Cationic networks in the structures of rare earth germanates and borogermanates: Typology of cationic networks in the structures of rare earth compounds

The cationic networks in the structure of binary and mixed-anion compounds forming in the Ln₂O₃-B₂O₃-GeO₂ systems have been studied. The collection of the known cationic networks consisting of two dominating pure forms, anisotropic and isotropic, as well as of the combined anisotropic-isotropic and multivector variety of anisotropic cationic networks, was enlarged by two new types of isotropic cationic networks, namely, combined isotropic-isotropic (in the Nd₄GeO₈ structure) and imperfect isotropic (in the structure of borogermanates Ln₁₄ (BO₆)₆ (GeO₄)2O(₈).     

Cationic networks in the structures of borates,tungstates, and borotungstates forming in the Ln<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-WO<Subscript>3</Subscript> systems

The influence of the structures of binary compounds on the formation of ternary mixed-anion compounds—borotungstates—in ternary Ln₂O₃-B₂O₃-WO₃ systems was studied. The structures of borotungstates either inherit the predominant type of cationic network from the structures of the binary compounds (anisotropic cationic networks in Ln (BO₂)(WO₄)) or represent a sum of equivalent anisotropic and isotropic cationic networks, forming combined cationic networks (in Ln₃BWO₉). In the lanthanide series, the ranges of existence of borotungstates coincide with the ranges of existence of the structure types most abundant in the binary compounds.     

Triangulation in the Li2O-Ln2O3-B2O3 (Ln = Nd, Eu, Dy, Yb, and Y) systems and the melting character of ternary compounds

Li₂O-Ln₂O₃-B₂O₃ (Ln = Nd, Eu, Dy, Yb, and Y) ternary systems were studied along their inner sections. Two types of ternary compounds were found: Li₃Ln₂(BO₃)₃ (Ln = Nd, Eu, Dy, and Yb) and Li₆Ln(BO₃)₃ (Ln = Dy and Yb). The systems were triangulated. Melts were chosen for growing single crystals of ternary compounds in multinary systems. Original Russian Text ? Sh.A. Gamidova, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 1, pp. 142–145.     

Structure of the La12GdEuB6Ge2O34 borogermanate as probed by NMR and IR spectroscopy

The structure of fine crystalline borogermanate La₁₂GdEuB₆Ge₂O₃₄ has been studied by NMR and IR spectroscopy. It has been demonstrated that this compound is isostructural to the homonuclear Ln₁₄B₆Ge₂O₃₄ compounds (Ln = Pr-Gd) and crystallizes in space group P3₁. The rare-earth elements have been distributed over the LnO _n polyhedra in La₁₂GdEuB₆Ge₂O₃₄ by analogy with the known structures. Lanthanum can occupy positions with CN 7–10, and the symmetry of these LnO _n coordination polyhedra is not higher than C _2v . In the La₁₂GdEuB₆Ge₂O₃₄ structure, the LnO _n coordination polyhedra are formed by oxygen atoms of oxo groups and anions, some of the oxygen atoms being shared by LnO _n polyhedra. The BO₃ and GeO₄ groups in the structure are also bridging, i.e., are involved in bonding of LnO _n polyhedra. One of the B-O bonds in La₁₂EuGd(BO₃)₆(GeO₄)₂O₈ is elongated as compared with the B-O bond lengths in homonuclear compounds Pr₁₄(BO₃)₆(GeO₄)₂O₈ and Nd₁₄(BO₃)₆(GeO₄)₂O₈. In the La₁₂GdEuB₆Ge₂O₃₄ structure, germanium is located in isolated GeO₄ tetrahedra with distorted T _d symmetry. The local symmetry of lanthanum in fine crystalline La₁₂GdEuB₆Ge₂O₃₄ have been assessed using ¹³⁹La NMR (B ₀ = 7.04 T, room temperature). For comparison, binary lanthanum compounds with a simpler structure— LaBO₃, La(BO₂)₃, and La₂GeO₅—have been used. The spectra of all compounds are rather broad (ν_1/2 = 180–240 kHz). The ¹³⁹La NMR spectra of the LaBO₃, La(BO₂)₃, and La₁₂GdEu(BO₃)₆(GeO₄)₂O₈ borates show a signal at (1080 ± 40) ppm, which is absent in the spectrum of La₂GeO₅. The shape of the ¹³⁹La NMR spectra of La₁₂GdEu(BO₃)₆(GeO₄)₂O₈ and LaBO₃ is characterized by the second-order quadrupole splitting with a downfield shoulder. The similarity of these spectra points to close ¹³⁹La NMR chemical shifts of La₁₂GdEu(BO₃)₆(GeO₄)₂O₈ and LaBO₃. No quadrupole splitting was observed in the spectra of La(BO₂)₃ and La₂GeO₅.     

