Effect of A-cation size variance on structural and physical properties of praseodymium manganites Pr-0.85(Na1-xKx)(0.15)MnO3

The effect of cationic mismatch at the A site on the structural, electrical and magnetic properties of alkali-substituted Pr(0.85)A(0.15)MnO(3) (A = N a, K) was investigated by varying the K/Na ratio at constant substitution l evel 0.15. This system encompasses remarkable structural changes: the pure sodium and potassium phases have O-type and O ' -type orthorhombic structur es, respectively, while intermediate compositions are almost undistorted (p seudo-cubic). All samples are ferromagnetic with a rather weak dependence o f T-c, on x (dT(c)/dx = 5 K) Resistivity measurements show a semiconducting -like hopping behaviour. No metal-insulator transition is observed at the m agnetic transition temperature T-c. The CMR effect reaches a maximum at T-c . The magnetic and transport properties in the paramagnetic regime are disc ussed in the framework of a magnetic polaron model. These results illustrat e the counterbalancing effect of the average A-site cation size < rA > and of the variance in < rA >. (C) 2001 Elsevier Science Ltd. All rights reserv ed.