Fluorite-related phases Ln3MO7, Ln = rare earth,Y, or Sc, M = Nb,Sb, or Ta. I. Crystal chemistry

Compounds Ln₃MO₇, where Ln = La, Nd, Gd, Ho, Er, Y, or Sc, and M = Nb, Ta, or Sb have been examined by powder X-ray diffraction, electron diffraction, and electron microscopy. For large Ln cations, an orthorhombic fluorite-related superstructure is formed, of probable space group Cmcm for Ln = La and C222₁ for Ln = Nd, Gd, Ho, or Y, while for the smaller Ln cations, Er, and under some conditions, Ho and Y, the structure is defect fluorite containing microdomains of ordered, but undetermined, structure. The composition Sc₃MO₇ was not single phase under the conditions used. Compounds of the type Ln₂ScMO₇ have the pyrochlore structure.     

Long- and short-range order in stuffed titanate pyrochlores

We report a structural study of the stuffed pyrochlore series Ln₂(Ti_2−xLn_x)O_7−x/2 (Ln=Ho, Yb; 0?x?0.67). Electron microscopy and Rietveld refinements of neutron powder diffraction data for the x=0.67 end members, Ho₂TiO₅ and Yb₂TiO₅, reveal that small domains (∼50 Å or less) exist where the Ln and Ti/Ln sublattices are pyrochlore like, while the average structure is fluorite like. Both the Ho and Yb stuffed pyrochlore series for 0.1?x?0.5 are shown to be a composite of long- and short-range-ordered pyrochlore phases. The relative fraction of long-range vs. short-range pyrochlore order decreases with increasing Ln doping. An additional complex structural modulation of the pyrochlore structure is observed in electron diffraction and high-resolution electron microscopy images.     

Ab initio study on structure and phase transition of A- and B-type rare-earth sesquioxides Ln2O3 (Ln=La-Lu, Y, and Sc) based on density function theory

Ab initio energetic calculations based on the density functional theory (DFT) and projector augmented wave (PAW) pseudo-potentials method were performanced to determine the crystal structural parameters and phase transition data of the polymorphic rare-earth sesquioxides Ln₂O₃ (where Ln=La-Lu, Y, and Sc) with A-type (hexagonal) and B-type (monoclinic) configurations at ground state. The calculated results agree well with the limited experimental data and the critically assessed results. A set of systematic and self-consistent crystal structural parameters, energies and pressures of the phase transition were established for the whole series of the A- and B-type rare-earth sesquioxides Ln₂O₃. With the increase of the atomic number, the ionic radii of rare-earth elements Ln and the volumes of the sesquioxides Ln₂O₃ reflect the so-called “lanthanide contraction”. With the increase of the Ln³⁺-cation radius, the bulk modulus of Ln₂O₃ decreases and the polymorphic structures show a degenerative tendency.     

A structural study of ternary lanthanide orthoscandate perovskites

Ternary lanthanide scandates (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho) have been synthesized at ambient pressure. Their structure has been investigated at room temperature by Rietveld analysis of powder X-ray diffraction data. The Ln-scandates are orthorhombic perovskites, adopting space group Pbnm (? 62), a≈b≈√2a_p, c≈2a_p, Z=4. Heavy lanthanides (Er-Lu), and Y do not form perovskites at ambient conditions. Compositionally driven phase transitions were not observed. The unit-cell parameters decrease with increasing ScO₆ octahedron rotation and atomic number of the Ln cation. In common with lanthanide orthoferrites, the uniform structural evolution is interrupted at the middle-heavy part of the lanthanide sequence. This is probably due to an interplay between: (i) enlargement of the ScO₆ octahedra relative to BO₆ in other perovskites (e.g., FeO₆ in GdFeO₃); (ii) reduction in size of the first coordination sphere of Ln³⁺ coincident with the lanthanide contraction; (iii) coincident expansion of the second coordination sphere due to screening effects of O^I1 on O^I2, and entry of Sc to the lanthanide coordination sphere; (iv) complex mixing between oxygen and lanthanide lanthanide f- and scandium d-orbitals. In the series studied, Ln³⁺ are in eight-fold coordination (tetragonal antiprism), and are considerably displaced from the center of the LnO₈ polyhedron along [001]. Evolution of the crystallochemical characteristics through the Ln orthoscandate series is complex due to both the antipathetic distortions of A- and B-site coordination polyhedra and interaction of the orbitals of oxygen, Ln and Sc. Empirically obtained limits of Goldschmidt and observed ^viiit_o tolerance factors for ternary LnBO₃ compounds adopting the Pbnm structure are 0.795 and 0.841, respectively.     

