In Situ Neutron Diffraction Studies of the Flux Crystal Growth of the Reduced Molybdates La₄Mo₂O₁₁ and Ce₄Mo₂O₁₁: Revealing Unexpected Mixed-Valent Transient Intermediates and Determining the Sequence of Events during Crystal Growth

Abstract

Exploratory flux-mediated crystal growth is one route to discovering novel inorganic compounds, including metastable ones; however, the method does not reveal the existence of intermediates or transient species, as only the final products are isolated at the end of the reaction. Uncovering the presence of any potential intermediates or transient species is, thus, of interest and can be achieved via in situ X-ray or neutron diffraction crystal growth studies to observe, in real time, the processes occurring, including phase formation and crystal growth. In this study we investigated the processes involved in the crystal growth of La 4 Mo 2 O 11 and Ce 4 Mo 2 O 11 , two reduced Mo 5+ -containing oxides synthesized in a NaCl/CsCl eutectic flux, and studied via in situ diffraction experiments at the POWGEN beamline at the Spallation Neutron Source. These in situ studies reveal the appearance and disappearance of staring materials, intermediates, and final products. Several transient phases that formed as either single crystals or polycrystalline powders were identified and the reaction conditions (temperature and time) for their formation in single-crystals form established, enabling the use of quenching experiments to isolate these singlecrystal transient species and to carry out their structural characterization. Additionally, by changing the reactants-to-flux ratio, different reaction pathways were observed to ultimately result in the same target phase. A higher flux-to-reagent ratio was shown to favor the formation of reduced molybdenum transient phases.

Size-Driven Stability of Lanthanide Thiophosphates Grown from an Iodide Flux

“…The presence of a reduced phosphorus species among the products, i.e., P 2 S 6 4– in Cs 4 La 2 (P 2 S 6 )­(PS 4 ) 2 (which is isostructural with Cs 4 Gd 2 (P 2 Se 6 )­(PSe 4 ) 2 and will be reported elsewhere), is an indication of phosphorus­(V) to phosphorus­(V) reduction even in the absence of iodide anions. The qualitative difference in the products that resulted from the two different fluxes reveals the influence of the flux on product formation, and highlights the importance of in situ flux crystal growth experiments for better understanding of the processes occurring during these reactions …”

New Compounds and Phase Selection of Nickel Sulfides via Oxidation State Control in Molten Hydroxides

“…In an effort to pursue a more targeted and rational approach toward the synthesis of new compounds, panoramic reactions using in situ diffraction techniques to monitor the dissolution, intermediates, and products in a flux reaction have provided great insight into reaction mechanisms with different fluxes. − For example, Shoemaker et al discovered that varying the [S] in the polysulfide flux of K x S y could lead to a change in the CuS 4 motifs in the local coordination . To achieve the goal of rational material synthesis with the knowledge gained from panoramic synthesis, a tunable flux is required to shift the reaction paradigm toward targeted compounds or structures and away from known compounds.…”

Investigation of Metastable Low Dimensional Halometallates

“…There are many ways to think about metastable materials and exactly what that term implies about the nature of a material, its composition, its structure and its stability. One way is to consider metastable materials as structures and compositions that form in a local energy minimum and that are not the most thermodynamically stable phase that exists at the global energy minimum [ 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ]. Synthesizing such metastable materials therefore requires reaction conditions that kinetically result in the formation of a metastable phase and that are mild enough, often by being carried out at low temperatures, to avoid sliding into the global energy minimum.…”