The L_2,3 ELNES of Mn (Mn²⁺, Mn³⁺, and Mn⁴⁺) shows multiplet structures of valences representing valence fingerprints, although Mn in some materials, such as sedimentary and surficial Mn minerals, is more likely to be Mn³⁺ and Mn⁴⁺. An isolated 3d transition Mn²⁺ ion has five degenerate 3d orbitals. The splitting of the L₃ edge gives the signature of multiple valence states in mixed valence Mn-bearing minerals. The coordination of Mn is almost always octahedral. Mn materials are often fine grained heterogeneous mixtures containing more than one oxidation state.

In Mn EELS profile, the L₃ and L₂ (L_2,3) white lines correspond to electron excitations from the core states 2p_3/2 and 2p_1/2 to unoccupied 3d atomic orbitals, respectively, as shown in Figure 4345a. The two white lines are separated by ~10–12 eV due to the spin-orbit splitting of the Mn 2p core hole. The energy difference between these two peaks, ΔE(L₂ - L₃)] changes with the valence change in the Mn³⁺ to Mn⁴⁺ range.

Figure 4345a. Schematic diagram of formation of L₃ and L₂ (L_2,3) Mn white lines.

Figure 4345b shows EELS spectrum of LiMn_1.5Ni_0.5O₄ electrode material. The O-K , Mn-L and Ni-L edges are clearly presented.

Figure 4345b. EELS spectrum of LiMn_1.5Ni_0.5O₄ electrode material. Adapted from [6]

Figure 4345c shows the EELS profiles of ramsdellite, pyrolusite, cryptomelane (for different valence states), bixbyite, and manganite materials. The two Mn L_2,3-edges shift to higher energy loss range with the increase of oxidation state. The Mn L₃-edge structure of Mn³⁺ compounds is narrower and more symmetric than the edges for the Mn⁴⁺ valence states. Note that the shoulder of the most intense peak at ~644 eV has not been well-understood yet. Unlike Mn³⁺ compounds, different structural types of MnO₂ (Mn⁴⁺ valence states) produce different widths of the Mn L₃ edge. The main L₃ peak from ramsdellite is apparently narrower than that of pyrolusite [2] because various arrangements of Mn octahedra give rise to different shapes of their EELS spectra [3]. The numbers beside the spectrum give the valence states of each material. The inset (A) in Figure 4345c shows the energy difference (ΔE(L₂-L₃)) between the two peaks of L_2,3-edges, corresponding to the different valence states and indicating a linear trend but with large errors as a result of the Coster-Kronig Auger decay effect on the broadening of Mn L₂ edges. Inset (B) shows the integrated Mn L_2,3 white-line intensity ratios, I(L₃)/I(L₂), as a function of the Mn valence state.

Moreover, Table 4345a shows the energies (eV) of the Mn L₃ and L₂ edge peak maximum of some materials and the separation of the L₃ and L₂ peak maxima because of spin orbit splitting.

Three obvious changes in the Mn spectra with an increasing oxidation state (valence state) of the Mn can occur: Move to higher energy losses of the L₃ peak maximum (See Table 4345a), a decrease in the I(L₃)/I(L₂) white line intensity ratio, and a characteristic shape change of the L_2,3 edges.

Figure 4345d shows L₃ edges taken from hausmannite. The L₃ edges are arranged from top to bottom with an increase in intensity of the Mn³⁺ peak corresponding to an increase in the amount of Mn³⁺.

Figure 4345d. L₃ edges taken from hausmannite. [5]

To minimize the electron-beam-induced chemical reduction of Mn⁴⁺ in Mn-containing minerals [1], each analysis has to be obtained in a low-dose condition using the lowest probe current while still producing a sufficient signal-to-noise ratio. This damage minimization can be done by spreading the electron beam for a low irradiation rate, e.g. smaller than 2 × 10⁴ e/nm²/s. At this condition, the acquisition time for each spectrum can be relatively long, e.g. 8–10 s. If the counts for a single spectrum are not sufficient to produce pronounced sharp peaks, a series of spectra should be summed to improve counting statistics such that the integrated dose for each accumulated spectrum is more than 2 × 10⁶ e/nm².

Table 4345b lists the distribution of Mn valence states from surface layer to bulk of a Li[Mn₂]O₄ (LMO) particle.

[1] Garvie, L.A.J. and Craven, A.J., (1994) Electron-beam-induced reduction of Mn⁴⁺ in manganese oxides as revealed by parallel EELS. Ultramicroscopy, 54, 83–92.
[2] Garvie, L.A.J. and Craven, A.J. (1994) High-resolution parallel electron energyloss spectroscopy of Mn L_2,3-edges in inorganic manganese compounds. Physics and Chemistry of Minerals, 21, 191–206.
[3] Manceau, A., Gorshkov, A.I., and Drits, V.A. (1992) Structural chemistry of Mn, Fe, Co, and Ni in manganese hydrous oxides: Part I. Information from XANES spectroscopy. American Mineralogist, 77, 1133–1143.
[4] Shouliang Zhang, Kenneth J.T. Livi, Anne-Claire Gaillot, Alan T. Stone, and David R. Veblen, Determination of manganese valence states in (Mn³⁺, Mn⁴⁺) minerals by electron energy-loss spectroscopy, American Mineralogist, 95(11-12) 1741-1746.
[5] Laurence A. J. Garvie, Alan J. Craven, Use of electron-energy loss near-edge fine structure in the study of minerals, American Mineralogist, 79, (1994) 411-425.
[6] F. Cosandey, Analysis of Li-Ion Battery Materials by Electron Energy Loss Spectroscopy, Microscopy: Science, Technology, Applications and Education, A. Méndez-Vilas and J. Díaz (Eds.), 1662, (2010).
[7] Charles D. Amos, Manuel A. Roldan, Maria Varela, John B. Goodenough, and Paulo J. Ferreira, Revealing the Reconstructed Surface of Li[Mn₂]O₄, Nano Lett. 2016, 16, 2899−2906.