Electronic Properties of Mixed Valence Manganates:  the Role of the Cationic Vacancies

Abstract

Single-phase perovskites in the solid solution series La1 - x Na x MnO3+ δ have been obtained using a soft treatment, which makes possible strict stoichiometric control. Under these conditions, it becomes possible to systematically study the influence of the sodium content on the electronic properties of materials in this series. As long as all the samples have practically the same Mn4+ content (33%), the number of vacancies at A and B sites of the perovskite structure depends on the sodium content, and it decreases as x increases. Susceptibility to alternating current, magnetization, resistivity, and magnetoresistivity measurements have allowed us to establish relevant points of the electronic phase diagram of this alkali-metal-doped lanthanide manganate system. These results, together with those previously obtained for La1 - x K x MnO3+ δ, reveal the existence of a correlation between the critical temperature for ferromagnetic ordering and the concentration of vacancies at the B sites, v B, in samples with a fixed concentration of Mn4+. Such a correlation can be understood in terms of a magnetic phase segregation model in which the materials are thought of as composed by clusters, formed by the vacancies (trapping centers for mobile holes) and neighboring Mn cations (on which holes are trapped), and a matrix, formed by the remaining Mn cations. Within this model, the decrease in the number of mobile holes in the matrix is the cause of the decrease in the critical temperature with v B.

Effects of cation vacancy distribution in doped LaMnO3+δ perovskites

“…In Figure 4 we have plotted the Curie temperatures against this quantity for the three samples (full symbols). A trend similar to those reported in the literature [13,32] Figure 4). As can be seen from the plot, the data now fall on a straight line.…”

Charge-Carrier Localization on Mn Surface Sites in Granular LaMnO3+δ Samples

“…This diagram, based on our samples and literature data (11,12,14), is basically similar to that reported for all the aliovalent-doped manganite systems (42,43): i.e., the antiferromagnetic insulator LaMnO changes into a metallic ferromagnet upon doping (Fig. 9).…”

Nanosized Sodium-Doped Lanthanum Manganites: Role of the Synthetic Route on Their Physical Properties

“…The results show that pure LaMnO 3+δ is oxygen over-stoichiometric (0.18±0.01), while for x = 0.17 a nearly correct oxygen content was found; for x = 0.24 the sample displays an oxygen under-stoichiometry of about 0.05. These results are in agreement with the general behaviour of this class of perovskites with respect to the variation of the oxygen content along with the cation doping [15][16][17]; in particular it was established that in air at temperature around 973 K the oxidation state of different series of doped-manganites has a common and mean value around 33-35% and that the effect of dopant content increase is a decrease of the oxygen content [18][19][20].…”