Local Lattice Distortion Caused by Short Range Charge Ordering in LiMn₂O₄

Abstract

We have performed powder neutron diffraction on 7 Li-enriched sample of LiMn 2 O 4 at 300 K. The crystal structure determined by Rietveld analysis is a cubic spinel with space group of Fd " 3m in which all Mn atoms are crystallograghically equivalent, consistent with many preceding studies. However, the atomic pair distribution function (PDF) of this compound can not be fitted by the cubic structure with space group of Fd " 3m satisfactorily, and it can be reproduced by the orthorhombic structure with Fddd. It corresponds with the structure of charge ordered phase below about 260 K, indicating a short range charge ordering. In the local structure determined by PDF analysis, two types of MnO 6 octahedra with long and short atomic distances between Mn and O atoms exist and their Mn-O distances are almost consistent with the distances in the charge ordered phase. From these results, valence electrons are localized at Mn sites like a glass even in the cubic phase, resulting in the non-metallic electrical conductivity.

Lattice‐Cell Orientation Disorder in Complex Spinel Oxides

“…The invariable i‐1 pair corresponds to the cation‐oxygen bonds, which suggests that they are the unchanged structural units in the lattice cell and not responsible for the evolution of the global lattice parameters (see Figure S2, Supporting Information). Specifically, the constant MnO and NiO bond lengths are not influenced by the varied TM arrangement, which not only refutes the Fdm model but also rules out the formation of Mn 3+ , as the latter should produce a shoulder or broadening at the negative MnO peak due to the larger trivalent ions and a decrease of the TMO population due to the compensatory oxygen vacancy. Therefore, the PDF data do not support the hypothesis of cation rearrangement as a function of the Mn 3+ concentration, although very small amount of Mn 3+ might preexist in the starting material.…”

“…It is noted that the local distortion of MnO 6 and NiO 6 octahedra persists throughout the crystal although Mn 3+ is almost eliminated by Ni substitution. It implies a different picture of the local structure from those arising from the Jahn–Teller distortion of MnO 6 or the Mn 3+ /Mn 4+ charge ordering in LiMn 2 O 4 .…”

“…The resulting global structures are often described as a disordered solid solution, a concept originating from alloy systems, which emphasizes the framework symmetry while ignoring different ions in the lattice sites. By overlooking the precise nature of MO ionic bonds, such a global structure cannot describe the true bonding and the topological and chemical environment of a particular ion, which are referred as local structures …”

Broken translational and rotational symmetries in LiMn1.5Ni0.5O4 spinel

“…Strong interaction between spin and lattice degrees of freedom, and associated charged and/or orbital ordering has also been observed in similar spinel compounds in low temperature range leading to magnetic and/or structural phase transition [52][53]. Nevertheless, our study shows a width, respectively at absolute zero temperature, and ph ph   corresponds to phonon-phonon coupling parameter.…”

Reactive Milling Induced Structure Changes in Phenylphosphonic Acid Functionalized LiMn2O4 Nanocrystals – Synthesis, Rietveld Refinement, and Thermal Stability

“…The models of ideal cubic spinel at room temperature and the orthorhombic structure below 10 °C have also been confirmed by single‐crystal X‐ray diffraction . A combined approach of Rietveld refinement and pair distribution function refinement for neutron diffraction data of LiMn 2 O 4 reveals that the orthorhombic structure is already present at room temperature in the form of a short‐range ordering . Cooling of the sample induces the expected transformation to an orthorhombic state with a long‐range ordering.…”