Structural Transformation of Ca-Arrangements and Carrier Transport Properties in Ca0.33CoO2Epitaxial Films

The electrical properties of Ca 0.33 CoO 2 epitaxial films are investigated as the Ca 2+ -arrangement is transformed. Despite identical Ca-content, annealing the film caused the Ca-arrangement to change from a √3 a × √3 a hexagonal type to a 2 a × √3 a orthorhombic type, which was confirmed by X-ray diffraction and scanning transmission electron microscopy. A hexagonal Ca-arrangement type, Ca 0.33 CoO 2 , exhibited a metallic resistivity as low as 7.5 × 10 -4 Ω·cm and a moderately large thermopower (∼100 µV·K -1 ), whereas the orthorhombic Ca-arrangement type displayed a thermally-activated resistivity ( 10 -2 Ω·cm) with a slightly increase in thermopower, which led to about a one order of magnitude difference in the thermoelectric power factor.