Memory and pressure studies in Na_xCoO₂ cobaltites

We present a detailed study on the memory effect results in Na _0.55CoO₂ single crystals. We analyze the temperature dependence of the nonvolatile current-pulse-induced resistance memory state. These results allow us to have more insight in the mobility of Na⁺ ions induced by current and their effect on the memory effect. We also developed X-ray diffraction studies under pressure at ambient temperature in the N _a0.5CoO₂ powder compound. An orthorhombic to hexagonal phase transition was observed at 9GPa. This transition can be explained taking into account the Na ions displacement between two allowed positions. These structural results allow us to confirm that the non-volatile resistive commutation can be interpreted by the displacement of the Na ions induced by the current pulses.