Thermal Expansion and Defect Chemistry of MgO-Doped Sm₂Zr₂O₇

A series of solid solutions of (Sm 2 - x Mg x )Zr 2 O 7 - x /2 pyrochlores were prepared by a solid-state reaction and characterized by X-ray diffraction, high-temperature dilatometry, Raman spectroscopy, and X-ray photoelectron spectroscopy. X-ray diffraction and Raman spectroscopy reveal that MgO doping does not break down the pyrochlore structure of Sm 2 Zr 2 O 7 for all of the samples, though it increases the degree of structure disorder. However, the thermal expansion coefficient is remarkably increased through doping up to x = 0.075 with a maximum value around 11.94 × 10 - 6 K -1 (room temperature to 1000 °C), which can mitigate the mismatches of thermal expansion in the high-temperature applications of Sm 2 Zr 2 O 7 . A new solid−solution mechanism that is different from previous research was proposed and confirmed by analyzing the variation of the lattice parameters, experimental density, and X-ray photoelectron spectroscopy of samples. It involves the transformation of the solid−solution model about Mg 2+ interstitial and substitution with a turning point at composition point x = 0.075. This transformation and consequently the variation of lattice energy may be responsible for the variation of the thermal expansion coefficient.