General Synthesis and Structural Evolution of a Layered Family of Ln₈(OH)₂₀Cl₄·nH₂O (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Y)

Abstract

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)](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 (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.

Recent progress in layered rare-earth hydroxide (LRH) and its application in luminescence

“…The composition is diverse as the general formula suggests, but all the components are isostructural and are derived from brucite [Mg(OH) 2 ], which consists of Mg(OH) 6 octahedra sharing edges to form infinite charge-neutral layers. Partial substitution of the M 2+ ions by M 3+ renders layers that acquire a positive charge, which is  balanced by an interlayer counter anion between the two brucite-like slabs [4].…”

“…Partial substitution of the M 2+ ions by M 3+ renders layers that acquire a positive charge, which is  balanced by an interlayer counter anion between the two brucite-like slabs [4]. Layered rare-earth hydroxides (LRHs), having the general formula of Ln 2 (OH) 6m (A x ) m/x ·nH 2 O (Ln: trivalent rare-earth ions; A: intercalated anion; 1.0 ≤ m ≤ 2.0), are a new family of anion-type inorganic layered compounds, but exclusively contain Ln 3+ cations in the host layers [5][6][7]. Due to the unique electronic, optical, magnetic, and catalytic properties of the Ln elements, the LRHs are attracting continuous attention since their emergence in 2006 [5], and extensive efforts have been paid to their synthesis, structural characterization, anion exchange, and exfoliation.…”

“…Due to the unique electronic, optical, magnetic, and catalytic properties of the Ln elements, the LRHs are attracting continuous attention since their emergence in 2006 [5], and extensive efforts have been paid to their synthesis, structural characterization, anion exchange, and exfoliation. Ln 2 (OH) 5 (A x ) 1/x ·nH 2 O (m = 1, n = 1-2, and typically n  1.5; termed as the 251-LRH phase) [5][6][7] and Ln 2 (OH) 4 (A x ) 2/x ·nH 2 O (m = 2, n = 0-2; 241-LRH phase) [8,9] are two important groups of the LRHs family. The anions (such as NO 3  , Cl  , and Br  ) in the 251-LRH phase of n  1.5 frequently exist in the interlayer gallery as free ones (not coordinated to the Ln 3+ center) and thus exhibit facile exchange with a wide range of carboxylate and sulfonate anions [10][11][12][13][14][15].…”

“…2 (x = 2), is mainly located in the interlayer space to support the layers, and the loss of coordinated water molecules will result in an abrupt layer contraction [10][11][12][13][14][15]. The hydration number tends to decrease with increasing atomic number for the chloride family (orthorhombic structure) while increase for the nitrate analogue (monoclinic structure) [6,11]. The synthesis of 251-LRHs for the full spectrum of lanthanides (including Y) was pursued by several research groups including us.…”

“…The synthesis of 251-LRHs for the full spectrum of lanthanides (including Y) was pursued by several research groups including us. A series of 251-LRHs, with single-phase products for Sm-Er and Y, have been synthesized by homogeneous precipitation, which involves refluxing an aqueous solution containing the corresponding Ln salt (nitrate or chloride) and hexamethylenetetramine precipitant [6]. Solvothermal reaction, however, is needed to extend the nitrate family of 251-LRHs to the bigger ions of Gd, Eu, Nd, and La [16,17].…”