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首页> 外文期刊>Chemistry: A European journal >Pd0.213Cd0.787 and Pd0.235Cd0.765 structures: Their long c axis and composite crystals, chemical twinning, and atomic site preferences
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Pd0.213Cd0.787 and Pd0.235Cd0.765 structures: Their long c axis and composite crystals, chemical twinning, and atomic site preferences

机译：Pd0.213Cd0.787和Pd0.235Cd0.765结构：长c轴和复合晶体，化学孪晶和原子位点偏好

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摘要

We present single-crystal studies of Pd0.213Cd0.787 and Pd0.235Cd0.765, synchrotron powder studies of Pd1-xCd, 0.755 >= x >= 0.800, and LDA-DFT and extended Huckel (eH) calculations on these or related phases. The two single-crystal structures have a, b, and c axis lengths of 9.9013(7), 14.0033(10), 37.063(24) and 9.9251(3), 14.0212(7), 60.181(3) angstrom, respectively and they crystallize in the space groups Ccme and F2mm, respectively (solved as (3+1)-dimensional crystals their most convenient superspace group is Xmmm(00 gamma)s00). The structures have two different structural components each with their own separate axis parameters. Powder data shows that the ratio of these separate axes (S/L) varies from 1.615 to 1.64, values near the golden mean (1.618). For Pd0.213Cd0.787, different Pd and Cd site occupancies lead to variation in the R factor from 2.6-3.6%. The site occupancy pattern with the lowest R factor (among the 26820 variants studied) is the exact site occupancy pattern predicted by LDA-DFT parameterized eH Mulliken charge populations. The phases can be understood through a chemical twinning principle found in gamma-brass, the parent structure for the above phases (a relation with the MgCu2 Laves phase is also noted). This twinning principle can be used to account for Cd and Pd site preferences. At the same time there is a clean separation among the Cd and Pd atoms for the two separate chain types at height b=0 and 1/2. These results indicate that Cd:Pd stoichiometry plays a role in phase stability.
机译：我们介绍了Pd0.213Cd0.787和Pd0.235Cd0.765的单晶研究，Pd1-xCd，0.755> = x> = 0.800的同步加速器粉末研究以及LDA-DFT和扩展Huckel（eH）计算的相关结果阶段。这两个单晶结构的a，b和c轴长度分别为9.9013（7），14.0033（10），37.063（24）和9.9251（3），14.0212（7），60.181（3）埃分别在Ccme和F2mm空间群中结晶（解析为（3 + 1）维晶体，它们最方便的超空间群是Xmmm（00 gamma）s00）。这些结构具有两个不同的结构组件，每个组件都有各自独立的轴参数。粉末数据显示，这些单独的轴的比率（S / L）从1.615到1.64，接近黄金平均值（1.618）的值。对于Pd0.213Cd0.787，不同的Pd和Cd位置占用会导致R系数从2.6-3.6％发生变化。 R因子最低的位点占用模式（在研究的26820变体中）是由LDA-DFT参数化的eH Mulliken电荷种群预测的确切位点占用模式。可以通过在γ-黄铜中发现的化学孪生原理来理解这些相，上述相的母体结构（还指出了与MgCu2 Laves相的关系）。此孪生原理可用于说明Cd和Pd站点的首选项。同时，两种单独的链类型在高度b = 0和1/2时Cd和Pd原子之间存在清晰的分隔。这些结果表明，Cd：Pd化学计量在相稳定性中起作用。

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