  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.
