Rattling Behavior in a Simple Perovskite NaWO₃

Rattling phenomena have been observed in materials characterized by a large cage structure but not in a simple ABO₃-type perovskite because the size mismatch, if it exists, can be relieved by octahedral rotations. Here, we demonstrate that a stoichiometric perovskite oxide NaWO₃, prepared under high pressure, exhibits anharmonic phonon modes associated with low-energy rattling vibrations, leading to suppressed thermal conductivity. The structural analysis and the comparison with the ideal perovskite KWO₃ without rattling behavior reveal that the presence of two crystallographic Na1 (2a) and Na2 (6b) sites in NaWO₃ (space group Im3̄) accompanied by three in-phase WO₆ octahedral (a⁺a⁺a⁺) rotations generates an open space Δ∼0.5 Å for the latter site, which is comparable with those of well-known cage compounds of clathrates and filled skutterudites. The observed rattling in NaWO₃ is distinct from a quadruple perovskite AA′₃B₄O₁₂ (A, A′: transition metals) where the A (2a) site with lower multiplicity is the rattler. The present finding offers a general guide to induce rattling of atoms in pristine ABO₃ perovskites.