P-03 Isomorphic Substitutions in the Electron-Ion Conductor Pr5Mo3O16+δ

Topic: Advances in solid state materials synthesis, new solids and compounds

L. I. Ardanova^1*, K. A. Chebyshev², E. I. Get`man^{2, 3}, A. V. Ignatov², L. V. Pasechnik², N. I. Selikova²

¹ Department of Biochemistry, Chemistry, and Geology, Minnesota State University, 241 Ford Hall, Mankato, Minnesota, 56001, USA

² Department of Inorganic Chemistry, Faculty of Chemistry, Donetsk National University, Donetsk 83001, Ukraine

³ Department of Analytical Chemistry, Faculty of Chemistry, Donetsk National University, Vinnytsia 21021, Ukraine

 ^* E-mail: lyudmyla.stackpool@mnsu.edu

Keywords: molybdates, rare-earth elements, fluorite, solid solutions, solid electrolytes, electrical conductivity

Abstract: Solid ionic oxygen conductors are interesting functional materials due to the possibility of using them in fuel cells, gas sensors, and devices for producing pure oxygen. Compounds Ln₅Mo₃O_16+δ exhibit high oxygen-ion conductivity at medium temperatures. These molybdates have a fluorite-like structure and crystallize in the Pn-3n space group. The initial fluorite structure is distorted as a result of the presence of cations significantly different in size and charge. It leads to the displacement of oxygen ions from the ideal fluorite positions and the formation of MoO₄ tetrahedra.

The conductivity of the oxidized Ln₅Mo₃O_16+δ molybdates is proportional to the REE size, which is due to an increase in interatomic distances and, as a consequence, to a decrease in steric obstacles due to the oxygen ions movement. The cubic fluorite-like structure is formed under oxidizing conditions only for praseodymium and neodymium molybdates. Praseodymium may have variable oxidation states in Pr₅Mo₃O_16+δ, which determines the presence of electronic conductivity and the possibility of using Pr₅Mo₃O_16+δ as electrode material in fuel cells [1]. The value of electronic conductivity is insufficient for practical use, but it can be improved through heterovalent isomorphic substitutions with charge compensation of elements in lower oxidation states for praseodymium and molybdenum according to the schemes: 2Pr⁺³ → M⁺² + Pr⁺⁴; Mo⁺⁶ + Pr⁺³ → M⁺⁵ + Pr⁺⁴. However, other schemes with charge compensation are valid: Pr⁺³ + 1/2O^-2→ M⁺² + 1/2V_o; Mo⁺⁶ + 1/2O^-2→ M⁺⁵ + 1/2V_o. To verify this, in this research we studied isomorphic substitution of calcium for praseodymium and niobium for molybdenum in the Pr₅Mo₃O_16+δ.

Samples of the Pr_5-xCa_xMo₃O_16+δ (x = 0 – 1) and Pr₅Mo_3-xNb_xO_16+δ (x = 0 – 2) systems were obtained by solid-phase synthesis. The unit cell parameters were calculated by full profile analysis, the conductive properties were measured, and the morphology of the ceramic surface was studied using scanning electron microscopy for the obtained samples. The elemental composition is confirmed by energy dispersive x-ray spectroscopy.

The phase with a cubic structure exists in the whole concentration range in the

Pr_5-xCa_xMo₃O_16+δ system after calcination at 1100ºС. Single-phase samples obtained in the composition up to x = 0.5. A phase with calcium molybdate structure appears at a higher concentration of the modifying element. The unit cell parameter decreases with the introduction of calcium to x = 0.5 and does not change at higher x values which indicates the substitutional limit of calcium for praseodymium in the structure of praseodymium molybdate. In the Pr₅Mo_3-xNb_xO_16+δ system, the homogeneous region of single-phase samples was found up to x = 1.2.

It was shown that the substitution of calcium for praseodymium and niobium for molybdenum leads to a decrease in total conductivity. It suggests that praseodymium stays in the oxidation state of +3 in the solid solutions and charge compensation is achieved through the formation of oxygen vacancies.

Reference

1. Istomin S.Ya. Kotova A.I., Lyskov N.V., Mazo G.N., and Antipov E.V. (2018) Pr₅Mo₃O_16+δ: A New Anode Material for Solid Oxide Fuel Cells. Russian Journal of Inorganic Chemistry. Vol. 63. pp.1291-1296.