Effect of disorder produced by cationic vacancies at the B sites on the electronic properties of mixed valence manganites

An alloy series of single-phased polycrystalline La1-xNaxMnO3+delta (0 less than or equal to x less than or equal to 0.15) has been synthesized in ord er to study the effect of disorder on the electronic properties of mixed va lence manganites. The synthetic variables allow one to maintain a constant proportion of Mn4+ in the samples (Mn3+/Mn4+ = 2.1+/-0.2), while the simila r size of La3+ and Na+ ions results in no appreciable change in the toleran ce factor of the perovskite structure throughout the series. In this way, t he sodium content x controls the concentration of cationic vacancies at the B (Mn) sites. The presence of these vacancies gives rise to a change in th e periodic potential at the Mn sites adjacent to such vacancies, thus influ encing the electronic band structure of these materials. All the samples un dergo a ferro- to paramagnetic transition, at temperatures that vary from 3 30 to 140 K as the disorder increases. Concomitantly, the residual resistiv ity in the low-temperature metalliclike regime increases by eight orders of magnitude. The x = 0.00 sample, i.e., the sample having the largest concen tration of vacancies in the series, presents a distinctive behavior: it sho ws semiconductorlike resistivity and a magnetic behavior reflecting an inho mogeneous magnetic state. These results have been explained on the basis of the effect of structural disorder on the electronic band structure. Above the transition temperature, thermopower and resistivity measurements sugges t a polaronic character of the conductivity. Polaron formation energies (ap proximate to 200 meV) are found to be nearly independent of the degree of d isorder in the samples. Our results suggest that metallic ferromagnetic reg ions and semiconducting cluster-glass zones coexist below T-c. With increas ing disorder, the semiconducting regions grow in volume, which modifies the transport and magnetotransport properties of these samples.