Structure of Ce_1-xSn_xO₂ and its relation to oxygen storage property from first-principles analysis

Abstract

CeO₂-SnO₂ solid solution has been reported to possess high oxygen storage/release property which possibly originates from local structural distortion. We have performed first-principles based density functional calculations of Ce_1-xSn_xO₂ structure (x = 0, 0.25, 0.5, 1) to understand its structural stability in fluorite in comparison to rutile structure of the other end-member SnO₂, and studied the local structural distortion induced by the dopant Sn ion. Analysis of relative energies of fluorite and rutile phases of CeO₂, SnO₂, and Ce_1-xSn_xO₂ indicates that fluorite structure is the most stable for Ce_1-xSn_xO₂ solid solution. An analysis of local structural distortions reflected in phonon dispersion show that SnO₂ in fluorite structure is highly unstable while CeO₂ in rutile structure is only weakly unstable. Thus, Sn in Ce_1-xSn_xO₂-fluorite structure is associated with high local structural distortion whereas Ce in Ce_1-xSn_xO₂-rutile structure, if formed, will show only marginal local distortion. Determination of M-O (M = Ce or Sn) bond lengths and analysis of Born effective charges for the optimized structure of Ce_1-xSn_xO₂ show that local coordination of these cations changes from ideal eightfold coordination expected of fluorite lattice to 4+4 coordination, leading to generation of long and short Ce-O and Sn-O bonds in the doped structure. Bond valence analyses for all ions show the presence of oxygen with bond valence ~1.84. These weakly bonded oxygen ions are relevant for enhanced oxygen storage/release properties observed in Ce_1-xSn_xO₂ solid solution.