Structural and magnetic study of self- doped La1-x-yCaxVacyMnO3

Vitor S. Amaral 1F. Figueiras 1Joao P. Araujo 2P. B. Tavares 3J. B. Sousa 2J. M. Vieira 4

1. Universidade de Aveiro, Departamento de Fisica and CICECO (UA), Campus de Santiago, Aveiro 3810-193, Portugal
2. Departamento de Fisica and IFIMUP, Universidade do Porto (UP), Rua do Campo Alegre, 687, Porto 4169-007, Portugal
3. Departamento de Química, Universidade de Trás-os-Montes e Alto Douro (UTAD), Vila Real 5001-911, Portugal
4. Universidade de Aveiro, Departamento de Engenharia Cerâmica e do Vidro and CICECO (UA), Campus de Santiago, Aveiro 3810-193, Portugal

Abstract

To understand the combined effect of divalent ion and A-site vacancy (Vac) (beyond the La1-xMnO3 system) self doping polycrystalline La1-x-yCaxVacyMnO3 samples were studied. Samples with excess Mn in the range x<0.33 and y<0.45 were prepared by standard solid-state reactions with several final conditions. Structural and phase analysis of the samples were done by X-ray diffraction with Rietveld refinement and High Resolution TEM. Electrical and magnetic properties were also studied. The manganite structure in the studied composition range includes rhombohedric and orthorhombic phases. Most samples contain haussmanite (Mn3O4) phase coexisting with perovskite manganite phase. The relative contents of Mn3O4 were determined by calibrating the area ratio of the two main diffraction peaks of each phase. The results were applied to determine the vacancy content y2 in the manganite phase which was found to accommodate a large fraction of vacancies, beyond y2=1/8, the reported limit for La1-xMnO3, which is confirmed in our x=0 sample. A large amount of A-site vacancies (y>0.12) leads to a strong distortion of the manganite phase. This orthorhombic cell distortion is characterized by an increase of the cell volume (up to about 5%), keeping the shorter a lattice parameter constant, and increasing b and c. This effect is associated with the formation of a vacancy structure ordering with Mn displacement to the A-sites, leading to the perovskite related AMn7O12 (A=La,Ca) structure as intergrowth or interleaved planes in the main phase.
For the compositions studied, ferromagnetic metallic behavior below Tc is found, confirming the stability of the Mn sublattice. Near the optimum (x=0.33) stoichiometric (y=0) system, high vacancy content does not degrade the properties: y2~ 0.10 (mainly La vacancies ) even leads to an increase of both Tc (by 4K) and MR peak (by 14%). Further y2~0.20 (Ca and La) gives properties very close to the stoichiometric sample.

 

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Presentation: poster at E-MRS Fall Meeting 2003, Symposium D, by Vitor S. Amaral
See On-line Journal of E-MRS Fall Meeting 2003

Submitted: 2003-05-27 18:06
Revised:   2009-06-08 12:55