Chemical, magnetic and electronic properties of NaxCoO2 and related compounds

Resumen

In the last years, NaxCoO2 has experienced a renewed interest in the field of solid state science. However, NaxCoO2 is not a novel material, it has been extensively studied for decades. In the 80´s, it was investigated due to its electrochemical properties (high ionic mobility, high electrical conduction) and tested as a cathod in reversible alkaline cells, as its analogous LixCoO2. In the 90’s, its thermoelectric properties raised the interest of this material for energy harvesting at high temperature and refrigeration. However, the discovery of novel properties (i.e. high thermoelectric power or, mainly, the occurrence of superconductivity below 5K) and their possible relationship with similar phenomena found in other materials boosted a renewed interest in this highly electronic correlated system. The structural, magnetic and electronic properties of NaxCoO2 have been studied in detail in order to understand deeply some of the most fundamental aspects which drive the chemical and physical behaviour of this system: presence of oxygen vacants, the role of water played in the occurrence of superconductivity, the proximity of the system to a quantum phase transition or the nature of the unconventional thermoelectric and magnetic properties with x, specifically at the half-doped x=0.5. All of them are kept under strong scientific discussions that, far from solving them, contribute to generate an even higher controversy. On other hand, the efficiency of topotactic reactions in order to exchange Na+ ions by other mono- or di-valent ions, such as Li+, Ca2+ or Sr2+, has been studied. The physical properties of the resulting compounds are shown and compared to those ones from the analogous NaxCoO2 precursor. Therefore, in the pages inside, the reader will can find our main results and conclusions achieved in each one of these subjects, in a modest attempt to explain the chemistry and physics involved in NaxCoO2 and related compounds.