155Gd Mössbauer spectroscopic and X-ray diffraction study of the Zr 1− xGd xO 2− x/2 (0< x≤1.0) system

Abstract

155Gd Mössbauer spectroscopy has been applied for the Gd x Zr 1− x O 2− x/2 system, to elucidate the local structure around the Gd 3+ over the entire 0< x≤1.0 range. Powder X-ray diffraction (XRD) measurements showed that below 1550 °C an intermediate pyrochlore (P) phase forms in the middle 0.45≤ x≤0.55 range, sandwitched with a disordered defect-fluorite (F) phase on both sides (0.18≤ x<0.45 and 0.55< x≤0.62), and upon heating above 1550 °C the former (P) is also disordered to the latter (F). 155Gd-Mössbauer spectra in the solid solution region (0.18≤ x≤0.62) apparently consisted of quadrupole split doublet which indicates the existence of one kind of distorted Gd site, in contrast to the well-resolved two doublets observed for the C-type Gd 2O 3 ( x=1.0). 155Gd-Mössbauer parameters in the solid solution region (0.18≤ x≤0.62) such as electric quadrupole coupling constant (e 2q Q), line width (2Γ) and relative absorption area (RAA) were found to exhibit a characteristic maximum around the ideal-pyrochlore composition ( x=0.50), while isomer shift ( δ) showed different behavior that this is nearly constant for 0.18≤ x≤0.50 and increases slightly for x>0.50. Their basic features were unchanged upon disordering above 1550 °C. These Mössbauer results suggest that the Gd x Zr 1− x O 2− x/2 solid solution (0.18≤ x≤0.62) has essentially a pyrochlore(P)-type local structure, whether long-range ordered or not, both the Gd–O bond strength and anisotropy attaining a maximum at x=0.50.