Low temperature phase transitions and crystal structure of Na0.5CoO2

Specific heat measurements on Na0.5CoO2 show that the transitions observed at 87 and 53 K in the resistivity and magnetic susceptibility are accompanied by changes in entropy, whereas the one near 20 K is not. Electron diffraction studies suggest that the 87 K transition has a structural component. The crystal structure of Na0.5CoO2, determined by powder neutron diffraction, consists of layers of edge-shared CoO6 octahedra in a triangular lattice, with Na ions occupying ordered positions in the interleaving planes. The Na ions form one-dimensional zigzag chains. Two types of Co ions are also found in chains.