Charge delocalization and structural response in layered ${\mathrm{La}}_{1.2}$${\mathrm{Sr}}_{1.8}$${\mathrm{Mn}}_2$${\mathrm{O}}_7$: Enhanced distortion in the metallic regime

Temperature-dependent neutron diffraction studies on melt-grown crystals of the layered compound ${\mathrm{La}}_{1.2}$ ${\mathrm{Sr}}_{1.8}$ ${\mathrm{Mn}}_2$ ${\mathrm{O}}_7$ reveal a dramatic structural response to the onset of ferromagnetism and the coincident insulator-metal transition. Unlike the related manganite perovskites, whose Jahn-Teller distorted octahedra become more regular at temperatures below the insulator-metal transition, the ${\mathrm{MnO}}_6$ octahedra of this layered material are more severely distorted when the charge is itinerant than when it is localized.