Structural Stability In Oxygen-deficient Perovskites From First Principles

Perovskite-type oxide（LaM03）can be used as an excellent redox reaction catalyst.Perovskite oxides with oxygen stoichiometry of 3/5 can be reduced easily by loss of lattice oxygen to form oxygen vacancy.Oxygen vacancy will affect the physical and chemical properties of perovskite oxides,and the structural stability limits their actual applications.On the basis of density functional theory calculations,we have analyzed the structural stability of LaMO_3-δ（M = Sc-Cu）with various bulk oxygen vacancies.First,the charge change of the two transition metal ions nearest to the oxygen vacancy is largest during the charge redistribution arising from the oxygen removal.The way of how the two extra electrons leave oxygen and occupy the d orbitals of the transition metal ions influences the formation of the oxygen vacancy.Then,the oxygen vacancy formation process is analyzed in detail.It is found that the formation energy of oxygen vacancy can be decomposed into relaxation energy and bonding energy,and the oxygen vacancy formation energy is determined largely by the latter.In addition,LaMO_3-δ（M = Sc-Cu）peroviskites with oxygen deficiency ranging from 0.125 to 1.25 have been examined to analyze the reducibility and the effect of the formation of oxygen vacancies to their structure stability.According to the calculated oxygen vacancy formation energies,it is apparent that the four LaM03（M = Sc,Ti,V and Cr）structures are hard to reduce because of their high oxygen vacancy formation energies（>4.80 eV）.Two quantities are used to measure the structural stability of perovskites,namely,an abrupt increase in the formation energy of per oxygen vacancy and the high formation energy（more than or close to 4.80 eV）.If the sharp rise in oxygen vacancy formation energy curve is used as the critical value,the stable structures at the maximum possible oxygen deficiencies are LaMnO₂.5,LaFeO₂.5,LaCoO₂.5,LaNiO₂.5,and LaCuO₂.It is found that the variable oxidation states of transition metals in LaMO₃ plays a key role in determining their structural stability.When LaMO₃ is reduced to LaMO₂.5,the transition metal ions is reduced from +3 to +2 to gain 1 e,and it can be reduced to LaMO₂ with the oxidation state decreased from +2 to+1.