Large in-plane deformation of RuO₆ octahedron and ferromagnetism of bulk SrRuO₃

SrRuO₃ is a ferromagnetic metal with several unusual physical properties such as zero thermal expansion below T_c, so-called Invar behavior. Another anomalous feature is that the a-axis lattice constant is larger than the b-axis lattice constant, a clear deviation from the predictions of the Glazer structural description with rigid RuO₆ octahedron motion. Using high resolution neutron diffraction techniques, we show how these two structural anomalies arise from the irregular in-plane deformation, i.e. plastic behavior of the RuO₆ octahedron, a weak band Jahn-Teller distortion. We further demonstrate that the ferromagnetic instability of SrRuO₃ is related to the temperature-induced localization of Ru 4d bands.