Molecular Analogues of the Structure and Spin Vector Ordering of LnMnO₃ Manganite Perovskites

Ashlyn R. Hale, Cédric Pécou, Khalil A. Abboud, George Christou

Department of Chemistry, University of Florida, Gainesville, FL 32611, USA

Molecular perspectives of vital homo- and heterometallic metal oxides have proven to be quite valuable. Utilizing bottom-up synthetic methodologies, our group has successfully generated a versatile library of ‘molecular metal oxide nanoparticles’ (MNPs), which are monodisperse metal-oxo clusters that mimic the structure of the parent oxide as confirmed by single-crystal X-ray crystallography. Furthermore, the true solubility of these materials enables their complete characterization via various solid-state and solution-phase techniques, allowing for atomically-precise interpretation of structure-property relationships unknown or ambiguous in the bulk and nanomaterials. Our first forays focused on cerium dioxide (CeO₂) with the fluorite structure and AMnO₃ manganites with the perovskite structure. The former is of widespread importance for industrial, environmental, and biomedical catalysis, while the latter offer ferromagnetic, ferroelectric, and multiferroic properties relevant to new technologies.

Since our initial discoveries of the largest CeO₂ MNP at the time (Ce₄₀) and the first molecular analogue of the CeMnO₃ perovskite repeating unit (Ce₃Mn₈), our MNP program has flourished. To date, we have generated a family of CeO₂ MNPs with varying nuclearities up to 100, while the CeMnO₃ project has seen the development of a family of Ce₃Mn₈ complexes featuring various ligands. The latter was inspired by the rich literature regarding the different spin vector ordering modes in bulk magnetic perovskites and DFT predictions of mode changes by external stimuli such as pressure. Our initial Ce₃Mn₈ cluster exhibited the same ground state spin vector ordering as C-type AF (antiferromagnetic) bulk perovskites with low-lying A-type AF excited states. This suggested that small structural perturbations via changes in the ligation environment might allow us to change the ground state to A-type AF. In this work, synthetic efforts have been extended to further corroborate this hypothesis. In addition, a new Ce/Mn/O complex has been synthesized, showing structural features relevant to both CeO₂ and CeMnO_3, providing further support for our rationalization of the perovskite-like spin vector ordering observed in our molecular mimics.