Superatomic crystal emerging in transition-metal oxides: Molybdenum hollandite K2Mo8O16

Abstract

Density-functional-theory-based electronic structure calculations are carried out to elucidate the origins of the observed electronic properties of molybdenum hollandite K${}_{2}$Mo${}_{8}$O${}_{16}$. We find that the Mo${}_{4}$ cluster in the double Mo chains behaves as a ``superatom,'' a hypothetical big atom with a single composite molecular orbital, and that the system can be regarded as a solid of the superatoms condensed into a simple monoclinic structure with four superatoms per unit cell, thereby yielding four energy bands near the Fermi level at half filling. Based on an effective model proposed, we argue that K${}_{2}$Mo${}_{8}$O${}_{16}$ is a Mott insulator with one electron per superatom, which exhibits strongly frustrated antiferromagnetic spin correlations in the superatomic crystal.