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21.01.2010

Synthesis and electrical properties of nanocrystalline Ca1 − xEuxMnO3 ± δ (0.1 ≤ x ≤ 0.4) powders prepared at low temperature using citrate gel method



C. Silveiraa, M.E. Lopesb, M.R. Nunesc and M.E. Melo Jorgec, Corresponding Author Contact Information, E-mail The Corresponding Author






a Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Portugal


b Departamento de Química, Centro de Química de Évora, Universidade de Évora, Évora, Portugal


c Departamento de Química e Bioquímica, Centro de Ciências Moleculares e Materiais, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal






Received 5 August 2009; 


revised 28 September 2009; 


accepted 23 October 2009. 


Available online 22 November 2009.






Abstract


Nanopowders of Ca1 − xEuxMnO3 (0.1 ≤ x ≤ 0.4) manganites were synthesized as a single phase using the auto gel-combustion method. The citrate method shows to be simple and appropriate to obtain single phases avoiding segregation or contamination. The Ca1 − xEuxMnO3 system has been synthesized at 800 °C during 18 h, against the conventional method of mixing oxides used to obtain these materials at higher temperatures of synthesis. The formation reaction was monitored by X-ray diffraction (XRD) analysis and an infrared absorption technique (FTIR). The polycrystalline powders are characterised by nanometric particle size, not, vert, similar 48 nm as determined from X-ray line broadening analysis using the Scherrer equation. Morphological analysis of the powders, using the scanning electron microscope (SEM), revealed that all phases are homogeneous and the europium-substituted samples exhibit a significant decrease in the grain size when compared with the undoped samples. The structure refinement by using the Rietveld method indicates that the partial calcium substitution by europium (for x ≥ 0.3) modifies the orthorhombic structure of the CaMnO3 perovskite towards a monoclinic phase. All manganites show two active IR vibrational modes around 400 and 600 cm− 1. The high temperature dependence of electrical resistivity (between 25 and 600 °C) allows us to conclude that all the samples exhibit a semiconductor behaviour and the europium causes a decrease in the electrical resistivity by more than one order of magnitude. The results can be well attributed to the Mn4+/Mn3+ ratio.





Keywords: Nanocrystalline manganites; Electron-doped manganites; Citrate gel method; Europium doping; Electrical properties





Article Outline



1. Introduction
2. Experimental
3. Results and discussion

3.1. Phase formation
3.2. X-ray powder diffraction and structure
3.3. Particle size and morphology
3.4. Fourier transform infrared spectroscopy
3.5. High temperature electrical resistivity measurements

4. Conclusions
References





























































Corresponding Author Contact InformationCorresponding author. Dept. Quimica e Bioquimica FCUL, Campo Grande C8, 1749-016 Lisboa, Portugal.










Solid State Ionics
Volume 180, Issue 40, 29 January 2010, Pages 1702-1709


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