Diamond-Shaped Rb₃Sb₂I₉ Microcrystals: Structural and Optical Properties

Diamond-shaped, several tens of micrometer-sized flat single crystals of Rb3Sb2I9 grown with (006) facets are studied for their structural and optical properties. These microcrystals are obtained through the facile solution process that yields a large fraction of these micron-sized single crystals. The crystals typically have areas around 500 μm2 and thicknesses ranging from 2 to 3 μm. X-ray diffraction shows only intense (006) and (004) reflections, indicating highly oriented crystals grown through solution processing. Thermogravimetry analyses show thermal stability up to 250 °C and characteristic dissociation of Rb3Sb2I9. An estimated band gap of 2.04 eV from diffuse reflectance spectra further validates the phase purity of these microcrystals confirmed from other structural characterization. These crystals exhibit exotic properties, including an anomalous phase transition and Fabry–Pérot like resonating cavity behavior. Temperature-dependent Raman spectral studies and self-trapped excitonic photoluminescence studies prompt phase transition ∼213 K, which coincide with an anomaly noted in the dielectric response. Highly reflective surfaces of the Rb3Sb2I9 microcrystals act as mirror planes and lead to Fabry–Pérot resonating conditions for the STE emissions, which manifest as a broad STE PL emission superposed with intensity modulations. Further, Rb3Sb2I9 exhibits strong electron–phonon coupling with Huang–Rhys parameter S = 48. The diamond-shaped flat microcrystals of Rb3Sb2I9 are promising for various optoelectronic properties.