Phase separation, effects of magnetic field and high pressure on charge ordering in y-Na0.5CoO2

Transmission electron microscopy observations reveal the presence of complex superstructures and remarkable phase separation in association with Na-ordering phenomenon in y-Na0.5CoO2. Resistivity and magnetization measurements indicate that three phase transitions at the temperatures of 25 K, 53 K and 90 K respectively appear commonly in y-Na0.5CoO2 samples. Under a high pressure up to 10 Kbar, the low-temperature transport properties show certain changes below the charge order transition; under an applied magnetic field of 7 Tesla, phase transitions at around 25 K and 53 K, proposed fundamentally in connection with alternations of magnetic structure and charge ordering maintain almost unchanged.