Magnetic and thermoelectric properties of layered LixNayCoO2

The magnetic, thermoelectric, and structural properties of LixNayCoO2, prepared by intercalation and deintercalation chemistry from the thermodynamically stable phase Li0.41Na0.31CoO2, which has an alternating Li/Na sequence along the c-axis, are reported. For the high Li-Na content phases Li0.41Na0.31CoO2 and Li0.40Na0.43CoO2, a sudden increase in susceptibility is seen below 50 K, whereas for Li0.21Na0.14CoO2 an antiferromagnetic-like transition is seen at 10 K, in spite of a change from dominantly antiferromagnetic to dominantly ferromagnetic interactions with decreasing alkali content. The Curie constant decreases linearly with decreasing alkali content, at the same time the temperature-independent contribution to the susceptibility increases, indicating that as the Co becomes more oxidized the electronic states become more delocalized. Consistent with this observation, the low alkali containing phases have metallic-like resistivities. The 300 K thermopowers fall between 30 μV/K (x+y=0.31) and 150 μV/K (x+y=0.83).