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Oxygen potential measurements of pyrochlore-type Zr$$_{2}$$M$$_{2}$$O$$_{7+x}$$7(M=Pu, Ce) by EMF method

Otobe, Haruyoshi; Nakamura, Akio; Yamashita, Toshiyuki; Ogawa, Toru

Pyrochlore-type (P-type) zirconia has been widely studied as a disposable form of the high-level radioactive nuclear waste. We have developed the EMF (electromotive force) measurement system using the cubic zirconia sensor, in order to investigate the oxygen potential ($$Delta$$g(O$$_{2}$$)) of P-type Zr$$_{2}$$Pu(Ce)$$_{2}$$O$$_{7+x}$$. For P-type Zr$$_{2}$$Ce$$_{2}$$O$$_{7+x}$$ at 1078 K, we have found that $$Delta$$g(O$$_{2}$$) vs. oxygen composition (x) relationship has different behavior between x more than 0.34 and x less than 0.34. This may suggest that the degree and way of the ordering of oxide ions (O$$^{2-}$$) and oxygen vacancies (V0) should change around x =0.34. We have also measured the $$Delta$$g(O$$_{2}$$) vs. temperature (T) relationship between 763 and 1078 K at various x, and derived the partial molar enthalpy (h(O$$_{2}$$)) and entropy (s(O$$_{2}$$)) of oxygen. It was found that h(O$$_{2}$$) and s(O$$_{2}$$) of P-type Zr$$_{2}$$Ce$$_{2}$$O$$_{7+x}$$ exhibit significantly different behavior from those of fluorite-type CeO$$_{2-x}$$. The similar experiments are in progress for P-type Zr$$_{2}$$Pu$$_{2}$$O$$_{7+x}$$, and the results are discussed in comparison with those of P-type Zr$$_{2}$$Ce$$_{2}$$O$$_{7+x}$$ mentioned above.

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