Enthalpy of Formation of Na_xCoO₂ and (Na, Mg)_xCoO₂ Systems: A First Principle Calculation Study

Enthalpy of formation of Na_xCoO₂ (NCO) compounds with 0 < x < 1 and partially substituted Na⁺ ions with Mg²⁺ ions, (Na, Mg)_xCoO₂ (NMCO) has been calculated using full potential-linear augmented plane wave method within density functional theory. The enthalpy of formation of NCO was found to decrease as sodium concentration, x increases. We found that the enthalpy of formation decreases more rapidly in region of 0 < x < 0.55 and slower in 0.55 < x < 1, which is indicating the presence of two different stability region against sodium concentration. The substitution of Mg²⁺ into Na⁺ sites decreases the enthalpy of formation of NCO compound and suggests Mg²⁺ substitution could increase the stability of NCO compound. The Mg²⁺ substitution also affect the lattice parameters. In the electronic structure, Mg²⁺ substitution is found to lower the position of unfilled Co-3d band for about 0.2 eV compared to the NCO compound. The computational results also indicate that the Mg²⁺ substitution will not affect Seebeck coefficient especially at high temperature. (paper)