Optical absorption induced by small polaron formation in transition metal oxides: The case ofCo3O4

Abstract

Small polarons (SPs) are known to exist in most important transition metal oxides (TMOs); however, the nature of small polaron formation remains enigmatic, and a fundamental understanding of how SPs impact the intrinsic electronic structure and optical properties of these materials is largely lacking. In this work, we employ first-principles calculations to investigate SP formation in Co3O4, a highly promising material for a wide range of emerging energy applications, and we resolve the conflicting findings that have been reported on the electronic structure of the system. We confirm that the intrinsic band gap of Co3O4 is 1.6 eV, and we show that the formation of hole small polarons significantly influences the optical absorption spectra, leading to a 0.8 eV transition that is often misinterpreted as the band edge that defines the fundamental gap. In addition, we discuss how uniaxial strain can be utilized to probe the Jahn-Teller distortion of SP states and in turn, effect their optical transitions. Beyond Co3O4, our study suggests a general roadmap for establishing a first-principles computational approach that can simultaneously achieve an accurate description of SP states, electronic band structure and optical transitions of polaronic magnetic oxides. * yuanping@ucsc.edu † ogitsu1@llnl.gov arXiv:1909.08653v1 [cond-mat.mtrl-sci]

Ultrafast Formation of Small Polarons and the Optical Gap in CeO2

“…Polarons are classified into two types depending on the spatial extension of the polarization field: large polarons are spread across several lattice unit cells while small polarons have the size of a single or few unit cells. The formation of small polarons has been observed in several TMOs such as Fe 2 O 3 9 , TiO2 10 , NiO 11 , and Co 3 O 4 12 . The textbook case of the small polaron model was proposed to explain electron mobility in partially reduced cerium oxide (CeO 2-x ) 5 .…”

“…The formation of small polarons has been observed in several TMOs such as Fe 2 O 3 , 9 TiO 2 , 10 NiO, 11 and Co 3 O 4 . 12 The textbook case of the small polaron model was proposed to explain electron mobility in partially reduced cerium oxide (CeO 2– x ). 5 In particular, the presence of small polarons in CeO 2 was suggested by the temperature dependence of the conductivity of single crystals and supported using the thermopower-conductivity relation.…”

Electron microscopy and spectroscopic study of structural changes, electronic properties and conductivity in annealed Li$_x$CoO$_2$

“…From previous literature, the prediction of decreasing gaps 54,55 is found to hold for Li x CoO 2 with varying x. In the extreme case of x = 0, pure Co 3 O 4 in the F d 3m phase also has been reported to have a smaller band gap (1.6 eV), 57 , which, however, is related to a small polaron formation. Note that this value of the gap is smaller than ours (1.724 eV) which does not include such polaronic effects.…”

“…On the other hand, as mentioned earlier, there are also indications that on heating the Li concentration may be decreasing in the sample due to out-diffusion. The poorer the system is in Li, the more correlated the electronic structure becomes with increasing polaronic effects 57 and this may also be part of the reason for the conductivity decrease. While for Li x CoO 2 in the R 3m and the closely related C2/m and P 2/m structures, a normal band picture still holds with some p-type hole doping due to x < 1, in the F d 3m and even more so, in the F m 3m phases, the starting picture of a band insulator becomes untenable because of the overlapping of the t 2g and e g bands.…”

Quantum Confinement Effects of Thin Co3O4 Films

“…[30,31], this was attributed to the first optical band gap of Co 3 O 4 . However, recently, it was shown by time-resolved optical spectroscopy measurements [32][33][34] and by rigorous calculations [35], which illustrated that this was due to localized polaron states in the material. Therefore, the first and second direct optical band gaps of Co 3 O 4 are placed at about 1.5 eV and just above 2 eV, where one may see strong increases of the absorbance.…”