Effect ofA-site cation size mismatch on charge ordering and colossal magnetoresistance properties of perovskite manganites

Resistivity and ac-susceptibility measurements demonstrate that the cation size mismatch at the $A$ site in perovskite manganites influences greatly the electrical and magnetic properties which are associated with colossal magnetoresistance. It has been reported that the Pr${}_{1-x}$Ca${}_x$MnO${}_3$ system does not exhibit an insulator-metal transition and remains insulating at all temperatures because of charge ordering at high temperatures. We show that the cation size mismatch at the $A$ site, introduced by replacing the Pr${}^{3+}$ ions by larger La${}^{3+}$ and smaller Y${}^{3+}$ ions with constant average $A$-site ionic radius $〈r_A〉$=1.18 Å, suppresses charge ordering and the samples with $x$=0.25 and 0.3 undergo insulator-metal transitions at 130 and 60 K, respectively, without applying a magnetic field. For 0.30 $<~x<~$ 0.5, they all show a spin-glass behavior.