Single crystals of the systems ${\mathrm{Pr}}_{0.5}({\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{)}_{0.5}{\mathrm{MnO}}_3,$ $\left({\mathrm{Pr}}_{1-y}{\mathrm{Y}}_y{)}_{0.5}\right({\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{)}_{0.5}{\mathrm{MnO}}_3,$ and ${\mathrm{Sm}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_3$ were grown to provide a series of samples with fixed ratio $\mathrm{Mn}\left(\mathrm{III}\right)/\mathrm{Mn}\left(\mathrm{IV}\right)=1\mathrm{}$ having geometric tolerance factors that span the transition from localized to itinerant electronic behavior of the ${\mathrm{MnO}}_3$ array. A unique ferromagnetic phase appears at the critical tolerance factor $t_c=0.975\mathrm{}$ that separates charge ordering and localized-electron behavior for $t<\mathrm{}{t}_c$ from itinerant or molecular-orbital behavior for $t>\mathrm{}{t}_c.$ This ferromagnetic phase, which has to be distinguished from the ferromagnetic metallic phase stabilized at tolerance factors $t>\mathrm{}{t}_c,$ separates two distinguishable type-CE antiferromagnetic phases that are metamagnetic. Measurements of the transport properties under hydrostatic pressure were carried out on a composition t a little below $t_c$ in order to compare the effects of chemical vs. hydrostatic pressure on the phases that compete with one another near ${t=t}_c.$

• Received 1 May 2002

DOI:https://doi.org/10.1103/PhysRevB.66.174432