Novel high-temperature phase and crystal structure evolution of CsCuBr3 halide identified by neutron powder diffraction

Abstract

The search for novel lead-free materials with potential optoelectronic applications is a main research topic nowadays for an environment-friendly energy transition. Based on promising all-inorganic CsPbBr₃ perovskite, we tested here the substitution of toxic Pb²⁺ with Cu²⁺ and explored the understudied CsCuBr₃ halide. We present a mechanically-synthesized CsCuBr₃ specimen obtained by ball milling, consisting of a well-crystallized and pure sample. Calorimetric measurements revealed a thermal event suggesting a structural phase transition around 422 K, immediately below the decomposition of the sample due to Br loss. A detailed structural analysis was carried out using neutron powder diffraction data from 20 to 420 K, focusing on the evolution of the orthorhombic phase (space-group: C222₁) up to 400 K; this crystal arrangement consists of dimer units of face-sharing [CuBr₆] octahedra containing Cu–Cu dimers with conspicuously short distances that account for the magnetic coupling between Cu²⁺ spins described before. Additionally, at 420 K, we identified a novel high-temperature phase as described in the hexagonal P6₅22 space group, where infinite chains of [CuBr₄] square-planar units were observed. An additional study considering the Debye model was carried out, providing information on the relative Cs–Br and Cu–Br chemical bonds. From diffuse reflectance UV-Vis measurements, an optical gap of ∼1.8 eV was estimated for the orthorhombic phase at room temperature. Magnetic measurements indicate that the effective magnetization behavior versus temperature appears close to linear and seems to extrapolate to zero (smaller than 10⁻⁵ μ_B/Cu atom) at temperatures above 420 K, coinciding with onset of phase transition.