Single-phase polycrystalline samples, La1-xNaxMnO3, where x ranged from 0 to 0.15, were prepared in order to study the effect of disorder. It was possible to maintain a constant proportion of Mn4+ in the samples (Mn3+/Mn4+ = 2.1), while the similar sizes of the La3+ and Na+ ions led to no appreciable change in the tolerance factor of the perovskite structure within the series. In this way, the Na content controlled the concentration of cationic vacancies at B (Mn) sites. The presence of these vacancies gave rise to a change in the periodic potential at the Mn sites adjacent to such vacancies; thus affecting the electronic band structure of the materials. All of the samples underwent a ferro- to paramagnetic transition, at temperatures ranging from 330 to 140K, as the disorder increased. At the same time, the residual resistivity in the low-temperature metal-like region increased by 8 orders of magnitude. The x = 0 sample, which contained the largest concentration of vacancies, exhibited semiconductor-like resistivity and a magnetic behavior which indicated an inhomogeneous magnetic state. The results were explained in terms of the effect of structural disorder upon the electronic band structure.

Effect of Disorder Produced by Cationic Vacancies at the B Sites on the Electronic Properties of Mixed Valence Manganites. J.Vergara, R.J.Ortega-Hertogs, V.Madurga, F.Sapiña, Z.El-Fadli, E.Martínez, A.Beltrán, K.V.Rao: Physical Review B, 1999, 60[2], 1127-35