First-principles calculations of structural and electronic properties of layered AxRhO2 (A = Li, Na, K, Rb, Cs)

Layered transition metal oxides, such as NaxCoO2, are known for their various interesting physical phenomena, which are mainly due to the strong correlation of the transition elements and tunable concentration of alkali metals. Here, we have systematically investigated the structural and electronic properties of 4d layered transition metal oxides AxRhO2 (A = Li, Na, K, Rb, Cs) by first-principles calculations. It is found that when the concentration (x) of alkali ions in AxRhO2 increases, the in-plane lattice constant (a) increases while the out-of-plane one (c) decreases. In the case of stoichiometric ARhO2 (i.e., x = 1), both lattice constants (a and c) increase when the alkali ions changes from Li to Cs. The calculated electronic band structures and density of states indicate that all the stoichiometric ARhO2 compounds are indirect band-gap semiconductors with band gaps ranging from 3 eV to 3.6 eV. Finally, we calculate the Fermi surfaces of KxRhO2 and demonstrate the Lifshitz transition, which could be triggered by potassiation/depotassiation in experiments. Despite the structural similarity between these materials, we have observed the difference in their band structures at the valence band maximum, which will possibly result in a different behavior of the Lifshitz transition. Our calculations point out the similarities and the subtle differences between different alkali rhodates, which give some useful information for future experimental works on these materials.