Abstract:
Manganates of the compositions Nd0.5-xLaxCa0.5MnO3 and Pr0.5-xLaxCa0.5 MnO3, with the average A-site cation radius, <r(A)>, in the range 1.17-1.20 Angstrom, are charge ordered at ordinary temperatures (T-CO similar to 240 K), and undergo a re-entrant transition to a ferromagnetic state on cooling (T-C < T-CO). The ferromagnetic Curie temperature, T-C, of the re-entrant transition increases markedly with x or <r(A)> in the two series of compounds. The plots of T-C and T-CO against <r(A)> show that the two curves intersect around a <r(A)> value of 1.195 Angstrom, below which T-CO > T-C. Site disorder due to the size mismatch of the A-site cations, sigma(2), has a marked effect on the re-entrant transition temperature, T-C. Thus, in a series of manganates of the type Ln(0.5)A(0.5)MnO(3) with a fixed <r(A)> value of 1.185 Angstrom, the T-C decreases markedly with increase in site disorder, suggesting that the re-entrant transition can be entirely suppressed at a sufficiently high value of sigma(2). Between T-CO and T-C, the CO and FM states are likely to coexist, the coexistence temperature regime decreasing with increasing <r(A)>, and increasing with sigma(2) at a fixed <r(A)>.
