Thermal expansion of lanthanum silicate oxyapatite (La9.33+2x(SiO4)6O2+3x), lanthanum oxyorthosilicate (La2SiO5) and lanthanum sorosilicate (La2Si2O7)

Four types of powder specimens of La9.33(SiO4)6O2 (space group P63/m and Z=1), La9.33+2x(SiO4)6O2+3x with 0.06≤x≤0.13 (P63/m and Z=1), La2SiO5 (P21/c and Z=4) and La2Si2O7 (P21/c and Z=4) were examined by high-temperature X-ray powder diffractometry to determine the changes in unit-cell dimensions up to 1473 K. The anisotropy of thermal expansion was demonstrated for the former two crystals to clarify the thermal behaviors of the highly c-axis-oriented polycrystals. With La9.33(SiO4)6O2, the linear expansion coefficient of the a-axis (αa) was 4.8×10−6 K−1 and that of the c-axis (αc) was 1.8×10−6 K−1 in the temperature range from 298 to 1473 K. The αa- and αc-values of La9.33+2x(SiO4)6O2+3x (0.06≤x≤0.13) were, respectively, 5.9×10−6 K−1 and 2.3×10−6 K−1. The coefficients of mean linear thermal expansion were 4.9×10−6 K−1 for La2SiO5 and 6.0×10−6 K−1 for La2Si2O7, which describe the thermal expansion behaviors of the randomly grain-oriented polycrystalline materials.