Realization of 2-2 elpasolites using atomic resolution STEM and degradation assisted XRD

In the previous decade, a lot of research has been done in the area of halide perovskite materials which has led them to be used in all different types of photonic applications. The low dimension crystals of this structure have also shown a lot of promise in quantum photonic devices. We synthesized perovskite Quantum Dots of composition Cs2PbSnI6 for photonic applications in Near Infra-red emission range. The atomic arrangement of B site cations in this composition dictated if the crystal structure formed was a double perovskite (elpasolite) or not. All the elpasolites reported in literature have different oxidation states of the two cations at the B site (+1, +3) for example in Cs2AgBiBr6 , where Ag is +1 and Bi is +3. But in our case, both the B site cations, Pb and Sn, are in +2 state. This increased the chances of having a disordered crystal structure where the B site cations do not follow any order. To verify the crystal structure a few different characterization methods were used, such as X-Ray Diffraction (XRD) or Selected Area Electron Diffraction (SAED) in Transmission Electron Microscope (TEM). But the active perovskite phase, the black phase, was only metastable at room temperature, any action of air, moisture, electron beam and solvent can lead to phase transition to inactive (yellow) phase. Also, the low signal to noise ratio in XRD and EDX led to inconclusive results in nanocrystals as small intensity peaks were not seen. Furthermore, the crystals formed were seen to be cubic in TEM and were always found to land on one of the facets. Since the crystal structure is also cubic, this led to presence of only (100) and (110) reflections and their multiples in both XRD and SAED patterns. The use of solvent also triggered coagulation of nanoparticles and formation of nanowires. Further the use of solvent also led to carbon contamination under electron beam. Due to a combined issue of all the degradation mechanisms, all the measurements were done in air-free conditions. The crystal structures of all possible arrangement of B site cations for both black and yellow phases were simulated using DFT. These structures were then used for Le-Bail refinement and refined with Rietveld refinement. These refined crystal structures were then used for matching with SAED ring pattern of ensemble of quantum dots. XRD was carried out in both air-free and ambient conditions to identify the peaks of interest, which indicated the possibility of ordered structure. Also, atomic resolution STEM images of multiple orientations of the crystals were carried out using high tiltilng holder to confirm the ordered double perovskite crystal structure. The resulting methodology for distinction between ordered and disordered structures can be extended to many more material systems. The possibility of 2-2 elpasolites mentioned in this work would help in understanding the structure and properties of wide range of materials under the umbrella of Perovskites.