Nobuo Iyi

Comparative study of defect structures in lithium niobate with different compositions

Structure refinements were conducted on LiNbO[sub 3] crystals with four different compositions, ranging from near stoichiometric (Li/(Li + Nb) = 0.498) to highly nonstoichiometric (Li/(Li + Nb) = 0.470), by the X-ray single-crystal diffraction and the TOF neutron powder diffraction methods to clarify the major defect mechanism of LiNbO[sub 3] governing its nonstoichiometry. Two models - Li-site vacancy model and Nb-site vacancy model - were chosen on the basis of density data and examined in the refinements. The former is expressed as [Li[sub 1-5x]Nb[sub x][open square][sub 4x]][Nb]O[sub 3] and the latter [Li[sub 1-5x]Nb[sub 5x]] [Nb[sub 1-4x][open square][sub 4x]]O[sub 3], where [open square] denotes a vacancy. The refinement results revealed that the amount of Nb occupancy was composition-independent and that Li[sup +] ions were replaced by the Nb[sup 5+] ions, creating vacancies at the Li site. Rietveld analysis of the neutron diffraction data was consistent with the X-ray refinement. These results strongly support the Li-site vacancy model.

Exfoliating layered double hydroxides in formamide: a method to obtain positively charged nanosheets

Exfoliation of layered double hydroxides (LDHs) into single layers provides a new type of nanosheet with ultimate two-dimensional anisotropy and positive charge. In this Highlight article, we briefly review the latest advances in this emerging field. In comparison with the previous studies, we show that micrometer-sized and well-defined LDH nanosheets can be readily attained by synthesizing large crystals of LDH-carbonate via so-called homogeneous precipitation and subsequent exfoliation of the nitrate form in formamide. Some general aspects including the exfoliating process and characterization, a plausible delaminating mechanism, and future challenges, are presented and discussed.

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)

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.

Increased optical damage resistance in Sc2O3‐doped LiNbO3

The optical damage of 1 mol % Sc2O3‐doped LiNbO3 was approximately two times smaller than an optical grade LiNbO3 measured as a function of Ar+ (λ=488 nm) irradiation time. Severe Ar+ beam distortion observed in the undoped LiNbO3 was not present in the Sc2O3‐doped LiNbO3. There was negligible shift in the OH− absorption band but a 10 nm blue shift was observed in the absorption edge, indicating that Sc3+ and Mg2+ incorporation may proceed by a different mechanism. This is the first of reported results, to the authors’ knowledge, of a trivalent dopant increasing the damage resistance level in LiNbO3.

New Layered Rare‐Earth Hydroxides with Anion‐Exchange Properties

We report the synthesis of a new series of layered hydroxides based on rare-earth elements with a composition of RE(OH)2.5Cl(0.5).0.8 H2O (RE: Eu, Tb, etc.) through the homogeneous precipitation of RECl3.x H2O with hexamethylenetetramine (HMT). Rietveld analysis combined with direct methods revealed an orthorhombic layered structure comprising a positively charged layer of [RE(OH)2.5-(H2O)0.8]0.5+ and interlayer Cl- ions. The Cl- ions were readily exchangeable for various anions (NO3-, SO4(2-), dodecylsulfonate, etc.) at ambient temperature. Photoluminescence studies showed that the compounds display typical RE3+ emission. With rare-earth-based host layers and tunable interlayer guests, the new compounds may be of interest for optoelectronic, magnetic, catalytic, and biomedical materials.

Adsorption of dodecyl- and octadecyltrimethylammonium ions on a smectite and synthetic micas

Adsorption of n-alkyltrimethylammonium ions with alkyl chain lengths of C12 and C18 on the natural Kunipia F smectite (KN) as well as on synthetic fluorotetrasilicic mica (TSM) and fluorotaeniolite (TN) whose CECs are 119, 170 and 215 meq/100 g, respectively, was studied. The C and N contents were determined and thermogravimetric and XRD analyses were carried out. When concentration of alkylammonium cations changed between 0.25 and 1.0 CEC, the maximum amount of cations adsorbed on the KN smectite was close to its CEC. In contrast, only part of inorganic (sodium or lithium) ions in the interlayer spaces of the TSM and TN fluoromicas were replaced by the organic ions, even if an excess of the surfactants was used. However, at a high concentration of the C18 salt, the salt molecules were additionally adsorbed on the silicates, resisting washing with water but being removable with ethanol. The XRD patterns of the organo-smectites showed that the differences in the charge densities of the KN layers are low. In contrast, TSM and TN are built of layers with various layer charges. Thus, the alkylammonium-exchanged micas represent polyphase systems consisting of regularly and randomly interstratified layers differing in the amount of alkylammonium cations adsorbed. A part of the interlayers presumably contain both organic and inorganic ions with spacings controlled by the arrangements of the organic ions.

