Large thermoelectric response in a diluted ferroelectric system: Ba0.7Eu0.3Ti1-xNbxO3

Abstract

We investigated the electrical conductivity, thermal conductivity and thermopower as a function of Nb content (x) in Ba0.7Eu0.3Ti1-xNbxO3 (x = 0.001- 0.10) in the temperature range T = 400-2 K. The substitution of Nb destabilizes the ferroelectric insulating ground state of Ba0.7Eu0.3TiO3 and transforms into a paramagnetic metal for x = 0.1. Thermopower is negative in the entire composition range (S = -613 microVolt/K at 400 K for x = 0.001) and its magnitude decreases with increasing Nb content which suggests doping of electrons into empty Ti-3d(t2g) conduction band. In this series, the dimensionless figure of merit (ZT) increases with temperature for all the compositions and the x = 0.03 composition exhibits the maximum ZT (= 0.12 at 400 K). The enhanced value of ZT is primarily due to the low thermal conductivity of samples in this series (~ 0.7 to 1 W/(m.K) at 400 K) compared to other potential high temperature n-type thermoelectric oxides such as carrier doped SrTiO3 and CaMnO3. The low thermal conductivity in our compounds most likely arises from heavy Eu2+ ion and lattice disorder introduced by Nb5+ which scatter phonons effectively.