No abstract

Single-phase perovskites in the solid solution series La 0.7+y A 0.3-y Mn 1-x M x O 3 (with 0.00 e x e 0.10; A ) Sr 2+ , M ) Cu 2+ , Zn 2+ , Sc 3+ , Cr 3+ , Co 3+ , and Ga 3+ ; A ) Ba 2+ , M ) Cu 2+ , Zn 2+ , and Sc 3+ ) have been prepared via the acetic acid solutions freeze-drying method. This soft procedure makes possible strict stoichiometric control, and the synthetic variables allow one to maintain a constant proportion of Mn 4+ (ca. 32%) in the 47 compounds prepared. In this way, the concentration of cationic vacancies at A and B sites is practically negligible in all cases. X-ray powder diffraction patterns corresponding to the 47 compounds have been completely indexed with rhombohedral perovskite cells. The crystal structures have been refined in space group R h 3c, in the hexagonal setting, from room-temperature data. The variation with x of a set of structural parameters (rhombohedral cell volume, rhombohedral cell angle, and B-O and A-O bond lengths) has been considered as a function of the mean sizes of cations at A and B sites. Manganates in these series present colossal magnetoresistance. The values of the critical temperature, T c , for the paramagnetic-ferromagnetic transition in La 0.7+y A 0.3-y Mn 1-x M x O 3 exhibit three different patterns, which clearly appear related to the mean size of cations at B sites. This fact has been interpreted by considering the variation of the electronic contribution to T c with the structural disorder introduced by the presence of cations with different sizes at B sites.

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.

Series of perovskite oxides with the composition LaMn 12x M x O 3zd (M~Cr, Co; 0¡x¡1) have been synthesized by thermal treatment of precursors obtained by freeze-drying of acetic acid solutions. The oxides have been characterized by X-ray diffraction, and the Mn 4z content and, thus, the oxygen excess, d, has been determined by redox back-titration. LaMnO 3.14 and LaCrO 3 phases have the rhombohedral-LaAlO 3 and the orthorhombic-GdFeO 3 structures, respectively. The LaMn 12x Cr x O 3zd phases have the rhombohedral structure for x¡0.3, and the orthorhombic structure for x¢0.5. LaCoO 3 has, as LaMnO 3.14 , the rhombohedral structure. However, the LaMn 12x Co x O 3zd phases have orthorhombic structure for 0.05¡x¡0.55. The evolution of the crystal symmetry has been interpreted by considering the effect of the size of the substituting ion on the capability of the perovskite network to accommodate the oxygen excess (d).

No abstract