Unusual thermoelectric power of single-crystal ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_2{\mathrm{O}}_7$

Anisotropic resistivity and thermoelectric power of a single crystal of the mixed-valent, bilayered perovskite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_2{\mathrm{O}}_7$ have been measured under hydrostatic pressure $P<~14\hspace{0.5em}\mathrm{kbar}.$ The transport properties measured parallel to an $a-b$ plane can be interpreted with a model of two-manganese Zener polarons, that, on cooling to $T_C,$ progressively condense out into clusters, but are released to form a polaron liquid below a $T_0<~T_C.$ Conduction along the $c$ axis can be described by conventional small-polaron transport that is unaltered by clustering in the basal planes.