Defect-Accommodating Intermediates Yield Selective Low-Temperature Synthesis of YMnO₃ Polymorphs

Abstract

In the synthesis of complex oxides, solid-state metathesis provides low-temperature reactions where product selectivity can be achieved through simple changes in precursor composition. The influence of precursor structure, however, is less understood in solid-state synthesis. Here we present the ternary metathesis reaction (LiMnO 2 + YOCl → YMnO 3 + LiCl) to target two yttrium manganese oxide products, hexagonal and orthorhombic YMnO 3 , when starting from three different LiMnO 2 precursors. Using temperature-dependent synchrotron X-ray and neutron diffraction, we identify the relevant intermediates and temperature regimes of reactions along the pathway to YMnO 3. Manganesecontaining intermediates undergo a charge disproportionation into a reduced Mn(II,III) tetragonal spinel and oxidized Mn(III,IV) cubic spinel, which lead to hexagonal and orthorhombic YMnO 3 , respectively. Density functional theory calculations confirm that the presence of Mn(IV) caused by a small concentration of cation vacancies (∼2.2%) in YMnO 3 stabilizes the orthorhombic polymorph over the hexagonal. Reactions over the course of 2 weeks yield o-YMnO 3 as the majority product at temperatures below 600°C, which supports an equilibration of cation defects over time. Controlling the composition and structure of these defect-accommodating intermediates provides new strategies for selective synthesis of complex oxides at low temperatures.

Selectivity in yttrium manganese oxide synthesis via local chemical potentials in hyperdimensional phase space

“…consistent with the predicted thermodynamic connectivity in Figure 3. In contrast, Li-based assisted metathesis reactions proceed via LiMnO 2 -based intermediates that result in the direct formation of YMnO 3 phase, 21,22 which is also predicted from the thermodynamic connectivity (Fig-…”

“…In contrast, LiMnO 2 has a narrower range of stability than its neighboring Li-Mn-O phases, which each prefer distinct non-layered structures (Figure S15b). In both the Li 2 CO 3 assisted metathesis reactions 21 and the direct reaction between LiMnO 2 and YOCl, 22 we observed formation of YMnO 3 and YMn 2 O 5 , suggesting that the decreased range of chemical potentials over which layered LiMnO 2 is stable and lack of structural homology between the Li x MnO 2 phases inhibits the accessibility of Y 2 Mn 2 O 7 (e.g., due to rearrangement of the phases into spinel-derived structures). Finally, in the K 2 CO 3 assisted metathesis reactions, no product selectivity was observed as the reaction produced a mixture of YMnO 3 , YMn 2 O 5 , and Y 2 Mn 2 O 7 products.…”

“…This sodium based reaction differs from the equivalent reactions with Li 2 CO 3 and K 2 CO 3 , which we previously showed instead yield YMnO 3 . 21,22 We attribute the selective formation of Y 2 Mn 2 O 7 to the distance of the Na-based intermediates in chemical potential space, which is shown visually on the chemical potential diagram and is numerically calculated between the stability regions of the reactants and products in a reaction. These methods, applied to easily retrievable materials data, now allow one to search for selective reactions over large, hyperdimensional compositional spaces.…”

nmfMapping: a cloud-based web application for non-negative matrix factorization of powder diffraction and pair distribution function datasets

“…A drawback to NMF is that the optimization is nonconvex and finding the global minimum is not assured. However, in practice proper initialization seems to make NMF results consistent for diffraction and PDF data (Liu et al, 2021;Long et al, 2009) where its application looks very promising (Todd et al, 2020).…”

Validation of non-negative matrix factorization for rapid assessment of large sets of atomic pair distribution function data

“…The appearance and disappearance of reaction components or phase transitions can be followed in real time as the experiment progresses without prior knowledge of what the components will be. Furthermore, chemical information can actually be extracted by applying refinement approaches to the components themselves (Todd et al, 2020).…”