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Polycrystalline samples of the series of triple perovskites Sr3−xPbxFe2TeO9 (0 ≤ x ≤ 2.25) (SPFTO) were synthesized using solid state reaction. These materials have been studied by a combination of X-ray powder diffraction (XRPD), Mössbauer spectrometry, Raman and UV–Vis spectroscopies. The crystal structures were resolved by the Rietveld refinement method, and revealed that this Sr3−xPbxFe2TeO9 (0 ≤ x ≤ 2.25) system shows one space group change from tetragonal I4/m (0 ≤ x ≤ 1) to another tetragonal form I4/mmm (1.25 ≤ x ≤ 1.88) and a second transition to hexagonal R 3 ¯ m (2.08 ≤ x ≤ 2.25). An anti-site disordering of Fe and Te on the B sites has been detected indicating the presence of a partial amount of Te at Fe positions and vice versa. The valence state of iron in the Fe site was determined to be Fe(III) by Mössbauer spectrometry, which also revealed two sites in a concordance with the XRPD measurements. 57Fe Mössbauer spectra measurements show paramagnetic and magnetic ordering behaviors. The observed Raman spectra as a function of composition show obvious changes on the positions (wavenumbers), the FWHM and the intensities of the modes confirming the phase transformations observed by the XRPD results. These structural transitions led to a distinct change in the optical band gap energy, varying from 2.14 to 1.85 eV. [ABSTRACT FROM AUTHOR] more...

A series of perovskites Sr 3– x Ca x Fe 2 TeO 9 (0 ≤ x ≤ 1) have been prepared in polycrystalline form by solid–state reaction method in air. These materials have been studied by X–ray powder diffraction method (XRPD) and Raman spectroscopy. An analysis of the XRD patterns at room temperature has shown that these compounds crystallize in a tetragonal system, space group I 4/ m. The structure contains alternating (Fe/Te) 2a O 6 and (Fe/Te) 2b O 6 octahedra, tilted in anti–phase in the basal ab–plane. The study of Raman spectroscopy at various temperatures shows a transition from tetragonal to cubic phase: I 4/ m → Fm 3 ¯ m . This phase transition occurs at high–temperature. Analysis of Raman spectra recorded at several temperatures shows that this phase transition appears near ∼375 °C for ( x = 0), ∼435 °C for ( x = 0.5) and ∼451 °C for ( x = 1). A proportional and gradual increase of temperature phase transition is observed as function of the calcium amount. [ABSTRACT FROM AUTHOR] more...