New solid electrolytes of the pyrochlore family

The class of oxygen-ion-conducting rare-earth pyrochlores has been considerably extended. New solid electrolytes, Ln₂Ti₂O₇ (Ln = Dy-Lu) and Ln₂Hf₂O₇ (Ln = Eu, Gd) pyrochlores, are intrinsic ionic conductors at elevated temperatures, as are the well known Ln₂Zr₂O₇ (Ln = Sm-Gd) zirconates, which suggests that oxygen ion conduction in the rare-earth pyrochlore family has a general character. The thermodynamic order-disorder transitions that yield a PII cation- and anion-disordered pyrochlore phase possessing high oxygen ion conductivity occur throughout the rare-earth pyrochlore family: Ln₂M₂O₇ (Ln = Sm-Lu; M = Ti, Zr, Hf). The composition-structure-oxygen-ionic conductivity relationship is analyzed for Ln₂(M_{2 − x} Ln _x )O_{7 − δ} (Ln = Sm-Lu; M = Ti, Zr, Hf) with x from 0 to 0.81.     

Crystal chemical aspect of synthesis of laser crystals with garnet structure in Ln2O3−Sc2O3−M2O3 systems (Ln=Y,Gd; M=Ga,Al)

Regions of existence were determined for various types of poly- and monocrystalline solid solutions (Ln₃[Sc_yM_2−y]M₃O₁₂; {Ln_3−xSc_x}[Sc_yM_2−y]M₃O₁₂; Ln₃[Ln_zSc_yM_2−y−z] M₃O₁₂; Ln=Y, Gd; M=Ga, Al) by analyzing the diagrams r^VIII−r^VI (r^VI are weighted mean dodecahedral and octahedral radii, respectively). We found the position of congruently melting compositions in r^VIII−r^VI coordinates and optimal compositions for obtaining Nd³⁺- and Cr³⁺-doped crystals. The structure of the congruently melting composition was found to be formed of “equilibrium” polyhedra, which need not be stabilized. It is shown that a congruently melting composition, which is absent in the original matrix, may be achieved by isomorphous substitutions at certain positions of the structure. The most probable mechanisms of formation of poly- and monocrystalline solid solutions with garnet structure are suggested using the calculated binodal curves of decomposition. M. V. Lomonosov Moscow Academy of Fine Chemical Technology. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 5, pp. 23–33, September–October, 1994. Translated by O. Kharlamova     

Enantioselective Self‐Assembly of Triangular Dy3 Clusters with Single‐Molecule Magnet Behavior

