  We systematically synthesized perovskite-type oxides Sr1-xCaxCoO3 containing
unusually high valence Co4+ ions by a high pressure technique, and investigated
the effect of systematic lattice change on the magnetic and electronic
properties. As the Ca content x exceeds about 0.6, the structure changes from
cubic to orthorhombic, which is supported by the first-principles calculations
of enthalpy. Upon the orthorhombic distortion, the ground state remains to be
apparently ferromagnetic with a slight drop of the Curie temperature.
Importantly, the compounds with x larger than 0.8 show antiferromagnetic
behavior with positive Weiss temperatures and nonlinear magnetization curves at
lowest temperature, implying that the ground state is noncollinear
antiferromagnetic or helimagnetic. Considering the incoherent metallic behavior
and the suppression of the electronic specific heat at high x region, the
possible emergence of a helimagnetic state in Sr1-xCaxCoO3 is discussed in
terms of the band-width narrowing and the double-exchange mechanism with the
negative charge transfer energy as well as the spin frustration owing to the
next-nearest neighbor interaction.
