  SrRuO3 is a ferromagnetic metal with several unusual physical properties such
as zero thermal expansion below Tc, 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.
