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

An alloy series of single-phased polycrystalline ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}{\mathrm{MnO}}_{3+\mathrm{\ensuremath{\delta}}} (0&lt;~x&lt;~0.15)$ has been synthesized in order to study the effect of disorder on the electronic properties of mixed valence manganites. The synthetic variables allow one to maintain a constant proportion of ${\mathrm{Mn}}^{4+}$ in the samples $({\mathrm{Mn}}^{3+}{/\mathrm{M}\mathrm{n}}^{4+}=2.1\ifmmode\pm\else\textpm\fi{}0.2),$ while the similar size of ${\mathrm{La}}^{3+}$ and ${\mathrm{Na}}^{+}$ ions results in no appreciable change in the tolerance factor of the perovskite structure throughout the series. In this way, the 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 the periodic potential at the Mn sites adjacent to such vacancies, thus influencing the electronic band structure of these materials. All the samples undergo a ferro- to paramagnetic transition, at temperatures that vary from 330 to 140 K as the disorder increases. Concomitantly, the residual resistivity in the low-temperature metalliclike regime increases by eight orders of magnitude. The $x=0.00$ sample, i.e., the sample having the largest concentration of vacancies in the series, presents a distinctive behavior: it shows semiconductorlike resistivity and a magnetic behavior reflecting an inhomogeneous 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 suggest a polaronic character of the conductivity. Polaron formation energies (\ensuremath{\approx}200 meV) are found to be nearly independent of the degree of disorder in the samples. Our results suggest that metallic ferromagnetic regions and semiconducting cluster-glass zones coexist below ${T}_{c}.$ With increasing disorder, the semiconducting regions grow in volume, which modifies the transport and magnetotransport properties of these samples.