Effect of A-cation size variance on structural and physical properties of praseodymium manganites Pr0.85(Na1−xKx)0.15MnO3

The effect of cationic mismatch at the A site on the structural, electrical and magnetic properties of alkali-substituted Pr0.85A0.15MnO3 (A=Na, K) was investigated by varying the K/Na ratio at constant substitution level 0.15. This system encompasses remarkable structural changes: the pure sodium and potassium phases have O-type and O′-type orthorhombic structures, respectively, while intermediate compositions are almost undistorted (pseudo-cubic). All samples are ferromagnetic with a rather weak dependence of Tc, on x (dTc/dx=5 K) Resistivity measurements show a semiconducting-like hopping behaviour. No metal-insulator transition is observed at the magnetic transition temperature Tc. The CMR effect reaches a maximum at Tc. The magnetic and transport properties in the paramagnetic regime are discussed in the framework of a magnetic polaron model. These results illustrate the counterbalancing effect of the average A-site cation size 〈rA〉 and of the variance in 〈rA〉.