The Origin of Enhanced High Temperature Electron Transport in Thermoelectric Ca$_{3}$Co$_{4}$O$_{9}$

Temperature dependent measurements of resistivity, crystal structure and heat capacity reveal significant hysteresis occurring near 400 K in Ca$_{3}$Co$_{4}$O$_{9}$. The largest changes in structure occur in the CoO$_{2}$ layer associated with electron transport: manifested mainly by $b_{2}$ axis changes. Application of magnetic fields up to 8 T reduces the area of the resistivity hysteresis loop with saturation at $\sim $4 T. Reduced resistivity associated with this first order phase transition from metallic to semiconducting behavior enhances the thermoelectric properties at high temperatures and points to the metal-insulator transition as a mechanism for improved ZT in high temperature thermoelectric oxide. This work is supported by DOE Grant DE-FG02-07ER46402. The Physical Properties Measurements System was acquired under NSF MRI Grant DMR-0923032 (ARRA award).