Giant spin-phonon coupling in a 5d NaOsO3*

The coupling of distinct properties offers avenues to multifunctional materials. A limiting factor, however, is the degree that one parameter has to be modified to sufficiently alter the coupled property. Through a neutron scattering and first-principles density functional theory study of the 5d perovskite NaOsO$_3$ we reveal that from only a 0.1\% lattice change an unprecedentedly large coupling emerges. The manifestation is a ``giant'' spin-phonon coupled mode shift of $\Delta \omega$=40 cm$^{-1}$, the largest observed in any material. By identifying the dominant phonon as the octahedral breathing mode we show isosymmetric ordering and cooperation between the lattice and the exotic magnetically driven Slater metal-insulator transition in this material. The occurrence of the dramatic spin-phonon-electronic coupling in NaOsO$_3$ is due to a property common to all 5d materials: the large spatial extent of the 5d ion. Consequently examining 5d materials in a new light offers novel routes for multifunctional devices with enhanced coupled phenomena.