Mixed valence states of Cr and Fe in La1−xSrxFe0.8Cr0.2O3−y
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M.D. Carvalhoa, , , T. Ramosa, 1, L.P. Ferreirab, c and A. Wattiauxd
a CCMM, Dep. Química e Bioquímica, Fac. Ciências, Universidade de Lisboa, Campo Grande, C8, 1749-016 Lisboa, Portugal
b CFMC-UL, Fac. Ciências, Universidade de Lisboa, Campo Grande, C8, 1749-016 Lisboa, Portugal
c Dep. Física, Fac. Ciências e Tecnologia, Universidade de Coimbra, 3004-516 Coimbra, Portugal
d ICMCB, CNRS, Université de Bordeaux1, 87 avenue du Dr. A. Schweitzer, Pessac, F-33608, France
Received 18 August 2009;
revised 6 December 2009;
accepted 8 December 2009.
Available online 21 December 2009.
Abstract
The La1−xSrxFe0.8Cr0.2O3−y (x = 0.2, 0.4, 0.6 and 0.8) phases were studied by X-ray photoelectron spectroscopy at room temperature and 57Fe Mössbauer spectroscopy at different temperatures. Mixed valence states were observed both for chromium and iron ions, justifying the complex magnetic behaviour exhibited by these compounds. The Mössbauer results indicate the simultaneous presence of Fe3+, Fe4+ and Fe5+ at 4.2 K and the co-existence of Fe3+ and Fe(3+n)+ at T = 293 K, with the latter fraction increasing with increasing strontium content. The presence of Cr3+/4+ is interpreted as being mainly responsible for the incomplete charge disproportionation reaction of iron at low temperature, as deduced from the Mössbauer results.
Parameters obtained from the Mössbauer spectra of La1−xSrxFe0.8Cr0.2O3−y collected at 4.2, 293 and 343 K (only for the x = 0.4 compound).
δ, isomeric shift, relative to α-Fe at 293 K; Bhf, mean magnetic hyperfine field; Δ, mean quadrupole splitting; , quadrupole shift; Γ, line width; %, relative area. The estimated errors are <0.01 for δ and , unless specified in the table. The values in parentheses in the Bhf and Δ columns correspond to the standard deviations of the distributions.
Binding energies, Eb, FWHM values, relative areas (%) and assigned oxidation states for the observed Cr 2p3/2 peaks on the XPS spectra of La1−xSrxFe0.8Cr0.2O3−y phases.
Bhf
, mean magnetic hyperfine field; 
, quadrupole shift; Γ, line width; %, relative area. The estimated errors are <0.01 for δ and 