Three pairs of enantiopure chiral triangular Ln₃ clusters, [Ln₃L_{RRRRRR/SSSSSS}(μ₃‐OH)₂(H₂O)₂(SCN)₄]?xCH₃OH?yH₂O ( R ‐Dy₃ , Ln=Dy, x=6, y=0; S ‐Dy₃ , Ln=Dy, x=6, y=1; R ‐Ho₃ , Ln=Ho, x=6, y=1; S ‐Ho₃ , Ln=Ho, x=6, y=1; R ‐Er₃ , Ln=Er, x=6, y=0; S ‐Er₃ , Ln=Er, x=6, y=1), have been successfully synthesized by a rational enantioselective synthetic strategy. The core of triangular Ln₃ is bound in the central N₆O₃ of the macrocyclic ligand, and the coordination spheres of Ln ions are completed by four SCN^? anions and two H₂O molecules in axial positions of the macrocycle. The circular dichroism (CD) and vibrational circular dichroism (VCD) spectra of the enantiomers demonstrate that the chirality is successfully transferred from the ligands to the resulting Ln₃ clusters. Ac susceptibility measurements reveal that single‐molecule magnet behavior occurs for both enantiopure clusters of R ‐Dy₃ and S ‐Dy₃ . This work is one of the few examples of the successful design of a pair of triangular Dy₃ clusters showing simultaneously slow magnetic relaxation and optical activity, and this might open up new opportunities to develop novel multifunctional materials.     

Heavy lanthanide phosphates: Nanostructuring during their solid-phase synthesis

Specific features of the textures (the preferred orientation of the nanometer building blocks) of the cationic and anionic components of the structures of phosphates forming in the Ln₂O₃-P₂O₅ systems (Ln = Er-Lu) have been studied. Nanostructuring upon the formation of phosphates is determined by two oppositely directed processes of fragmentation of corresponding infinite cationic and anionic frameworks to elementary blocks, LnO_n polyhedra and [PO₄] tetrahedra.     

2D l‐Di‐toluoyl‐tartaric acid Lanthanide Coordination Polymers: Toward Single‐component White‐Light and NIR Luminescent Materials

A series of five l ‐di‐p‐toluoyl‐tartaric acid (l ‐DTTA) lanthanide coordination polymers, namely {[Ln₄K⁴ L₆(H₂O)_x]?yH₂O}_n, [Ln=Dy ( 1 ), x=24, y=12; Ln=Ho ( 2 ), x=23, y=12; Ln=Er ( 3 ), x=24, y=12; Ln=Yb ( 4 ), x=24, y=11; Ln=Lu ( 5 ), x=24, y=12] have been isolated by simple reactions of H₂L (H₂L= L ‐DTTA) with LnCl₃?6 H₂O at ambient temperature. X‐ray crystallographic analysis reveals that complexes 1 – 5 feature two‐dimensional (2D) network structures in which the Ln³⁺ ions are bridged by carboxylate groups of ligands in two unique coordinated modes. Luminescent spectra demonstrate that complex 1 realizes single‐component white‐light emission, while complexes 2 – 4 exhibit a characteristic near‐infrared (NIR) luminescence in the solid state at room temperature.     

Nanostructuring in the process of formation of rare-earth tungstates

Studying the textures (the preferred orientation of the nanometer building blocks) in the structures of neodymium tungstates shows that infinite dimeric bands {[LnO_n]-[LnO_n]}-{[LnO_n]-[LnO_n]} in a fluoritelike structure field of the Nd₂O₃-WO₃ binary system undergo fragmentation and modification. The structural background (staggered arrangement of polyhedra) is determined by the like effects of both the basic oxide Ln₂O₃ (a fluorite-type structure) and the acid oxide WO₃ (a ReO₃-type structure).     

Infrared,Raman and XPS spectroscopic studies of Bi2O3–B2O3–GeO2 glasses

Glasses with composition 50Bi₂O₃–(50 ? x) B₂O₃–xGeO₂ (x = 0, 5, 10, 15 mol%) were prepared by conventional melting method. The thermal properties were investigated by differential thermal analysis (DTA) and the structures of the glasses were probed by Infrared, Raman and X-ray photoelectron spectroscopy (XPS). The results show that density, refractive index and optical basicity increase with the increase of GeO₂. The glass transition temperature (T_g), onset crystallization temperature (T_x) and ΔT (T_x ? T_g) increase as well. The cut-off edges in ultraviolet and infrared shift to longer wavelength by the addition of GeO₂. Infrared, Raman and XPS results indicate that the glass network consists of [Bi–O₆] octahedron, [BO₃] triangle, [BO₄] tetrahedron and [GeO₄] tetrahedron and borate oxide mainly exists in [BO₃] units. XPS result indicates Ge⁴⁺ ions form steady [GeO₄] tetrahedra units in the glass network and the number of non-bridging oxygens decreases with the addition of GeO₂.     

