NaMoO₂: a Layered Oxide with Molybdenum Clusters

NaMoO₂ was synthesized as a layered oxide from the reaction between the layered oxide Na_2/3MoO₂ and metal sodium. Its structure was determined from high-resolution powder X-ray diffraction, and it can be described as an α-NaFeO₂ distorted structure in which sodium ions and molybdenum atoms occupy octahedral interstitial sites. Chains of “diamond-like” clusters of molybdenum were evidenced in the [MoO₂] layers resulting from the Peierls distortion expected in a two-dimensional triangular lattice formed by transition metal atoms with a d³ electronic configuration. Molybdenum–molybdenum distances as short as 2.58 Å were found in these clusters. The magnetic moment recorded at low temperatures and at room temperature showed that NaMoO₂ presents a very low magnetic susceptibility compatible with the localization of the 4d electrons in the Mo–Mo bonds. This localization was confirmed by DFT calculation that showed the NaMoO₂ was diamagnetic at 0 K. A sodium battery was built using NaMoO₂ as the positive electrode material, and we found that sodium ions can be reversibly deintercalated and intercalated in NaMoO₂, indicating that this compound is one of the many phases existing in the Na_xMoO₂ system.