Substitutional solid solutions of bismuth-containing lanthanide dititanates,Ln2−xBixTi2O7

The formation of solid solutions Ln_2?xBi_xTi₂O₇, where Ln = La to Lu and Y, except Ce, Pm, and Eu, has been studied by Raman spectroscopy and to a lesser extent by X-ray diffraction. It has been established that the solubility of bismuth increases with decreasing ionic radius of the lanthanide element. No evidence was experimentally found in this work for the existence of Bi₂Ti₂O₇.     

Lanthanum pyrochlores and the effect of yttrium addition in the systems La2−xYxZr2O7 and La2−xYxHf2O7

The crystal structures of the compounds La_2−xY_xZr₂O₇ and La_2−xY_xHf₂O₇ with x=0.0, 0.4, 0.8, 1.2, 1.6, and 2.0 have been studied using neutron powder diffraction and electron microscopy to determine the stability fields of the pyrochlore and fluorite solid solutions. The limits of pyrochlore stability in these solid solutions are found to be close to La_0.8Y_1.2Zr₂O₇ and La_0.4Y_1.6Hf₂O₇, respectively. In both systems the unit cell parameter is found to vary linearly with Y content across those compositions where the pyrochlore phase is stable, as does the x-coordinate of the oxygen atoms on the 48f (x,,) sites. In both systems, linear extrapolations of the pyrochlore data suggest that the disordering is accompanied by a small decrease in the lattice parameter of approximately 0.4%. After the pyrochlore solid solution limit is reached, a sharp change is observed from x∼0.41 to 0.375 as the disordered defect fluorite structure is favoured. Electron diffraction patterns illustrate that some short-range order remains in the disordered defect fluorite phases.     

Cationic (metallic) “Memory” in double condensed phosphates: Metals Ln-oxides Ln<Subscript>2</Subscript>O<Subscript>3</Subscript>-phosphates MLn(PO)<Subscript>3</Subscript>)<Subscript>4</Subscript> and MLnP<Subscript>4</Subscript>O<Subscript>12</Subscript>

The cationic networks that fix the distribution of cations in planar sections parallel to basis planes of the unit cell of crystal structures have been studied. Topologically identical cationic networks have been shown to be the carriers of deep structure-forming “memory” that successively relates the structures of rare earth metals (La ST) and oxides Ln₂O₃ (A-and B-Ln₂O₃ ST) to the structures of double condensed phosphates MLn(PO₃)₄ and MLnP₄O₁₂.     

LnSrScO4 (Ln=La, Ce, Pr, Nd and Sm) systems and structure correlations for A2BO4 (K2NiF4) structure types

The compounds LnSrScO₄, where Ln=La, Ce, Pr, Nd and Sm, have been synthesized. Rietveld profile analysis of powder X-ray diffraction data collected at room temperature reveal that the compounds possess a modified K₂NiF₄-type structure with orthorhombic cell symmetry formed by tilting of the ScO₆ octahedra. Variable temperature (25-1200 °C) powder X-ray diffraction data show that at the highest temperatures the structures of LaSrScO₄ and PrSrScO₄ transform to the regular tetragonal K₂NiF₄-structure type but the degree of orthorhombicity (c/a) in the unit cells initially increases on heating for all materials, reaching a maximum near 300 °C. This structural behavior is analyzed in terms of relative ionic radii of the various lanthanides and scandium. A general structural model based on tolerance factors has been developed for the family of materials A₂BO₄ with various A and B cation sizes.     

Preparation, characterization, magnetic susceptibility (Eu, Gd and Sm) and XPS studies of Ln2ZrTiO7 (Ln=La, Eu, Dy and Gd)

Bulk and nanosized pyrochlore materials Ln₂ZrTiO₇ (Ln=La, Eu, Dy, Gd and Sm) have been prepared by the sol-gel method. All the samples were characterized by powder X-ray diffraction, Raman and X-ray photoelectron spectroscopy. Magnetic susceptibility (χ) measurements of Gd₂ZrTiO₇, Sm₂ZrTiO₇ and Eu₂ZrTiO₇ were carried out by vibrating sample magnetometer in the temperature range 2-320 K. The variation of χ⁻¹ (or χ) with temperature of Gd₂ZrTiO₇, Sm₂ZrTiO₇ and Eu₂ZrTiO₇ follows the Curie law, intermediate formula and the Curie-Weiss law, respectively. From the linear portion of χT vs. T⁻¹ plot of Eu₂ZrTiO₇ from 2 to 15 K, the classical nearest neighbor exchange (J^cl) and dipolar interactions (D_nn) are obtained. The XPS of Ln₂ZrTiO₇ (Ln=La, Eu, Dy and Gd) gave characteristic peaks for Ln, Ti, Zr and O. The satellite peaks are observed only for 3d La of La₂ZrTiO₇.     