Synthesis and characterization of new borate-vanadate mixed glasses

Summary It has been found that a series of M_xO_y-V₂O₅-B₂O₃glasses (M_xO_y=Li₂O, Na₂O, K₂O and MgO) containing 10 mol%<span style='font-size:12.0pt;font-family: Symbol;mso-bidi-font-family:Symbol'>a-Fe₂O₃exhibited glass-forming regions that shifted with the content of network modifier (NWM) compared to B₂O₃and V₂O₅glasses. The M?ssbauer spectra of a series of M_xO_y-V₂O₅-B₂O₃glasses showed increased quadrupole splitting (D) with increasing NWM content. This suggests that the coordination numbers of the V⁴⁺and V⁵⁺are fixed and that the formation number of non-bridging oxygens (NBO) is considered to increase with increasing NWM content, and with increasing formation number of NBO, the Fe³⁺ion site changes from VO₄to BO₄tetrahedra. Consequently, the quadrupole splitting increases with increasing NWM content.     

Phase diagrams of the SrF2(Y,Ln)F3 systems part I.—X-ray characteristics of phases

The phase equilibria in the subsolidus part of 14 binary systems of SrF₂(Y, Ln)F₃ type (Ln—all lanthanides except Pm and Eu) were studied at temperatures over 850°C in equilibrated and quenched specimens by the X-ray analysis method. The oxygen concentration in the specimens before and after the thermal treatment was checked. The crystallographic characteristics of phases formed in the systems i.e. nonstoichiometric phases Sr_1?xLn_xF_2+x with fluorite type structure, phases with fluorite derived type structure, nonstochiometric phases Ln_1?ySr_yF_3?y with tysonite (LaF₃) type structure are given in this paper.     

Nanostructuring phenomena in oxygen-conducting complex oxides of heavy REE

In complex oxides of REE (Ln₄M₃O₁₂ (Ln = Tm, Lu; M = Zr, Hf), Ln₂TiO₅ (Ln = Er-Yb)) and Ho₂TiO₅, the following phase transitions of the order-disorder type are studied for different cooling rates: rhombohedral δ-phase-defective fluorite in Ln₄M₃O₁₂ (Ln = Tm, Lu; M = Zr, Hf), pyrochlor-like phasedefective fluoride in Ln₂TiO₅ (Ln = Er-Yb), and hexagonal β-phase-pyrochlor in Ho₂TiO₅. The presence of nanostructuring phenomena typical of fluorite-like polymorphous modifications of complex oxides in the Ln₂O₃-MO₂ (Ln = Ho-Lu; M = Ti, Zr, Hf) systems is confirmed. The conductivity of polymorphous modifications of Ln₄Zr₃O₁₂ (Ln = Tm, Lu;) and Ln₂TiO₅ (Ln = Ho-Yb) with different thermal prehistory is studied. The comparative studies of the oxygen-ionic conductivity of fluorite- and pyrochlor-like Ln₂TiO₅ (Ln = Ho-Yb), pyrochlor Ho₂TiO₅, and β-Ho₂TiO₅ and also of the conductivity of fluorite-like compounds and δ-Ln₄Zr₃O₁₂ (Ln = Tm, Lu) are carried out. The oxygen-ionic conductivity of complex oxides in the Ln₂O₃-MO₂ (Ln = Er-Lu; M = Ti, Zr, Hf) system is shown to decrease in the following series: defective pyrochlor-defective fluorite-rhombohedral δ-phase ∼ hexagonal β-phase.     

