Low temperature synthesis, structure and magnetic properties of La0.85(Na1–xKx)0.15MnO3 perovskites: the role of A cation size disparity in the electronic properties of mixed-valence manganates

Abstract

Single-phase perovskites in the solid solution series La _0.85 (Na _{1– x} K _x ) _0.15 MnO ₃ (0≤x≤1) have been obtained using a soft treatment, which makes possible strict stoichiometric control. X-Ray powder diffraction patterns of these compounds have been completely indexed with a rhombohedral perovskite cell. The crystal structures have been refined in space group R-3c, in the hexagonal setting, from room-temperature data. Substitution of Na ⁺ by larger K ⁺ ions produces a cell expansion and a decrease in the structural distortion from the ideal cubic structure. The critical temperature for the paramagnetic-ferromagnetic transition, T _c , is found to be practically constant, ca. 333 K, along the entire series. This behaviour is unexpected, taking into account previous correlations established for the alkaline-earth La _0.7 (Ca _{1– x} Sr _x ) _0.3 MnO ₃ series (T _c increases with the mean size of cations at the A positions, 〈r _A 〉) which expands 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, σ ² (〈r _A 〉)] would cause a decrease in T _c . An approach to the understanding of the contribution that the electronic energy makes to this last effect is advanced.