Synthesis of Metastable Inorganic Solids with Extended Structures

Abstract

The number of known inorganic compounds is dramatically less than predicted due to synthetic challenges, which often constrains products to only the thermodynamically most stable compounds. Consequently, a mechanism‐based approach to inorganic solids with designed structures is the holy grail of solid state synthesis. This article discusses a number of synthetic approaches using the concept of an energy landscape, which describes the complex relationship between the energy of different atomic configurations as a function of a variety of parameters such as initial structure, temperature, pressure, and composition. Nucleation limited synthesis approaches with high diffusion rates are contrasted with diffusion limited synthesis approaches. One challenge to the synthesis of new compounds is the inability to accurately predict what structures might be local free energy minima in the free energy landscape. Approaches to this challenge include predicting potentially stable compounds thorough the use of structural homologies and/or theoretical calculations. A second challenge to the synthesis of metastable inorganic solids is developing approaches to move across the energy landscape to a desired local free energy minimum while avoiding deeper free energy minima, such as stable binary compounds, as reaction intermediates. An approach using amorphous intermediates is presented, where local composition can be used to prepare metastable compounds. Designed nanoarchitecture built into a precursor can be preserved at low reaction temperatures and used to direct the reaction to specific structural homologs.

Observing and Modeling the Sequential Pairwise Reactions that Drive Solid‐State Ceramic Synthesis

“…By decomposing the overall phase evolution into a sequence of pairwise reactions, we can calculate the thermodynamics and analyze the kinetics of each reaction step separately, providing a simplified theoretical picture to conceptualize and navigate ceramic synthesis. [ 15,17–20 ]…”

Thermally-driven reactivity of Li0.35La0.55TiO3solid electrolyte with LiCoO2cathode

“…For example, it has been shown that the elements in the ultrathin layer can diffuse into the bulk material at the interface, before nucleation of the most thermodynamically stable phase. 64 Hence, it is possible that what we nd here, as Co-diffusion into LLTO and loss of LCO in the absence of any other secondary phases, is due to such a metastable path, instead of formation of the phases that were predicted by DFT to be thermodynamically stable. It is also possible that, even if such secondary phases formed, they were in too small amounts or not well crystallized, that we could not detect them with XRD up to 600 C. Our results from XRD are in agreement with the observed stability of the LCOjLLTO mixed powders by Liao et al, who also did not observe secondary phase formation till 700 C. 48 Importantly, our ndings in this work show that the LCOjLLTO system undergoes signicant interaction, especially in the form of elemental mixing (of Co in particular) upon thermal treatment, even if no secondary phases were detectable by XRD or XAS up to 700 C.…”

Hydrogenation Reaction Pathways and Crystal Structures of La2H2Se, La2H3Se and La2H4Se

“…In situ characterization techniques offer many opportunities in solid‐state research, such as exploration of crystal structure and properties at non‐ambient conditions [22–24] . Especially time‐resolved studies of chemical reactions involving solids are very beneficial; they unveil reaction pathways and add dynamics to the otherwise often static picture of solid‐state reactions [25,26] . Sapphire single‐crystal based gas‐pressure cells allow for time‐resolved in situ neutron powder diffraction studies under hydrogen (deuterium) gas pressure and elevated temperature.…”