Influence of Isovalent Partial Sulfur Substitution on the Structural, Thermal, Electrical and Spectroscopic Properties of La2Mo2O9 Oxide Ion Conductors

Abstract

Partial substitution of molybdenum by sulfur on the solid solution La2Mo2-ySyO9 (0.1≤y≤0.3) is carried out by using the lanthanum oxysulfate (La2SO6) via a solid-state reaction. Substitution rates are 5 mol % S, 10 mol % S and 15 mol % S. Structural and vibrational characteristics of La2Mo2-ySyO9 are revealed via XRD, thermal analysis, electrical properties as well as spectroscopic investigations. Results show the alteration of the reversible first order phase transition that commonly happens in La2Mo2O9 at around 580 °C from a monoclinic ordered α form at low temperature to a cubic disordered β phase at high temperature. Crystallization of the studied LAMOX specimens occurs in the non-centrosymmetric space group P213. The sintered samples’ annealing occurs at 850 °C and they are shaped into pellets for electrical properties determination. The Arrhenius law is activated below 580 °C. Above this thermal threshold, a Vogel Tamman Fulcher behavior is depicted. In addition, conductivity decreases in all samples as compared to the undoped composite. Ultimately, the observed vibrational modes in Raman and IR are assigned to the vibrations of molybdate MoO42− and sulfate SO42− groups.