Despite the rapid development and enormous success of organic–inorganic hybrid halide perovskites (AB′X₃), such as CH₃NH₃PbI₃ as absorbers for perovskite-based solar cells (PSCs), the commercial applications of photovoltaic techniques still face several challenges, such as decomposition when exposed to light and moisture, and lead toxicity. On the other hand, the double perovskites (A₂B′B″X₆) are derived from the AB′X₃ when half of the octahedrally coordinated B′-cations are partially replaced by the suitable B″-cations. They are attracting attention due to a new design strategy to replace Pb²⁺ ions with the couple of a monovalent M⁺ ion and a trivalent M³⁺ ion, leading to a new family of quaternary double perovskites. In this way, we aim to synthesize and characterize Cs₂AgSbCl₆ powdered samples, designed for solar cell applications. The crystalline phase and morphological features are investigated by X-ray powder diffraction (XRPD), neutron powder diffraction (NPD), scanning electron microscopy (SEM) in complement with UV–vis spectroscopy, showing a suitable band gap of 2.7 eV. The solution synthesis method proved to be efficient in obtaining polycrystalline-Cs₂AgSbCl₆ samples in a cubic ordered phase. DFT calculations also provided insights on the vibrational properties of Cs₂AgSbCl₆, corroborating the experimental data and elucidating the optical activity of Raman and infrared modes.

Among the hybrid organic–inorganic perovskites MAPbX₃ (MA: methyl-ammonium CH₃–NH₃⁺, X = halogen), the triiodide specimen (MAPbI₃) is still the material of choice for solar energy applications. Although it is able to absorb light above its 1.6 eV bandgap, its poor stability in humid air atmosphere has been a major drawback for its use in solar cells. However, we discovered that this perovskite can be prepared by ball milling in a straightforward way, yielding specimens with a superior stability. This fact allowed us to take atomic-resolution STEM images for the first time, with sufficient quality to unveil microscopic aspects of this material. We demonstrated full Iodine content, which might be related to the enhanced stability, in a more compact PbI₆ framework with reduced unit-cell volume. A structural investigation from neutron powder diffraction (NPD) data of an undeuterated specimen was essential to determine the configuration of the organic MA unit in the 100–298 K temperature range. A phase transition is identified, from the tetragonal structure observed at RT (space group I4/mcm) to an orthorhombic (space group Pnma) phase where the methyl-ammonium organic units are fully localized. Our NPD data reveal that the MA changes are gradual and start before reaching the phase transition. Optoelectronic measurements yield a photocurrent peak at an illumination wavelength of 820 nm, which is redshifted by 30 nm with respect to previously reported measurements on MAPbI₃ perovskites synthesized by crystallization from organic solvents.