Study on the relation between the crystal structure and thermal stability of FeUO
and CrUO
FeUO
およびCrUO
の結晶構造と熱的安定性に関する研究
秋山 大輔*; 日下 良二
; 熊谷 友多
; 中田 正美
; 渡邉 雅之
; 岡本 芳浩
; 永井 崇之
; 佐藤 修彰*; 桐島 陽*
Akiyama, Daisuke*; Kusaka, Ryoji; Kumagai, Yuta; Nakada, Masami; Watanabe, Masayuki; Okamoto, Yoshihiro; Nagai, Takayuki; Sato, Nobuaki*; Kirishima, Akira*
ウラン酸鉄,ウラン酸クロム、およびその固溶体を合成し、これらのウラン酸塩が異なる熱的安定性を示すメカニズムを研究した。熱的安定性を評価するため、ウラン酸塩試料の熱重量分析を実施した結果、ウラン酸クロムの分解温度(約1250
C)に対してウラン酸鉄は低温(約800
C)で分解するが、クロムを含む固溶体では熱分解に対する安定性が高まることが分かった。この熱的安定性と結晶構造との関係性を調べるため、エックス線結晶構造解析,エックス線吸収微細構造測定,メスバウアー分光測定,ラマン分光分析による詳細な結晶構造と物性の評価を行ったが、本研究で用いたウラン酸塩試料の間に明瞭な差異は観測されなかった。そのため、熱的安定性の違いは結晶構造に起因するものではなく、鉄とクロムとの酸化還元特性の違いによるものと推定した。クロムは3価が極めて安定であるのに対して、鉄の原子価は2価と3価を取ることができる。このため、ウラン酸鉄の場合には結晶中でウランと鉄との酸化還元反応が起こり、低温での分解反応を誘起したものと考えられる。
FeUO
, CrUO
, and Fe
Cr
UO
are monouranates containing pentavalent U. Even though these compounds have similar crystal structures, their formation conditions and thermal stability are significantly different. To determine the factors causing the difference in thermal stability between FeUO
and CrUO
, their crystal structures were evaluated in detail. A Raman band was observed at 700 cm
in all the samples. This Raman band was derived from the stretching vibration of the O-U-O axis band, indicating that Fe
Cr
UO
was composed of a uranyl-like structure in its lattice regardless of its "x"' value. M
ssbauer measurements indicated that the Fe in FeUO
and Fe
Cr
UO
were trivalent. Furthermore, Fe
Cr
UO
lost its symmetry around Fe
with increasing electron densities around Fe
, as the abundance of Cr increased. These results suggested no significant structural differences between FeUO
and CrUO
. Thermogravimetric measurements for UO
, FeUO
, and CrUO
showed that the temperature at which FeUO
decomposed under an oxidizing condition (approximately 800
C) was significantly lower than the temperature at which the decomposition of CrUO
started (approximately 1250
C). Based on these results, we concluded that the decomposition of FeUO
was triggered by an "in-crystal" redox reaction, i.e., Fe
U
Fe
U
, which would not occur in the CrUO
lattice because Cr
could never be reduced under the investigated condition. Finally, the existence of Cr
in FexCr
UO
effectively suppressed the decomposition of the Fe
Cr
UO
crystal, even at a very low Cr content.