Stabilizing fluorite structure in ceria-based high-entropy oxides: Influence of Mo addition on crystal structure and transport properties

Abstract

(Ce,Gd,Nd,Sm,Pr)O2-δ and (Ce,Gd,La,Nd,Pr)O2-δ rare-earth high-entropy oxides (HEOx) with the oxygen vacancy-ordered Ia-3 structure are doped with a small amount of molybdenum, to prepare two series of materials, (Ce,Gd,Nd,Sm,Pr,Mox)O2-δ and (Ce,Gd,Nd,Sm,Pr,Mox)O2-δ with x = (0, 0.1, 0.2, 0.3 and 0.5). The first-ever stabilization of the fluorite-type Fm-3m structure (i.e. with disordered vacancies) in the ceria-based HEOx is achieved. Structural, microstructural and transport properties of the compounds are systematically studied. The substitution level of x ≥ 0.3 allows to fully stabilize Fm-3m symmetry, with samples exhibiting good homogeneity. Influence of Mo on the oxygen vacancy concentration and charge state of the elements is studied with the use of Raman spectroscopy and XPS methods, showing decrease of the vacancy content and dominating presence of Mo6+, Ce4+ and mixed Pr3+/4+ states. Measured for the first time in this group of materials, the electrical conductivity of the Mo-doped oxides indicates the mixed ionic-electronic behavior.