Thermoelectric properties and their relation to crystal structure of rare earth (RE = La, Nd, Sm and Gd)-doped SrO(SrTiO3)2 Ruddelsden-Popper phase

We herein report the significant relationship between the thermoelectric properties and the crystallographic characteristics, especially the local symmetry of TiO₆ octahedra, in the n = 2 Ruddlesden-Popper (RP)-type phases of 5 at.%-rare earth (RE³⁺ = La³⁺, Nd³⁺, Sm³⁺ and Gd³⁺)-doped SrO(SrTiO₃)₂. It was observed that doped RE³⁺ ions occupied preferentially the Sr-site (C.N. = 9) in SrO layer, which had a favorable restoration effect on the distorted TiO₆ octahedra which was found more effective in smaller-sized RE³⁺-doped compounds, as partially proven by the increase in O3-Ti-O3 bond angle with decreasing in ionic radius of RE³⁺. The Sm³⁺ and Gd³⁺-doped SrO(SrTiO₃)₂ exhibited larger Seebeck coefficient (|S|) than those of La³⁺ and Nd³⁺-doped SrO(SrTiO₃)₂ over the whole measured temperature range (300~1000 K). This is considered to be due to the enhancement of the density of states (DOS) in Ti 3d orbitals originated from improved symmetry in TiO₆ octahedra by smaller RE³⁺ doping.