Solubility of Lanthanide Chlorides ( Ln Cl3) in Alkali Metal Chlorides ( M Cl): Thermodynamics and Electrical Conductivity of the M 3 Ln Cl6 Compounds

Summary.  Several compounds may exist in LnCl₃–MCl mixtures. Those corresponding to the M ₂ LnCl₅ and MLn ₂Cl₇ stoichiometries are formed in a few systems only, with diverse stability strongly dependent on both the corresponding lanthanide and alkali metal. On the other hand, M ₃ LnCl₆ that occur in most systems have a far larger stability range and melt congruently. These latter compounds were investigated in the present work by differential scanning calorimetry and electrical conductivity measurements. The thermodynamic and transport properties were correlated to structural features and related to the mechanism of compound formation. Corresponding author. E-mail: Marcelle.Gaune-Escard@polytech.univ-mrs.fr Received October 2, 2002; accepted November 6, 2002 Published online April 24, 2003 RID="a" ID="a" This paper is dedicated to Professor H. Gamsj?ger on the occasion of his birthday     

The New Phosphates Ln1/3Zr2(PO4)3 (Ln = Rare Earth)

The new phases Ln_1/3 Zr₂(PO₄)₃ (Ln = Rare Earth) crystallize with the Nasicon-type structure. The rare earth is located in the usually labeled M₁ site with rather ionic Ln -O bonds. The ceramics resulting from the decomposition of these phosphates have been characterized in the case of lanthanum and europium. They exhibit a very low thermal expansion between room temperature and 1340°C.     

On the crystal structures of Ln3MO7 (Ln=Nd, Sm, Y and M=Sb, Ta)—Rietveld refinement using X-ray powder diffraction data

We have investigated, using X-ray powder diffraction data, the crystal structures of some fluorite derivatives with the formula Ln₃MO₇ (Ln=lanthanide or Y and M=Sb and Ta). In these compounds ordering of Ln and M occurs, leading to a parent structure in Cmmm. Tilting of the MO₆ octahedra causes doubling of one of the cubic axes, leading to a number of non-isomorphic subgroups, e.g. Cmcm, Ccmm and Cccm. We have identified an alternative space group Ccmm instead of C222₁ for those compounds containing a medium sized lanthanide or Y and M being Sb or Ta. Interestingly this is an alternative setting for the space group of the structure obtained when Ln is large (Cmcm). However, there tilting of the octahedra is around the a-axis of the parent structure, rather than around the b-axis as it is found in the compounds which we are reporting on here.In one compound, Nd₃TaO₇, both tilts occur. The phase transition between the two possible structures is a slow and difficult process above 80 K, allowing both phases to coexist.     

Synthesis and magnetic properties of ALnO2 (A=Cu or Ag; Ln=rare earths) with the delafossite structure

Synthesis, structures, and magnetic properties of ternary rare earth oxides ALnO₂ (A=Cu or Ag; Ln=rare earths) have been investigated. CuLnO₂ (Ln=La, Pr, Nd, Sm, Eu) were synthesized by the direct solid state reaction of Cu₂O and Ln₂O₃, and AgLnO₂ (Ln=Tm, Yb, Lu) were obtained by the cation-exchange reaction of NaLnO₂ and AgNO₃ in a KNO₃ flux. These compounds crystallized in the delafossite-type structure with the rhombohedral 3R type (space group: R-3m). Magnetic susceptibility measurements showed that these compounds are paramagnetic down to 1.8 K. Specific heat measurements down to 0.4 K indicated that CuNdO₂ ordered antiferromagnetically at 0.8 K.     

Crystal growth and structural study of the new series Ln(OH)CrO4 (Ln = Y, Dy---Lu): Crystal structure of Er(OH)CrO4

Single crystals of the new series Ln(OH)CrO₄ (Ln = Y, Dy---Lu) have been obtained by hydrothermal procedures. The structure of Er(OH)CrO₄ has been determined by single-crystal X-ray techniques. The compound has monoclinic symmetry, space group P2₁/n, Z = 8, with a = 8.106(3), B = 11.324(2), C = 8.251(1) Å, β = 94.14(2)° and V = 755.4(3) ų. Final R values were R = 0.034, R_w = 0.049, for 2207 observed reflections. X-ray powder data show that all compounds of the title series are isomorphous. The coordination polyhedron of the lanthanide cations can be considered a square antiprism, with hydrogen bonds linking CrO₄ and LnO₈ groups. The X-ray data in this series provide evidence for the lanthanide contraction.     

