The effect of residual carbon on the phase stability of LaPO4 at high temperatures

Samples containing LaPO4 and polymeric binders were heated or hot-pressed in oxygen-poor environments to temperatures as high as 1585°C. Carbon analysis, powder X-ray diffraction and thermogravimetric analysis revealed that the residual carbon from the binders caused the LaPO4 to decompose into a lanthanum-rich phosphate compound, La3PO7, or a lanthanum oxide, La2O3, at temperatures above 750°C. These results agree with thermodynamic data, indicating that the driving force responsible for this carbothermic reduction reaction is the free energy of formation of P2O5, which is higher than the free energy of formation of CO above 750°C. It was also observed that this phenomenon would be detrimental to composites of LaPO4 and Al2O3, because LaAl11O18 and LaAlO3 were found to form at the interface, unless the samples were annealed in air after binder burnout.