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

A series of P 5+ - doped La 2 Mo 2 O 9 phases with different concentrations of P 5+ were prepared using conventional solid-state reactions. The formation of phase-pure P 5+ -doped La 2 Mo 2 O 9 has been monitored by powder X-ray diffraction, thermal analysis, conductivity measurements, Raman, and FT-IR absorption techniques. The structure and lattice parameters of La 2 Mo 2− y P y O 9− y /2 are obtained from Rietveld refinement. The effect of substituting P for Mo reveals that the phase transition which occurs in La 2 Mo 2 O 9 around 560 °C disappears when y > 0.02, as demonstrated by thermal analysis. Pure P 5+ -doped phases with monoclinic structure (α-form, the space group P 2 1 ) were observed for the concentration of optically active ions up to y = 0.02. When the concentration of P 5+ ions is higher, a cubic structure (β-form, the space group P 2 1 3) starts to appear. However, up to the concentration of y = 0.03 of the P 5+ 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. Combining XRD, DTA, FTIR, Raman and conductivity measurements, a thorough investigation of the solid solutions La 2 Mo 2 y P y O 9− y /2 ( y = 0.01 to 0.05) was revealed.