Synthesis and structural studies of lanthanide substituted bismuth-titanium pyrochlores

The identity of the pyrochlore phase seen during the synthesis of ferroelectric Bi_4−xLn_xTi₃O₁₂ Aurivillius oxides is shown to be Bi_2/3Ln_4/3Ti₂O₇. This pyrochlore is only stable for Ln³⁺=Sm³⁺ or smaller. For larger lanthanides the layered Aurivillius oxide is favoured. The presence of six-fold disorder, associated with the Bi 6s² lone pair electrons, is believed to stabilise the unexpected stoichiometry of this oxide. Precise structures, obtained by Rietveld refinement from synchrotron X-ray diffraction data, of three examples Ln³⁺=Eu, Ho and Yb are presented.     

Synthesis and structural study of the new rare earth magnesium borates LnMgB5O10 (Ln = La, …, Er)

To obtain rare earth luminescent materials with weak concentration quenching, the B₂O₃-rich portion of the ternary diagram Ln₂O₃MgOB₂O₃ (Ln = rare earth) has been investigated. A ternary phase of composition LnMgB₅O₁₀ has been found for Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er. These compounds all crystallize in the monoclinic space group $\text{P2}{}_1\text{c}$. The structure has been determined on a LaMgB₅O₁₀ crystal. A full-matrix least-squares refinement leads to R = 0.039. The structure can be described as being made of (B₅O₁₀^5?)_n two-dimensional layers linked together by the lanthanum and magnesium ions. The rare earth atom coordination polyhedra form isolated chains. These borates are isostructural with some rare earth cobalt borates.     

Lanthanide complexes of the monovacant Dawson polyoxotungstate [α2-As2W17O61] with 1D chain: Synthesis, structures, and photoluminescence properties

Six new lanthanide complexes, (H₃O)[Ln₃(H₂O)₁₇(α₂-As₂W₁₇O₆₁)]·nH₂O ((1) Ln=Ce^III and n≈13; (2) Ln=Pr^III and n≈9; (3) Ln=Nd^III and n≈14; (4) Ln=Sm^III and n≈8; (5) Ln=Eu^III and n≈4; (6) Ln=Gd^III and n≈7), have been isolated by conventional solution method and characterized by elemental analysis, IR spectroscopy and single crystal X-ray diffraction. All the complexes are isomorphic and crystallize in the triclinic space group P-1. These complexes are 1D chain-like structures constructed by lanthanide cations and monovacant Dawson-type [α₂-As₂W₁₇O₆₁]¹⁰⁻ polyoxoanions. The striking feature of the structures is that there are three kinds of coordination environments for lanthanide cations, which are responsible for the formation of polymeric structures. Photoluminescence measurements reveal that 4 and 5 exhibit orange and red fluorescent emission at room temperature, respectively.     

Crystal growth, structure and magnetic properties of the double perovskites Ln2MgIrO6 (Ln=Pr, Nd, Sm-Gd)

Single crystals of double-perovskite type lanthanide magnesium iridium oxides, Ln₂MgIrO₆ (Ln=Pr, Nd, Sm-Gd) have been grown in a molten potassium hydroxide flux. The compounds crystallize in a distorted 1:1 rock salt lattice, space group P2₁/n, consisting of corner shared MO₆ (M=Mg²⁺ and Ir⁴⁺) octahedra, where the rare earth cations occupy the eight-fold coordination sites formed by the corner shared octahedra. Pr₂MgIrO₆, Nd₂MgIrO₆, Sm₂MgIrO₆, and Eu₂MgIrO₆ order antiferromagnetically around 10-15 K.     

