Phosphorus-substitution effect on the phase stabilization, electrical and spectroscopic properties of LAMOX-based electrolyte for solid oxide fuel cells

Abstract

A series of P⁵⁺ – doped La₂Mo₂O₉ phases with different concentrations of P⁵⁺ were prepared using conventional solid–state reactions. The formation of phase-pure P⁵⁺-doped La₂Mo₂O₉ has been monitored by powder X-ray diffraction, thermal analysis, conductivity measurements, Raman, and FT-IR absorption techniques. The structure and lattice parameters of La₂Mo_2−yP_yO_9−y/2 are obtained from Rietveld refinement. The effect of substituting P for Mo reveals that the phase transition which occurs in La₂Mo₂O₉ around 560 °C disappears when y > 0.02, as demonstrated by thermal analysis. Pure P⁵⁺-doped phases with monoclinic structure (α-form, the space group P2₁) were observed for the concentration of optically active ions up to y = 0.02. When the concentration of P⁵⁺ ions is higher, a cubic structure (β-form, the space group P2₁3) starts to appear. However, up to the concentration of y = 0.03 of the P⁵⁺ ion a mixture of the monoclinic and cubic phases has been observed. From infrared and Raman analysis it is confirmed that different vibration modes arise from the vibration of molybdenum–oxygen bands. Mo–O bond lengths are also found to be independent of P-doping.