The Mn²⁺/Mn³⁺state of La_0.7Ce_0.3MnO₃by oxygen reduction and photodoping

Abstract

Films of cerium-doped LaMnO3, which has been intensively discussed as an electron-doped counterpart to hole-doped mixed-valence lanthanum manganites during the past decade, were analyzed by x-ray photoemission spectroscopy with respect to their manganese valence under photoexcitation. The comparative analysis of the Mn 3s exchange splitting of La0.7Ce0.3MnO3 (LCeMO) films in the dark and under illumination clearly shows that both oxygen reduction and illumination are able to decrease the Mn valence towards a mixed 2+/3+ state, independently of the film thickness and the degree of CeO2 segregation. Charge injection from the photoconductive SrTiO3 substrate into the Mn eg band with carrier lifetimes in the range of tens of seconds and intrinsic generation of electron-hole pairs within the films are discussed as two possible sources of the Mn valence shift and the subsequent electron doping.

Quantifying the electrical transport characteristics of electron-doped La0.7Ce0.3MnO3 thin films through hopping energies, Mn valence, and carrier localization length

“…Between 300 K and the plateau temperature the APH model gives hopping energies of 140 meV and 165 meV for B and C, respectively, which are higher than in the as-prepared cases and correspond to Mn valences of 3.18 (sample B) and 3.14 (sample C). These values are in good agreement with those derived from previous XPS results [17]. For sample D the same hopping energy as in the as-prepared case (144 meV) is extracted from the APH model, while the Mn valence from the present transport data (3.17) is lower than the value from XPS (3.31).…”

“…a mixed Mn 2+/3+ valence, this would result in Mn valences of 2.68 or 2.70, which is -in contrast to the cases of samples B and C -lower than the XPS-derived value [17] of 2.83. A possible explanation might be that the high defect density in sample D supports a more uniform oxygen diffusion during the annealing process and not a reduction limited to the surface as for B and C. For temperatures lower than 260 K the APH model fails to fit the experimental data, while, as shown in fig.…”

“…(12) these hopping energies would be equivalent to a Mn 3+ concentration of 96% for C, which is clearly different from the 81% from the former XPS investigation [17], while for B we get an unphysically high Mn 3+ concentration of 105%, which causes some doubts on the validity of eq. (12), at least for the case of the highly reduced LCeMO films.…”

“…As in our previous works on LCeMO [14,17], which focused on the investigation of the Mn valence by photoemission spectroscopy, the samples were investigated in different states of oxygen content:…”

“…[8,14,17], the transport measurements of the as-prepared samples were compared to the results of a 10-nm LCeMO thin film, referred to as sample A, which was already subject of these former studies.…”