Geng Fengxia, Matsushita Yoshitaka, Ma Renzhi, Xin Hao, Tanaka Masahiko, Izumi Fujio, Iyi Nobuo, Sasaki Takayoshi
International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
J Am Chem Soc. 2008 Dec 3;130(48):16344-50. doi: 10.1021/ja807050e.
The synthesis process and crystal structure evolution for a family of stoichiometric layered rare-earth hydroxides with general formula Ln(8)(OH)(20)Cl(4) x nH(2)O (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Y; n approximately 6-7) are described. Synthesis was accomplished through homogeneous precipitation of LnCl(3) x xH(2)O with hexamethylenetetramine to yield a single-phase product for Sm-Er and Y. Some minor coexisting phases were observed for Nd(3+) and Tm(3+), indicating a size limit for this layered series. Light lanthanides (Nd, Sm, Eu) crystallized into rectangular platelets, whereas platelets of heavy lanthanides from Gd tended to be of quasi-hexagonal morphology. Rietveld profile analysis revealed that all phases were isostructural in an orthorhombic layered structure featuring a positively charged layer, Ln(8)(OH)(20)(H(2)O)(n), and interlayer charge-balancing Cl(-) ions. In-plane lattice parameters a and b decreased nearly linearly with a decrease in the rare-earth cation size. The interlamellar distance, c, was almost constant (approximately 8.70 A) for rare-earth elements Nd(3+), Sm(3+), and Eu(3+), but it suddenly decreased to approximately 8.45 A for Tb(3+), Dy(3+), Ho(3+), and Er(3+), which can be ascribed to two different degrees of hydration. Nd(3+) typically adopted a phase with high hydration, whereas a low-hydration phase was preferred for Tb(3+), Dy(3+), Ho(3+), Er(3+), and Tm(3+). Sm(3+), Eu(3+), and Gd(3+) samples were sensitive to humidity conditions because high- and low-hydration phases were interconvertible at a critical humidity of 10%, 20%, and 50%, respectively, as supported by both X-ray diffraction and gravimetry as a function of the relative humidity. In the phase conversion process, interlayer expansion or contraction of approximately 0.2 A also occurred as a possible consequence of absorption/desorption of H(2)O molecules. The hydration difference was also evidenced by refinement results. The number of coordinated water molecules per formula weight, n, changed from 6.6 for the high-hydration Gd sample to 6.0 for the low-hydration Gd sample. Also, the hydration number usually decreased with increasing atomic number; e.g., n = 7.4, 6.3, 7.2, and 6.6 for high-hydration Nd, Sm, Eu, and Gd, and n = 6.0, 5.8, 5.6, 5.4, and 4.9 for low-hydration Gd, Tb, Dy, Ho, and Er. The variation in the average Ln-O bond length with decreasing size of the lanthanide ions is also discussed. This family of layered lanthanide compounds highlights a novel chemistry of interplay between crystal structure stability and coordination geometry with water molecules.
描述了通式为Ln(8)(OH)(20)Cl(4)·nH(2)O（Ln = Nd、Sm、Eu、Gd、Tb、Dy、Ho、Er、Tm和Y；n约为6 - 7）的一系列化学计量比层状稀土氢氧化物的合成过程及晶体结构演变。通过用六亚甲基四胺使LnCl(3)·xH(2)O均匀沉淀来实现合成，得到了Sm - Er和Y的单相产物。对于Nd(3+)和Tm(3+)观察到一些少量共存相，表明该层状系列存在尺寸限制。轻镧系元素（Nd、Sm、Eu）结晶为矩形片状，而从Gd开始的重镧系元素的片状趋于准六边形形态。Rietveld全谱分析表明，所有相在正交层状结构中是同构的，其特征为带正电荷的层Ln(8)(OH)(20)(H(2)O)(n)和层间电荷平衡的Cl(-)离子。面内晶格参数a和b随着稀土阳离子尺寸的减小几乎呈线性下降。对于稀土元素Nd(3+)、Sm(3+)和Eu(3+)，层间距c几乎恒定（约8.70 Å），但对于Tb(3+)、Dy(3+)、Ho(3+)和Er(3+)，它突然降至约8.45 Å，这可归因于两种不同程度的水合作用。Nd(3+)通常采用高水合相，而Tb(3+)、Dy(3+)、Ho(3+)、Er(3+)和Tm(3+)则倾向于低水合相。Sm(3+)、Eu(3+)和Gd(3+)样品对湿度条件敏感，因为高水合相和低水合相在临界湿度分别为10%、20%和50%时可相互转化，X射线衍射和重量法作为相对湿度的函数均支持这一点。在相转化过程中，由于H(2)O分子的吸收/解吸，层间也可能发生约0.2 Å的膨胀或收缩。细化结果也证明了水合差异。每式量配位水分子数n从高水合Gd样品的6.6变为低水合Gd样品的6.0。此外，水合数通常随着原子序数的增加而减少；例如，高水合Nd、Sm、Eu和Gd的n = 7.4、6.3、7.2和6.6，低水合Gd、Tb、Dy、Ho和Er的n = 6.0、5.8、5.6、5.4和4.9。还讨论了平均Ln - O键长随镧系离子尺寸减小的变化。这一系列层状镧系化合物突出了晶体结构稳定性与水分子配位几何之间相互作用的新化学性质。