Synthesis and Crystal Structure of Commensurate Polymorph of Ln4AlCu2B9O23 (Ln=Lu,Ho) and Refinement of Cu2Al6B4O17

Single crystals of new Cu,Lu(Ho)–alumoborate and known Cu,Al–borate were synthesized through reaction between CuB₂O₄ and LnBO₃ on the Al₂O₃ surface by annealing at 1100 °C. Structure of commensurate modification of Ln₄AlCu₂B₉O₂₃, (Ln = Lu,Ho), sp. gr. , was solved at room temperature. It was found that a low–temperature (110 K) modification possesses incommensurate modulations with modulation vector q =(0, 0, 0.132). The nonaborate block – [B₉O₂₃]^19– – 9[6T+3Δ] forms an isolated unique dense closed anionic unit. This block is terminated by Al–tetrahedrons in the chessboard pattern, resulting in formation of complex alumoborate layer [AlB₉O₂₃]^16–. Apical oxygen of central BO₃ triangle of the nonaborate block seems to be the source of modulations observed in low temperature polymorph. Cationic layers with the Ln and Cu atoms are alternating along c axis with anionic layers. The structure Cu₂Al₆B₄O₁₇, previously studied by the Rietveld method, was corroborated by single crystal data and was compared with LiAl₇B₄O₁₇.     

O MAS NMR study of the oxygen local environments in the anionic conductors Y2(B1−xB′x)2O7(B, B′=Sn, Ti, Zr)

The local environments for oxygen in yttrium-containing pyrochlores and fluorites, Y₂(B_1−xB′_x)₂O₇ (B=Ti, B′=Sn, Zr) are investigated by using solid state ¹⁷O MAS NMR spectroscopy. The quadrupolar coupling constants of the nucleus, ¹⁷O are sufficiently small for these ionic oxides, that high-resolution spectra are obtained from the MAS spectra. Different oxygen NMR resonances are observed due to local environments with differing numbers of metal cations (Y³⁺, Sn⁴⁺, Ti⁴⁺ and Zr⁴⁺), allowing the numbers of different local environments to be quantified and cation mixing to be investigated. Evidence for pyrochlore-like local ordering is detected for Y₂Zr₂O₇, which nominally adopts the fluorite structure.     

K3Ln[OB(OH)2]2[HOPO3]2 (Ln=Yb, Lu): Layered rare-earth dihydrogen borate monohydrogen phosphates

Two isotypic layered rare-earth borate phosphates, K₃Ln[OB(OH)₂]₂[HOPO₃]₂ (Ln=Yb, Lu), were synthesized hydrothermally and the crystal structures were determined by single-crystal X-ray diffraction (R3?, Z=3, Yb: a=5.6809(2) Å, c=36.594(5) Å, V=1022.8(2) ų, Lu: a=5.6668(2) Å, c=36.692(2) Å, V=1020.4(1) ų). The crystal structure can be described in terms of stacking of Glaserite-type slabs consisting of LnO₆ octahedra interlinked by phosphate tetrahedra and additional layers of [OB(OH)₂]^- separated by K⁺ ions. Field and temperature dependent measurements of the magnetic susceptibility of the Yb-compound revealed Curie-Weiss paramagnetic behavior above 120 K (μ_eff=4.7 μ_B). Magnetic ordering was not observed down to 1.8 K.     

Crystal structure and magnetism of layered Ln2Ca2MnNiO8 (Ln=Pr,Nd, Sm,and Gd) compounds

We report the syntheses, crystal structure, and magnetic properties of a series of distorted K₂NiF₄-type oxides Ln₂Ca₂MnNiO₈ (Ln=Pr, Nd, Sm, and Gd) in which Ln/Ca and Mn/Ni atoms randomly occupy the K and Ni sites respectively. The Ln=La compound does not form. These compounds show systematic distortions from the ideal tetragonal K₂NiF₄ structure (space group I4/mmm) to an orthorhombic structure (space group Pccn) with buckled MO₂ (M=Mn/Ni) layers. The degree of distortion is increased as the size of Ln decreases. Based on the magnetic data and X-ray absorption near edge spectra, we assigned Mn^IV and Ni^II. The Curie–Weiss plots of the high temperature magnetic data suggest strong ferromagnetic interactions probably due to Mn^IV–O–Ni^II linkages, implying local ordering of Mn/Ni ions to form ferromangnetic clusters in the MO₂ layers. At low temperatures below 110–130 K, these compounds show antiferromagnetic behaviors because of Mn^IV–O–Mn^IV and/or Ni^II–O–Ni^II contacts between the ferromagnetic clusters. The Ln=Pr and Nd compounds show additional antiferromagnetic signals that we attribute to the interlayer interactions between the clusters mediated by the Pr³⁺ and Nd³⁺ ions in the interlayer spaces. The present compounds show many parallels with the previously reported Ln₂Sr₂MnNiO₈ compounds.     