Specific features of phase equilibriums in Ln–Ba–Fe–O systems

Formation of five-layered Ln_2–εBa_3+εFe₅O_15–δ phases [exhibiting nanoscale ordering with layer-by-layer location of the cations in the Ln–Ba–(Ln,Ba)–(Ln,Ba)–Ba–Ln perovskite-type structure] has occurred in the Ln–Ba–Fe–O (Ln = Y, Pr, Nd, Sm, Eu, and Gd) systems at 1100°С in air. Partial substitution of iron with cobalt (Ln_2–εBa_3+εFe_5–yCo_yO_15–δ, Ln = Nd, Sm, Eu) has stabilized formation of the ordered structure. The oxygen content in the complex oxides has been determined in air over a wide temperature range by means of high-temperature thermogravimetry and iodometric titration. The change in oxygen content with temperature for the phases with five-layered ordering was significantly smaller than for the disordered phases.     

Phase composition, electroconductivity, oxygen ion transport number, and microhardness of Ln1 ? xSrxGa0.5 ? y/2Al0.5 ? y/2MgyO3 ? δ (Ln = La, Pr, Nd; x, y = 0.10, 0.15)

Phase composition, electroconductivity, oxygen ion transport number, and microhardness of samples of Ln_{1 − x} Sr_xGa_{0.5 − y/2}Al_{0.5 − y/2}Mg_yO_{3 − δ} (Ln = La, Pr, Nd; x, y = 0.10, 0.15) synthesized by a ceramic methods are studied. Methods of x-ray diffraction analysis and scanning electron microscopy reveal the La-containing samples to be homogeneous and have a perovskite structure. Magnesium does not dissolve in Pr-and Nd-containing systems but forms an individual phase based on magnesium oxide. Apart from magnesium oxide, in these systems there form extrinsic phases, specifically, LnSrGa₃O₇ and an unknown phase. The electroconductivity of La_{1 − x} Sr_xGa_{1 − y} Mg_yO_{3 − δ} decreases after substituting Al for Ga. Ceramic La_{1 − x} Sr_xGa_{0.5 − y/2}Al_{0.5 − y/2}Mg_yO_3−δ is a purely ionic conductor in the temperature interval 500 to 1000°C; Nd_xSr_xGa_{0.5 − y/2}Al_{0.5 − y/2}Mg_yO_{3 − δ} has predominantly ionic conduction; and the predominant type of conduction in Pr_{1 − x} Sr_xGa_{0.5 − y/2}Al_{0.5 − y/2}Mg_yO_{3 − δ} is electronic below 700–800°C, with the contribution of ionic conduction increasing at higher temperatures. Substituting Al for Ga raises the hardness of ceramics under study. Among the compositions studied, La_0.85Sr_0.15Ga_0.45Al_0.45Mg_0.10O_{3 − δ} and La_0.85Sr_0.15Ga_0.425Al_0.425Mg_0.15O_{3 − δ} exhibit a combination of electroconductivity and hardness that is optimal for application as electrolyte at reduced temperatures (600–800°C). The Pr_{1 − x} Sr_xGa_{0.5 − y/2}Al_{0.5 − y/2}Mg_yO_{3 − δ} system possesses mixed ionic-electronic conduction and high hardness, which makes it appealing for application as oxygen-penetrable membranes. Original Russian Text ? Yu.V. Danilov, A.D. Neuimin, L.A. Dunyushkina, L.A. Kuz’mina, N.S. Zybko, Z.S. Martem’yanova, A.A. Pankratov, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 1, pp. 57–65.     

Trends in the cation distribution in the structures of the compounds forming in the Ln<Subscript>2</Subscript>O<Subscript>3</Subscript>-GeO<Subscript>2</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript> systems

The cationic networks in the structures of the initial oxides and all binary and ternary compounds forming in the Ln₂O₃-GeO₂-P₂O₅ systems have been studied. In the phase diagrams of the Nd₂O₃-GeO₂-P₂O₅ and Er2O₃-GeO₂-P₂O₅ systems, the regions of the structural influence of individual compounds with topologically identical cationic networks—anisotropic (A), combined (C), and isotropic (I)—are united into common areas. The A: C: I area ratio is 1: 1: 1 in the neodymium system and 1.7: 1: 3.4 in the erbium system.     

