Structural, Magnetic and Optical Properties Study of Tellurium-Based Perovskites: Sr3−xPbxFe2TeO9 (0 ≤ x ≤ 2.25)

Abstract

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), Mossbauer 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 $$R{\overline{3}}m$$ (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 Mossbauer spectrometry, which also revealed two sites in a concordance with the XRPD measurements. 57Fe Mossbauer 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.