  We predict high thermoelectric efficiency in the layered perovskite
La$_2$Ti$_2$O$_7$, based on calculations (mostly ab-initio) of the electronic
structure, transport coefficients, and thermal conductivity in a wide
temperature range. The figure of merit $ZT$ computed with a
temperature-dependent relaxation time increases monotonically from just above 1
at room temperature to over 2.5 at 1200 K, at an optimal carrier density of
around 10$^{20}$ cm$^{-3}$. The Seebeck thermopower coefficient is between 200
and 300 $\mu$V/K at optimal doping, but can reach nearly 1 mV/K at low doping.
Much of the potential of this material is due to its lattice thermal
conductivity of order 1 W/(K m); using a model based on ab initio anharmonic
calculations, we interpret this low value as due to effective phonon
confinement within the layered-structure blocks.
