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 P2(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 P2(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.