Crystal structure analysis of Ln₂Zr₂O₇ (Ln = Eu and La) with a pyrochlore composition by high-temperature powder X-ray diffraction

Abstract

We synthesized Eu₂Zr₂O₇ and La₂Zr₂O₇ using high purity ZrO₂ of extremely low Hf content. The crystal structures of Eu₂Zr₂O₇ and La₂Zr₂O₇ were refined by the Rietveld analysis of the powder X-ray diffraction data (indexed as a cubic pyrochlore-type structure, space group: No.227, Fd3m) measured from 1173 to 298 K in a dry condition and from 1173 to 323 K in a wet condition. The crystal structure of La₂Zr₂O₇ is near to an ideal pyrochlore-type structure, whereas that of Eu₂Zr₂O₇ is a distorted pyrochlore-type structure. The thermal expansion coefficients of Eu₂Zr₂O₇ were larger than those of La₂Zr₂O₇. The value of the equivalent isotropic atomic displacement parameters (B_eq) calculated from the anisotropic atomic displacement parameter for the Zr in Eu₂Zr₂O₇ at 298 K was around 1.4 Ų, and the increase of this value in Eu₂Zr₂O₇ with increasing temperature was smaller than that in La₂Zr₂O₇.