Structures of the reduced niobium oxides Nb12O29 and Nb22O54
Structures of the reduced niobium oxides Nb12O29 and Nb22O54
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还原铌氧化物 Nb12O29 和 Nb22O54 的结构
DOI：
10.1016/j.jssc.2007.08.013
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发表时间：
2007
影响因子：
3.3
通讯作者：
R. Cava
中科院分区：
文献类型：
--
作者：
T. McQueen;Q. Xu;E. N. Andersen;H. Zandbergen;R. Cava
首次报道了Nb{sub22}O{sub54}的晶体结构，并重新研究了正交晶系Nb{sub12}O{sub29}的结构，解决了以往的歧义。采用单晶X射线衍射法和电子衍射法。这些化合物的空间群分别为P2/m(a=15.7491(2)A，b=3.8236(3)A，c=17.8521(2)A，{beta}=3.8320(3)A，b=20.7400(9)A，c=28.8901(13)A)和cCMM(a=3.8320(2)A，b=20.7400(9)A，c=28.8901(13)A)，并共享一个共同的结构单元，即4x3块共角NbO{sub6}八面体。尽管对称性施加了不同的限制，但这两种化合物中的这些块非常相似。在块体内部，发现Nb原子并不位于氧八面体的中心，而是向块体中心内移，形成明显的反铁电态。键价和键长没有显示出电荷有序的存在，这表明在所研究的温度下，所有的4d电子在这些化合物中都是离域的，T=200K。-图形摘要：报道了Nb{sub22}O{sub54}的晶体结构，并重新考察了正交晶系Nb{sub12}O{sub29}的结构。键价和没有显示电荷有序的存在，这表明在T=200K时，所有的4d电子在这些化合物中都是离域的，尽管这些材料的磁性是众所周知的。
The crystal structure of Nb{sub 22}O{sub 54} is reported for the first time, and the structure of orthorhombic Nb{sub 12}O{sub 29} is reexamined, resolving previous ambiguities. Single crystal X-ray and electron diffraction were employed. These compounds were found to crystallize in the space groups P2/m (a=15.7491(2)A, b=3.8236(3)A, c=17.8521(2)A, {beta}=102.029(3){sup 0}) and Cmcm (a=3.8320(2)A, b=20.7400(9)A, c=28.8901(13)A), respectively and share a common structural unit, a 4x3 block of corner sharing NbO{sub 6} octahedra. Despite different constraints imposed by symmetry these blocks are very similar in both compounds. Within a block, it is found that the niobium atoms are not located in the centers of the oxygen octahedra, but rather are displaced inward toward the center of the block forming an apparent antiferroelectric state. Bond valence sums and bond lengths do not show the presence of charge ordering, suggesting that all 4d electrons are delocalized in these compounds at the temperature studied, T=200 K. - Graphical abstract: The crystal structure of Nb{sub 22}O{sub 54} is reported, and the structure of orthorhombic Nb{sub 12}O{sub 29} is reexamined. Bond valence sums do not show the presence of charge ordering, suggesting that all 4d electrons are delocalized in these compounds at T=200 K, despite the well-known magnetic behavior of these materials.