Experimental protocols for the assessment of redox thermodynamics of nonstoichiometric oxides: A case study of YMnO_3-δ

The thermodynamics of reduction in variable valence oxides are important in a vast number of fields in which point defects, and in particular oxygen vacancies, control material functionality. Here, we present, by way of an example measurement of the material YMnO_3-δ, for which the vacancy formation energy has not been previously reported, best practices for material characterization. Sample mass is recorded by thermogravimetric analysis at temperatures from 600 to 1500°C under five different gas atmospheres, with oxygen partial pressure ranging from 0.081 to 7.66 × 10^–5 atm. Because YMnO_3-δ displays relatively small nonstoichiometry, less than 0.09, over the range of conditions examined, a large sample was required to record the modest mass changes. In the more oxidizing conditions, equilibrium behavior was recorded using a finite heating rate of 10°C min^–1. In the more reducing conditions, absolute mass changes with temperature were large such that the evolved oxygen increased the oxygen partial pressure in the sample vicinity, precluding equilibration under finite heating. Accordingly, a temperature-stepped protocol with long isothermal holds was employed. Analysis of the results from these stepped measurements required interpolation, which was carried out on the ln(δ) versus 1/T plane. In the nonpolar, centrosymmetric phase (P6₃/mcm and δ ≥ 0.016), the enthalpy of reduction was found to be 304 ± 4 kJ (mol-O)^–1, as averaged over the δ range 0.020–0.035. While YMnO_3-δ is known to incorporate oxygen excess in its ambient temperature, polar phase (P6₃cm), the conditions leading to such behavior were not included in this study.