Pressure-induced structural phase transitions on Na(0.5)CoO(2): a first principles study.

First principles calculations have been carried out to study the pressure effects on the structural, electronic and magnetic properties of Na(0.5)CoO(2). An unexpected Na ion structural transformation has been predicted from our calculations. The main factor for such a transformation has been attributed to the increasing Coulomb repulsion between the Na1 and Co ions with the increase of pressure. The experimentally observed insulator-metal transition has been found to be induced by this structural transformation. Furthermore, due to the strong coupling between Na ordering and Co magnetic ordering, a magnetic phase transition follows after the structural transformation.