Oxygen Nonstoichiometry in (Ca₂CoO₃)_0.62(CoO₂): A Combined Experimental and Computational Study

Abstract

The oxygen nonstoichiometry in the misfit calcium cobaltite (Ca 2 CoO 3 ) 0.62 (CoO 2 ) has been studied experimentally and by density functional theory (DFT) calculations. The standard oxidation enthalpy ΔH 0 Ox of oxygen deficient (Ca 2 CoO 3 ) 0.62 (CoO 2 ) was measured directly using simultaneous thermogravimetry and differential scanning calorimetry. ΔH 0 Ox was found to be in agreement with the prediction from a previously published defect chemical model based on purely thermogravimetrical analysis. A series of samples with different oxygen vacancy concentration was prepared by annealing in air, followed by rapid quenching. Room-temperature Raman spectroscopy showed a sharp mode at 700 cm −1 decreasing in intensity with increasing vacancy concentration. We discuss this observation as evidence for oxygen vacancies being preferably formed within the central layer of the Ca 2 CoO 3 subsystem. DFT calculations demonstrated that the calculated electronic structure is sensitive to the chosen model of the crystal structure. Still, for all investigated models, the standard formation enthalpy of oxygen vacancies within the Ca 2 CoO 3 moiety was much lower than that for a site within the CoO 2 layer, in agreement with the presented experimental data.

Chemical tracer diffusion of Sr and Co in polycrystalline Ca-deficient CaMnO3−δwith CaMn2O4precipitates

“…Similarly, we propose direct p-n junctions between oxide thermoelectric materials to develop high-temperature TEGs without the use of metallic interconnects. Cobalt-based oxide thermoelectric materials exhibit the best p-type properties comprising, for example, the misfit layered oxide Ca 3 Co 3.92 O 9-δ (CCO) 4,5 . Among the few n-type oxide-based thermoelectric materials we find the perovskite-structured CaMnO 3-δ (CM) 6 .…”

Hall effect measurements on thermoelectric Ca3Co4O9: On how to determine the charge carrier concentration in strongly correlated misfit cobaltites

“…This result indicates that the carrier mobility within the COL is indeed relatively independent of the interlayer strain between the RSL and COL, which is known to change systematically with the oxygen vacancy concentration. 37 Thereby, our simple discussion of different doping studies is justified.…”

“…3). Oxygen vacancies are formed mainly on oxygen sites within the RSL, 8,37 while charge transport is assumed to occur within the COL. Hence, charge carriers within the COL are not scattered efficiently by the oxygen defects, leaving the mobility effectively unchanged upon oxygen vacancy formation, as observed experimentally.…”

“…7,8,35,36 In contrast, the COL is stable towards oxygen nonstoichiometry. 8,37 By the controlled formation of oxygen vacancies, it is therefore possible to vary the carrier concentration in CCO via Eq. (1) in a significant range.…”

Influence of a mechano-chemical treatment on the synthesis and characteristics of p-type thermoelectric Ca 3 Co 4 O 9 ceramics

“…The observed differences in S among the samples could be attributed to the complex influences of their microstructural features, including density, grain morphology, grain size and connectivity, the nature of the grain boundaries and also the mechanical stress in the grains, which can all affect the carrier concentration and their mobility [32,54e56]. However, the influence of the oxygen stoichiometry of the Ca 3 Co 4 O 9þd compound, which is sensitive to the preparation conditions, and is known to affect the electrical conductivity and the Seebeck coefficient, could also not be excluded [57,58].…”