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

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 single-crystal 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.