Concerning the Structure of Hydrogen Molybdenum Bronze Phase III. A Combined Theoretical−Experimental Study

The structure of hydrogen molybdenum bronze phase III, H x MoO 3 ( x ≈ 5 / 3 ) has been studied on the basis of a combined experimental−theoretical approach. Our study of the average structure allows us to qualify previous contradictory results concerning the octahedral distortions and show no sign of hydrogen occupation of the intralayer sites. A first-principles theoretical study of this average structure leads to a clear understanding of the nature of the band structure as well as the importance and requirements for metal−metal bonding. The nature of the calculated Fermi surface suggests that a 3 b × 2 c superstructure may be a better description for phase III, which would be compatible with powder X-ray diffraction (XRD) results. Since the information available from powder XRD data does not allow a direct determination of the superstructure, the problem has been approached by means of first-principles geometry optimizations and molecular dynamics studies. The main structural features of this superstructure (distribution of Mo−Mo bonds, main distortions of the Mo−O network and hydrogen distribution) are described.