Low temperature synthesis, structure and magnetic properties of La-0.85(Na1-xKx)(0.15)MnO3 perovskites: the role of A cation size disparity in the electronic properties of mixed-valence manganates

Single-phase perovskites in the solid solution series La-0.85(Na1-xKx)(0.15 )MnO3 (0 less than or equal to x less than or equal to 1) have been obtaine d using a soft treatment, which makes possible strict stoichiometric contro l. X-Ray powder diffraction patterns of these compounds have been completel y indexed with a rhombohedral perovskite cell. The crystal structures have been refined in space group R-3c, in the hexagonal setting, from room-tempe rature data. Substitution of Na+ by larger K+ ions produces a cell expansio n and a decrease in the structural distortion from the ideal cubic structur e. The critical temperature for the paramagnetic-ferromagnetic transition, T-c, is found to be practically constant, ca. 333 K, along the entire serie s. This behaviour is unexpected, taking into account previous correlations established for the alkaline-earth La-0.7(Ca1-xSrx)(0.3)MnO3 series (T-c in creases with the mean size of cations at the A positions, < r(A)>) which ex pands over a similar < r(A)> range. We can therefore discuss these results in terms of two counterweighting contributions: increasing T-c values could be expected as a consequence of the increase with x of the < r(A)> value, but the concomitant disorder introduced at the A positions [as represented by the variance of the A cations radial distribution, sigma(2)(< r(A)>)] wo uld cause a decrease in T-c. An approach to the understanding of the contri bution that the electronic energy makes to this last effect is advanced.