Thermoelectric properties and their relation to crystal structure of rare earth (RE = La, Nd, Sm and Gd)-doped SrO(SrTiO<inf>3</inf>)<inf>2</inf> Ruddelsden-Popper phase

Abstract

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.