Phase relationships in the quasi-ternary LaO_1.5–SiO₂–MgO system at 1773 K

Phase relationships in the LaO_1.5-SiO₂-MgO quasi-ternary system at 1773 K were investigated by powder x-ray diffraction (XRD) analysis applying single- and multiple-phase Rietveld methods. Most of the formed phases satisfied the Gibbs' phase rule, except for the samples containing LaO_1.5 and a liquid phase at 1773 K. The detection of segregated MgO phases was difficult in the XRD profiles of the compositional samples around the oxyapatite single phase because the MgO peaks were weak and heavily overlapped by peaks from the oxyapatite and La(OH)₃ phases. The solid solubility limit of MgO in oxyapatite was determined not only from the chemical composition of the oxyapatite phase, which was confirmed by XRD, but also from several phase boundary compositions among the two-phase and three-phase regions based on the Gibbs' phase rule. Formation of a liquid phase at 1773 K was observed in a wide range of compositions and considered when constructing the phase diagram.