Structure of Ce(1-x)Sn(x)O(2) and its relation to oxygen storage property from first-principles analysis

CeO(2)-SnO(2) 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-x)Sn(x)O(2) 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 SnO(2), and studied the local structural distortion induced by the dopant Sn ion. Analysis of relative energies of fluorite and rutile phases of CeO(2), SnO(2), and Ce(1-x)Sn(x)O(2) indicates that fluorite structure is the most stable for Ce(1-x)Sn(x)O(2) solid solution. An analysis of local structural distortions reflected in phonon dispersion show that SnO(2) in fluorite structure is highly unstable while CeO(2) in rutile structure is only weakly unstable. Thus, Sn in Ce(1-x)Sn(x)O(2)-fluorite structure is associated with high local structural distortion whereas Ce in Ce(1-x)Sn(x)O(2)-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-x)Sn(x)O(2) 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 approximately 1.84. These weakly bonded oxygen ions are relevant for enhanced oxygen storage/release properties observed in Ce(1-x)Sn(x)O(2) solid solution.