First-principles density-functional calculations on HCr3O8: An exercise to better understand the ACr3O8(A = alkali metal) family

Accurate ab initio density-functional calculations are performed to understand structural stability, electronic structure, and magnetic properties of the ACr3O8(A = H, Li, Na, K, Rb, Cs) series. The ground-state structures for the compounds with A = Li−Rb take the same KCr3O8-type atomic arrangement (space group C2/m), whereas CsCr3O8adopts a modified atomic architecture (prototype; space group Pnma), in agreement with available experimental findings. The hypothetical compound HCr3O8is found to stabilize in an LiV3O8-type structure (space group P21/m), with an unexpectedly large equilibrium volume. The electronic structures of the ACr3O8compounds are analyzed using density-of-states, charge-density, and electron-localization-function plots, and all are found to exhibit semiconducting (insulating at 0 K) properties with very narrow band gaps. The Cr atoms occur in two different valence states and all compounds (except NaCr3O8) are found to exhibit antiferromagnetic ordering of magnetic moments at 0 K.