Stuffed rare earth pyrochlore solid solutions

Synthesis and crystal structures are described for the compounds Ln₂(Ti_2−xLn_x)O_7−x/2, where Ln=Tb, Dy, Ho, Er, Tm, Yb, Lu, and x ranges from 0 to 0.67. Rietveld refinements of X-ray powder diffraction data indicate that in the Tb and Dy titanate pyrochlores, the extra Ln³⁺ cations mix mainly on the Ti⁴⁺ site with little disorder on the original Ln³⁺ site. For the smaller rare earths (Ho-Lu), stuffing additional lanthanide ions results in a pyrochlore to defect fluorite transition, where the Ln³⁺ and Ti⁴⁺ ions become completely randomized at the maximum (x=0.67). Initial magnetic characterization for the fully stuffed x=0.67 samples for Ln=Tb-Yb shows no long range ordering down to 2 K, and only partial saturation of the full expected magnetic moment under applied fields up to 5 T. In all of these Ln-Ti-O pyrochlores, the addition of magnetic Ln³⁺ in place of non-magnetic Ti⁴⁺ adds edge sharing tetrahedral spin interactions to a normally corner sharing tetrahedral network of spins. The increase in spin connectivity in this family of solid solutions represents a new avenue for investigating geometrical magnetic frustration in the rare earth titanate pyrochlores.     

Darstellung,Kristallstrukturen und Eigenschaften von Ln4Au2O9 (Ln = Nd,Sm, Eu)

Syntheses, Crystal Structures, and Properties of Ln₄Au₂O₉ (Ln = Nd, Sm, Eu) The compounds Ln₄Au₂O₉ (Ln = Nd, Sm, Eu) have been prepared from amorphous Au₂O₃ · 2–3 H₂O and Ln₂O₃ (Ln = Nd, Sm, Eu) via solid state reaction under elevated oxygen pressure adding KOH as mineralising agent. They are isostructural with La₄Au₂O₉ (Nd₄Au₂O₉: a = 11.9813(3), b = 6.1474(1), c = 11.9641(4); 453 powder intensities, R_p = 3.75%; Sm₄Au₂O₉: a = 11.8689(4), b = 6.0360(1), c = 11.8469(4) Å; 812 unique reflections, R₁ = 2.75%; Eu₄Au₂O₉: a = 11.8241(3), b = 5.9922(1) Å, c = 11.8013(3) Å; 1315 unique reflections, R₁ = 7.83%). The crystal structure of Nd₄Au₂O₉ was refined from powder diffraction data. The structures of Sm₄Au₂O₉ and Eu₄Au₂O₉ were solved and refined from single crystal data. The isolated square planar AuO₄ units are stacked as columns and are linked to each other by LnO₇‐polyhedra. One of the oxygen atoms is exclusively connected to the trivalent lanthanides in tetrahedral geometry. Ln₄Au₂O₉, Bi₂CuO₄, Bi₂AuO₅ and Bi₄Au₂O₉ are closely related, structurally. The lanthanoid aurates decompose between 700 and 800 °C into Ln₂O₃, Au and O₂. The effective magnetic moments 3.64 μ_B (Nd₄Au₂O₉), 1.7 μ_B (Sm₄Au₂O₉) and 3.3 μ_B (Eu₄Au₂O₉) confirm that the lanthanides are trivalent. The UV/VIS absorption spectra can be interpreted at assuming free ions.     

Fluorite-related phases Ln3MO7, Ln = Rare-earth,Y or Sc, M = Nb,Sb or Ta. III. Structure of the non-stoichiometric Y3TaO7 phase

The orthorhombic fluorite-related superstructure phase Y₃TaO₇ is non-stoichiometric; the Y₂O₃ content may be varied from 75.0 to about 72.5 mole% without incurring structural changes. For overall Y₂O₃ contents from 72.0 to 70.5 mole%, the crystal symmetry changes from C222₁ to Cmmm, and the c axis becomes halved. The structure of the low-Y₂O₃ material has been determined from powder X-ray diffraction intensities supplemented by single crystal electron-optical data. The relationship of this structure to that of stoichiometric Y₃TaO₇ is discussed.