High-Temperature Thermoelectric Property of Layered La2-2xCa1+2xMn2O7 Manganites ($0.75 \leq x \leq 1.0$)

La 2-2x Ca 1+2x Mn 2 O 7 manganites with a doping concentration range of $0.75 \leq x \leq 1.0$ have been investigated to evaluate their thermoelectric properties in the high-temperature range from 400 to 1200 K. The materials, which are well known as $n = 2$ Ruddlesden--Popper compounds, have been confirmed as n-type semiconductors from the observation of the temperature dependence of the electrical conductivity and Seebeck coefficients. The electrical conductivity increases with increasing La concentration, suggesting hopping conduction. The thermal conductivity of the materials examined in this study is less than 1.0 W$\cdot$m -1 $\cdot$K -1 at room temperature. The dimensionless figure of merit ($ZT$) was drastically enhanced by La doping, and had the value of 0.018 at 1173 K for La 0.05 Ca 2.95 Mn 2 O 7 . This behavior is similar that of the well-known layered structure such as the p-type misfit-layered Ca 3 Co 4 O 9 where high Seebeck coefficient and low thermal conductivity.