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 center dot nH(2)O (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Y; n approximate to 6-7) are described. Synthesis was accomplished through homogeneous precipitation of LnCl(3)center dot xH(2)O with hexamethylenetetramine to yield a single-phase product for Sm-Er and Y. Some minor coexisting phases were observed for Nd3+ and Tm3+, indicating a size limit for this layered series. Light lanthanides (Nd, Sm, Eu) crystallized into rectangular platelets, whereas platelets of heavy lanthanides from Gel 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)(H2O)(n)](4+), 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 (similar to 8.70 angstrom) for rare-earth elements Nd3+, Sm3+, and Eu3+, but it suddenly decreased to similar to 8.45 angstrom for Tb3+, Dy3+, Ho3+, and Er3+, which can be ascribed to two different degrees of hydration. Nd3+ typically adopted a phase with high hydration, whereas a low-hydration phase was preferred for Tb3+, Dy3+, Ho3+, Er3+, and Tm3+. Sm3+, Eu3+, and Gd3+ 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 similar to 0.2 angstrom also occurred as a possible consequence of absorption/desorption of H2O 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.