Crystal chemistry of hexaaluminates: β-alumina and magnetoplumbite structures

Abstract Hexagonal aluminates are known to have a layer structure composed of spinel blocks and conduction layers which are stacked alternately. The structural parameters are influenced by the large cations in the conduction layer. Two typical types of hexagonal aluminates, β-alumina and magnetoplumbite, are studied and reviewed from this point of view. The conclusions are that the structure type of hexaaluminates is determined by the charge and radius of the large cations in the conduction layer, and that the conduction layer thickness decreases as the radii of the large cations in the conduction layer decreases and as the population increases. The spinel block thickness increases according to the increase in the amount of Al 3+ defect within the spinel block.

Water-Swellable MgAl−LDH (Layered Double Hydroxide) Hybrids: Synthesis, Characterization, and Film Preparation

LDH hybrids were synthesized from Cl (-)MgAl-LDHs by anion exchange with short-chain alkyl carboxylate intercalants: C n H 2 n+1 COO (-) ( n = 0-3). Among them, LDH3 (LDH with Mg/Al = 3) hybrids containing acetate ( n = 1) and propionate ( n = 2) exhibited swelling behavior in water. The action of water on acetate-LDH3 (AcO-LDH3) and propionate-LDH3 (PrO-LDH3) led to semitransparent suspensions via a viscous gel state. From the X-ray diffraction profiles of the gels, only a broad feature was observed by the loss of the sharp reflections. The reflections reappeared for the films obtained by drying the gel, indicating the restacking of the LDH nanosheets into the original stacked structure. Observation using atomic force microscopy revealed delaminated nanosheets with a thickness of 1.1-1.5 nm with the same morphological features as the starting LDHs. XRD measurement and AFM observation supported the formation of unilamellar LDH sheets. Semitransparent self-standing LDH films were obtained by peeling off the films formed on a PE (polyethylene) substrate by drying the colloidal suspension thereon. The thickness of the obtained flexible films ranged from 10 to 25 microm, and they could be anion exchanged with inorganic and organic anions in the film state.

The crystal structure of barium hexaaluminate phase I (barium β-alumina)

Abstract The crystal structure of barium hexaaluminate phase I (Ba0.79Al10.9O17.14) was determined by single crystal X-ray reflection data. The refinements were carried out by the least-square method to give a final R-value of 0.023. The structure was revealed to be essentially of a β-alumina type and the Ba ion was detected only at the 6h site near the Beevers-Ross site (2d site). The charge compensation for nonstoichiometry was found to be principally effected by the interstitial oxygen due to Frenkel defects of Al ions. From the structural point of view, phase I was referred to as “barium β-alumina.”

The aggregation of methylene blue in montmorillonite dispersions

Abstract Theories concerning the optical properties of cationic dyes adsorbed on clay surfaces are analysed in detail. An investigation of the aggregation of methylene blue (MB) in montmorillonite dispersions is conducted using visible (VIS) spectroscopy. The effects of the dye/ clay ratio and of the swelling properties of the montmorillonite substrate on dye aggregation are compared in terms of the effect of clay layer charge. The observed influence on dye aggregation was almost negligible for both swelling and dye loading. The layer charge of the silicate determines the extent and the type of dye aggregation in freshly prepared MB/montmorillonite dispersions. Observed spectral changes with time indicate a rearrangement and redistribution of dye H-aggregates (band close to 570 nm) to monomers (660 nm), dimers (605 nm) and J-aggregates (760 nm). Dye aggregates are probably already formed during dye cation migration in the vicinity of clay colloid particles. The extent and the type of initially formed species are probably affected by the electric double layer of clay layers. After reaching the clay surface, dye cation assemblies are rearranged and decompose as described above. Reaching chemical equilibrium, dye cations adjust the distribution of the layer charge, in order that each cation could balance the charge due to one unequivalent substitution.