X‐ray Diffraction and 151Eu Mössbauer Spectroscopic Study of the Hf1−xEuxO2−x/2(0 ≤x≤ 1.0) System

Abstract

Powder X‐ray diffractometry (XRD) and 151Eu Mössbauer spectroscopy were performed for samples prepared in the temperature range 1500–1500°C for the hafnia–europia (HfO2–Eu2O3) system EuxHf1−xO2−x/2(0 ≤x≤ 1.0). The XRD results showed that two types of solid solution phases formed in the composition range 0.25 ≤x≤ 0.725: an ordered pyrochlore‐type phase in the middle‐composition range (0.45 < x < 0.575) and a disordered fluorite‐type phase, enveloping the pyrochlore‐type phase on both sides, in the composition ranges 0.25 ≤x≤ 0.40 and 0.60 ≤x≤ 0.725. 151Eu Mössbauer spectroscopy sensitively probes local environmental changes around trivalent europium (Eu3+) caused by the formation of these solid solution phases. In addition to the broad, single Mössbauer spectra observed in this study for the Eu3+, the values of isomer shift (IS) exhibited a pronounced minimum in the neighborhood of the stoichiometric pyrochlore phase (x≈ 0.5). Such IS behavior of Eu3+ was interpreted based on the XRD crystallographic information that the ordered pyrochlore phase has a longer (in fact, the longest) average Eu–O bond length than those of the disordered fluorite phases on both sides or the monoclinic (and C‐type) Eu2O3at x= 1.0.