Synthesis of schafarzikite-type (PbBi)(Fe _{1− <i>x</i>} Mn _<i>x</i> )O ₄ : a study on structural, spectroscopic and thermogravimetric properties

Abstract In the mineral schafarzikite (FeSb 2 O 4 ) the stereochemical active 5 s 2 lone electron pairs (LEPs) play pivotal roles to diverse crystal-physico-chemical properties. The complete substitution of Sb(III) with an equivalent amount of Pb(II) and Bi(III) leads to higher oxidation state of the transition element (e.g., Fe 3+ ) and offers a mixed stereochemical activity of the 6 s 2 LEPs. Here we report the synthesis and characterization of a solid solution (PbBi)(Fe 1− x Mn x )O 4 ( x = 0–1.0; Δ x = 0.1). The polycrystalline samples are synthesized in sealed quartz tubes at 923 K and a pressure of about 1.0 Pa. The crystal structure of each member of the solid solution has been analyzed by X-ray power diffraction data Rietveld refinements. The metric and the geometric parameters (bond lengths, angles, distortion of the polyhedra) have been followed with respect to the chemical composition. The Wang-Liebau eccentricity (WLE) parameter, which measures the strength of the stereochemical activity, has been found to be associated with the over-bonding nature of the LEP cations. The vibrational properties are characterized using Raman and infrared spectroscopy, supporting the structural features. The optical band gap is evaluated from the UV/Vis diffuse reflectance spectra using both conventional Tauc’s and derivation of absorption spectrum fitting (DASF) methods. This study particularly highlights the miscibility between Fe(III) and Mn(III) cations on a single Wyckoff site within the schafarzikite-type structures, where the LEP driven void channels seem to be more suitable for a small cation (e.g., Li + ) insertion than the mineral schafarzikite.