Solid electrolytes based on CeO<Subscript>2</Subscript> for medium-temperature electrochemical devices

CeO₂-based solid solutions with a fluorite structure are promising materials as electrolytes of medium-temperature electrochemical devices: electrolytic cells, oxygen sensors, and solid oxide fuel cells. In this work, studies are presented of the effect of the dopant cation radius and its concentration on the physico-chemical properties of the Ce_{1 − x} Ln _x O_{2 − δ} solid solutions (x = 0–0.20; Ln = La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb) and also of multicomponent solid solutions of Ce_{1 − x} Ln _x/2Ln′ _x/2O_{2 − δ} (x = 0–0.20; Ln = Sm, La, Gd and Ln′ = Dy, Nd, Y) and Ce_{1 − x − y} Sm _x M _y O_{2 − δ} (M = Ca, Sr, Ba) obtained using the solid-phase synthesis technique. Electric properties of the samples were studied in the temperature range of 623–1173 K and in the oxygen partial pressure range of 0.01–10⁻²² MPa. The values of oxygen critical pressure ( p_O2 ^* )\left( {p_{O_2 }^* } \right) are presented, at which the ionic and electron conductivity values are equal. The values were calculated on the basis of experimental dependences at 1023 K at the assumption that the ionic conductivity value is determined only by the dopant concentration and its effective ionic radius and is independent of the oxygen partial pressure.     

Phase equilibria and crystal structure of the complex oxides in the Ln-Ba-Co-O (Ln=Nd, Sm) systems

The phase equilibria in the Ln-Ba-Co-O (Ln=Nd, Sm) systems were systematically studied at 1100 °C in air. The homogeneity ranges and crystal structure of the solid solutions: Ln_2−xBa_xO_3−δ (0<x≤0.1 for Ln=Nd and 0<x≤0.3 for Ln=Sm), Nd_3−yBa_yCo₂O₇ (0.70≤y≤0.80), BaCo_1−zSm_zO_3−δ (0.1≤z≤0.2) were determined by X-ray diffraction of quenched samples. The values of oxygen content (5+δ) for slowly cooled LnBaCo₂O_5+δ (Ln=Nd, Sm) samples were estimated as 5.73 for Ln=Nd, and 5.60 for Ln=Sm. The unit cell parameters were refined using Rietveld full-profile analysis. It was shown that NdBaCo₂O_5.73 possesses tetragonal structure and SmBaCo₂O_5.60 - orthorhombic structure. The projections of isothermal-isobaric phase diagrams for the Ln-Ba-Co-O (Ln=Nd, Sm) systems to the compositional triangle of metallic components were presented.     

Homogeneity regions of neodymium,samarium, and europium manganites in air

XRD phase analysis of homogeneous phases and heterogeneous compositions of general formula Ln_2?x Mn_xO_3±δ (Ln = Nd, Sm, Eu; 0.90 ≤ x ≤ 1.20; Δx = 0.22) prepared by ceramic synthesis from oxides in air at 900–1400°C was used to determine the solubility boundaries for Ln₂O₃ oxides and maganese oxides in LnMnO_3±δ. The results were represented as fragments of the phase diagrams for the Ln-Mn-O systems in air. It was assumed that the solubility of Ln₂O₃ oxides in LnMnO_3±δ is determined by lattice defects, while that of manganese oxides, in addition to above mechanism, by the disproportionation reaction 2Mn³⁺ = Mn²⁺ + Mn⁴⁺ followed by the partial substitution of divalent magnesium for Ln³⁺ at cuboctahedral positions of the perovskitelike crystal lattice.