Structural and magnetic inhomogeneity and the NMR of Mn55 and La139 in the magnetoresistive ceramics La0.7Ba0.3−xSnxMnO3→La0.7−xBa0.3−xMnO3+0.5xLa2Sn2O7

Abstract

The effects of substitution of barium by tin on the phase composition, structural imperfection, and properties of lanthanum manganite perovskites La0.7Ba0.3−xSnxMnO3 (x=0, 0.1, 0.15, 0.2, 0.3) are established by comprehensive studies done by x-ray diffraction, resistive, and magnetic (including NMR on Mn55 and La139) methods. It is shown that the introduction of Sn leads to the formation of a pyrochloric phase La2Sn2O7, to an increase in the density of lattice defects of the manganese-enriched main lanthanum manganite phase, and to a substantial decrease of the magnetoresistive effect. The smearing of the metal–semiconductor phase transition temperature is explained by an increase in the inhomogeneity and imperfection of the perovskite structure. The low activation energy is confirmed by a high degree of inhomogeneity and imperfection of the crystal lattice of the samples studied. The broad, asymmetric NMR spectra of Mn55 and La139 attest to high-frequency electron exchange between Mn3+ and Mn4+ and nonequivalence of the environment of those ions and La3+ due both to heterovalent ions and to vacancies and clusters.

Structural and magnetic heterogeneities, phase transitions, and magnetoresistance and magnetoresonance properties of the composition ceramic La0.7Pb0.3 − x Sn x MnO3

“…After this cycle, the real perovskite structure contains anion and cation vacan cies and nanostructured clusters with Mn 2+ in the deformed A positions of the perovskite structure in the presence of superstoichiometric manganese (x > 0). The existence of vacancies and clusters in rare earth manganites is supported by the data in [16,24,25], and the presence of Mn 2+ is supported by the 55 Mn NMR spectra recorded at T = 4.2 K [26] and by reso nance photoemission [27]. Therefore, the molar formulas presented in Table 2 correspond to the most probable imperfection of the La 0.7 -x Pb 0.3 -x MnO 3 perovskite structure.…”

“…NMR investigations give valuable information on the local magnetic and valence states of ions [19,20,24]. Figure 4 shows the NMR spectra of 55 Mn (x = 0.1) and 139 La (x = 0-0.3).…”