Oxygen Absorption in Nanocrystalline h-RMnO3 (R = Y, Ho, Dy) and the Effect of Ti Donor Doping

Abstract

Hexagonal manganites, RMnO3 (R = Sc, Y, Ho-Lu), can reversibly store and release large quantities of oxygen at temperatures in the range of 150-400 °C. The oxygen storage properties can be tuned by combining different R3+ cations, aliovalent dopants, and crystallite sizes in the nanometer range. Here, we study the oxygen absorption of nanocrystalline RMn1-xTixO3 (R = Y, Ho, Dy; x = 0, 0.15) using thermogravimetric analysis (TGA) and high-temperature X-ray diffraction (HT-XRD) in O2 and N2 atmospheres. The maximum oxygen storage capacity increases from R = Y through Ho and Dy and even further with Ti4+ as a donor dopant. Density functional theory (DFT) calculations show that the observed trends in oxygen absorption capacity are correlated with the enthalpy of oxidation and the lattice parameters. Ti4+ also increases the thermal stability of absorbed oxygen and thereby extends the operation range to higher temperatures where the absorption and desorption kinetics are faster. Reducing the size of the crystallites improves the oxygen storage capacity as well as the absorption kinetics due to shorter diffusion distances. Finally, a thermodynamic model for the oxidation of RMnO3 is presented and fitted to TGA data and the implications for the microscopic understanding of the oxidation process are discussed.