Charge ordering and elastic anomalies in NaxCoO2

We report in-plane resistivity and elastic constant C33 measurements on the Na0.8CoO2 and Na0.5CoO2 systems. An ordering transition is found at T0=280 K for Na0.8CoO2. The temperature dependence of the elastic constant C33 propagating perpendicular to the CoO2 layers is interpreted, in a phenomenological approach, as being due to the anharmonicity of atomic vibrations. The effective Grüneisen parameter γeff deduced directly from the temperature dependence of C33 exhibits important changes at the charge ordering, magnetic and metal-insulator phase transitions.