Noritake Tatsuo, Aoki Masakazu, Towata Shin-ichi, Takeuchi Tsunehiro, Mizutani Uichiro
Toyota Central R&D Laboratories Inc., Nagakute, Aichi 480-1192, Japan.
Acta Crystallogr B. 2007 Oct;63(Pt 5):726-34. doi: 10.1107/S0108768107039092. Epub 2007 Sep 14.
The crystal structure of the Ag(36)Li(64) gamma-brass was determined by analyzing the powder diffraction pattern taken using a synchrotron radiation beam with wavelength 0.50226 A. It turned out that the compound contained 52 atoms in its unit cell with the space group I43m and that the Li atom enters exclusively into inner tetrahedral (IT) and cubo-octahedral (CO) sites, whereas the Ag atom enters into those on outer tetrahedral (OT) and octahedral (OH) sites in the 26-atom cluster. Small amounts of Li also exist in OT and OH sites, resulting in chemical disorder. We discovered that the volumes of the IT and CO polyhedra shrink, while those of the OT and OH polyhedra expand relative to those of the corresponding polyhedra in the original b.c.c. (body-centered cubic) structure. This feature is universal and is found in other gamma-brasses such as Cu(5)Zn(8) and Al(8)V(5), for which the structure data are available. Among these gamma-brasses, we revealed the unique bond-length distribution for pairs connecting the atom on OH sites and that on CO sites, depending on the degree of d-p orbital hybridization between the transition metal elements such as Ag, Cu and V on OH sites, and the non-transition metal elements such as Li, Zn and Al on CO sites. It is suggested that this may hold a clue to resolving why some gamma-brasses such as the present Ag-Li and Cu-Zn possess a finite solid solution, but others such as Al(8)V(5) and Mn(3)In exist as line compounds.
通过分析使用波长为0.50226埃的同步辐射光束拍摄的粉末衍射图谱，确定了Ag(36)Li(64)γ黄铜的晶体结构。结果表明，该化合物的晶胞中含有52个原子，空间群为I43m，Li原子仅进入内部四面体（IT）和立方八面体（CO）位置，而Ag原子进入26原子簇中外部四面体（OT）和八面体（OH）位置的那些位置。少量的Li也存在于OT和OH位置，导致化学无序。我们发现，相对于原始体心立方（b.c.c.）结构中相应多面体的体积，IT和CO多面体的体积缩小，而OT和OH多面体的体积扩大。这一特征是普遍存在的，在其他γ黄铜如Cu(5)Zn(8)和Al(8)V(5)中也有发现，对于这些γ黄铜，结构数据是可用的。在这些γ黄铜中，我们揭示了连接OH位置上的原子和CO位置上的原子的对的独特键长分布，这取决于OH位置上的过渡金属元素如Ag、Cu和V与CO位置上的非过渡金属元素如Li、Zn和Al之间的d-p轨道杂化程度。有人认为，这可能为解决为什么一些γ黄铜如目前的Ag-Li和Cu-Zn具有有限固溶体，而其他一些如Al(8)V(5)和Mn(3)In以线状化合物形式存在提供线索。
