  Ferromagnetism and its evolution in the orthorhombic perovskite system
Sr1-xCaxRuO3 have been widely believed to correlate with the structural
distortion. The recent development of high-pressure synthesis of the Ba
substituted Sr1-yBayRuO3 makes it possible to study ferromagnetism over a
broader phase diagram, which includes new orthorhombic Imma and the cubic
phases. However, the chemical substitutions introduce the A-site disorder
effect on Tc, which complicates determination of the relationship between
ferromagnetism and the structural distortion. By clarifying the site disorder
effect on Tc in several new series of ruthenates in which the average bond
length <A-O> keeps the same, but the bond-length variance varies, we are able
to demonstrate a parabolic curve of Tc versus mean bond length <A-O>. A much
higher Tc ~ 177 K than that found in orthorhombic SrRuO3 can be obtained from
the curve at a bond length <A-O> which makes the geometric factor t =
<A-O>/(sqrt(2)<Ru-O>) = 1. This new result reveals not only that the
ferromagnetism in the ruthenates is extremely sensitive to the lattice strain,
but also that it has an important implication for exploring the
structure-property relationship in a broad range of oxides with perovskite or
perovskite-related structure.
