Reduction of the Jahn-Teller distortion at the insulator-to-metal transitionin mixed valence manganites

The insulator-to-metal transition in the manganite ${\mathrm{La}}_{0.52}$ ${\mathrm{Y}}_{0.15}$ ${\mathrm{Ca}}_{0.33}$ ${\mathrm{MnO}}_3$ (${\mathrm{T}}_{\mathrm{IM}}$≈115 K) has been studied by high-resolution neutron powder diffraction. The cell volume contraction at the Curie point is accompanied by a remarkable decrease of the Jahn-Teller distortion in ${\mathrm{MnO}}_6$ octahedra. The change of the Mn-O bond lengths at ${\mathrm{T}}_{\mathrm{IM}}$ is anisotropic and brings about a drop out of the basal-plane collective distortion mode ${\mathrm{Q}}_2$, proposed to be the deformation responsible for the band split of ${\mathrm{e}}_{\mathrm{g}\mathrm{\uparrow }}$ orbitals. This is consistent with the double-exchange picture, and precludes simple ferromagnetic exchange.