Mechanistic Insight of KBiQ 2 (Q = S, Se) using Panoramic Synthesis towards Synthesis-by-Design

2021 
Solid-state synthesis has historically focused on reactants and end products; however, knowledge of reaction pathways, intermediate phases and their formation may provide mechanistic insight of solid-state reactions. With an increased understanding of reaction progressions, design principles can be deduced, affording more predictive power in materials synthesis. In pursuit of this goal, in-situ powder X-ray diffraction is employed to observe crystalline phase evolution over the course of the reaction, thereby constructing a “panoramic” view of the reaction from beginning to end. We conducted in-situ diffraction studies in the K-Bi-Q (Q = S, Se) system to understand the formation of phases occurring in this system in the course of their reactions. Powder mixtures of K2Q to Bi2Q3 in 1:1 and 1.5:1 ratios were heated to 800°C or 650°C, while simultaneously collecting diffraction data. Three new phases, K3BiS3, β-KBiS2, and β-KBiSe2, were discovered. Panoramic synthesis showed that K3BiQ3 serves an important mechanistic role as a structural intermediate in both chalcogen systems (Q = S, Se) in the path to form the KBiQ2 structure. Thermal analysis and calculations at the Density Functional level (DFT) show that the cation-ordered β-KBiQ2 polymorphs are the thermodynamically stable phase in this compositional space, while Pair Distribution Function (PDF) analysis shows that all α-KBiQ2 and β-KBiQ2 structures have local disorder due to stereochemically active lone pair expression of the bismuth atoms. The formation of the β-KBiQ2 structures, both of which crystallize in the α-NaFeO2 structure type, show a boundary where the structure can be disordered or ordered with regards to the alkali metal and bismuth. A cation radius tolerance for six-coordinate cation site sharing of  ~ 1.3 is proposed. The mechanistic insight the panoramic synthesis technique provides in the K-Bi-Q system is progress towards the overarching goal of synthesis-by-design.
    • Correction
    • Source
    • Cite
    • Save
    • Machine Reading By IdeaReader
    99
    References
    4
    Citations
    NaN
    KQI
    []