calculations of the density of states confirm that the electronic properties are governed by the electronic states of Bi(III)Br6 octahedra and reveal that electron and hole migration occur between neighboring chains, being quenched along the inorganic part. The optical band gap Eg is 2.8 eV.