Structure and phase stability of nanocrystalline Ce1−xLnxO2−x/2−δ (Ln = Yb Lu) in oxidizing and reducing atmosphere

摘要

The structure and phase evolution of nanocrystalline Ce1>−xLnxO2>−x/2>−δ (Ln = Yb, Lu, x = 0 − 1) oxides upon heating in H2 was studied for the first time. Up to 950 °C the samples were single-phase, with structure changing smoothly with x from fluorite type (F) to bixbyite type (C). For the Lu-doped samples heated at 1100 °C in the air and H2, phase separation into coexisting F- and C-type structures was observed for ~0.40 < x < ~0.70 and ~0.25 < x < ~0.70, respectively. It was found also that addition of Lu³⁺ and Yb³⁺ strongly hinders the crystallite growth of ceria during heat treatment at 800 and 950 °C in both atmospheres. Valency of Ce and Yb in Ce0.1Lu0.9O1.55>−δ and Ce0.95Yb0.05O1.975>−δ samples heated at 1100 °C was studied by XANES and magnetic measurements. In the former Ce was dominated by Ce⁴⁺, with small contribution of Ce³⁺ after heating in H2. In the latter, Yb existed exclusively as 3+ in both O2 and H2.