Syntheses and Structures of the Quaternary Copper Tellurides K3Ln4Cu5Te10 (Ln=Sm, Gd, Er), Rb3Ln4Cu5Te10 (Ln=Nd, Gd), and Cs3Gd4Cu5Te10

Six quaternary alkali-metal rare-earth copper tellurides K₃Ln₄Cu₅Te₁₀ (Ln=Sm, Gd, Er), Rb₃Ln₄Cu₅Te₁₀ (Ln=Nd, Gd), and Cs₃Gd₄Cu₅Te₁₀ have been synthesized at 1123 K with the use of reactive fluxes of alkali-metal halides ACl (A=K, Rb, Cs). All crystallographic data were collected at 153 K. These compounds crystallize in space group Pnnm of the orthorhombic system with two formula units in cells of dimensions (A₃Ln₄, a, b, c (Å)): K₃Sm₄, 16.590(2), 17.877(2), 4.3516(5); K₃Gd₄, 16.552(4), 17.767(4), 4.3294(9); K₃Er₄, 16.460(4), 17.550(4), 4.2926(9); Rb₃Nd₄, 17.356(1), 17.820(1), 4.3811(3); Rb₃Gd₄, 17.201(2), 17.586(2), 4.3429(6); Cs₃Gd₄, 17.512(1), 17.764(1), 4.3697(3). The corresponding R₁ indices for the refined structures are 0.0346, 0.0315, 0.0212, 0.0268, 0.0289, and 0.0411. The three K₃Ln₄Cu₅Te₁₀ structures belong to one structure type and the Rb₃Ln₄Cu₅Te₁₀ (Ln=Nd, Gd) and Cs₃Gd₄Cu₅Te₁₀ structures belong to another one, the difference being the location of one of the three unique Cu atoms. Both structure types are three-dimensional tunnel structures that contain similar Ln/Te fragments built from LnTe₆ octahedra and CuTe₄ tetrahedra. The CuTe₄ tetrahedra form ¹_∞[CuTe⁵⁻₃] and ¹_∞[CuTe³⁻₂] chains. The alkali-metal atoms, which are in the tunnels, are coordinated to seven or eight Te atoms.     

Magnetic and Crystallographic Properties of LnCrO4 (Ln=Nd, Sm, and Dy)

Zircon-type compounds LnCrO₄ (Ln=Nd, Sm, and Dy) were prepared. Their precise crystal structures at room temperature were determined from X-ray diffraction measurements. These compounds have a tetragonal system with space group I4₁/amd. Magnetic susceptibility and specific heat measurements have been performed for all the compounds in the temperature range between 1.8 and 300 K. For NdCrO₄, an antiferromagnetic transition was found at 25.2 K. SmCrO₄ and DyCrO₄ show magnetic transitions at 15.0 and 22.8 K, respectively. In addition, structural phase transitions were observed at 58.5 and 31.2 K, respectively. For DyCrO₄, the crystal structure below the transition temperature was determined by low-temperature powder X-ray diffraction measurements to be orthorhombic with space group Imma.     

High-pressure synthesis and structures of lanthanide germanides of LnGe5 (Ln=Ce, Pr, Nd, and Sm) isotypic with LaGe5

A series of lanthanide penta-germanides LnGe₅ (Ln=Ce, Pr, Nd and Sm) has been prepared by high-pressure (5–13 GPa) and high-temperature (500–1200 °C) reaction. CeGe₅ crystallizes in an orthorhombic unit cell (S.G. Immm (71)) with a=4.000(5) Å, b=6.192(5) Å, c=9.86(1) Å, and V=244.1(5) Å³. The new germanides are isotypic with LaGe₅ consisting of a Ge covalent network with tunnels where guest ions Ln³⁺ are situated. The network is composed of sublayers with edge-sharing Ge six-membered rings with only boat conformation. The sublayers are connected by rare eight-coordinated Ge atoms. The cell volume of the compounds systematically decreases from La to Sm compounds, except for CeGe_5, owing to the lanthanide contraction. The lattice constants of CeGe₅ are smaller than those of the Pr compound because it contains Ce⁴⁺ ions. CeGe₅ is paramagnetic above 2 K, but does not obey the Curie–Weiss law. PrGe₅ and NdGe₅ are Curie–Weiss type paramagnets with Weiss temperatures of –3.3 and –18.4 K. SmGe₅ shows an antiferromagnetic transition at 10.4 K.     

Ternary Chlorides of the Trivalent Early Lanthanides. Phase diagrams,crystal structures and thermodynamic properties

A comprehensive review on phase diagrams, crystal structures and thermodynamics of ternary chlorides formed in systems ACl/LnCl₃ (A=Cs, Rb, K, Na; Ln=La−Gd) is presented. The review summarizes the author’s own studies, published since 1985, and original papers of other scientists. With the larger alkali metal ions compounds such as A₃LnCl₆, A₂LnCl₅ and ALn₂Cl₇were obtained. With sodium additional compounds NaLnCl₄ and Na₃Ln₅Cl₁₈ were obtained. The crystal structures are discussed with the concept of ionic radii, which determine the coordination numbers of Ln³⁺ and A⁺ cations against Cl⁻ anions. The formation enthalpies of the compounds from ACl and LnCl₃ were determined by solution calorimetry. Gibbs’ free energies and entropies for these reactions were obtained by e.m.f. measurements vs. temperature. The stability of a ternary chloride in a systemACl−LnCl₃ is given by the ‘free enthalpy of synproportionation’, that is, the formation of a compound from its neighbour compounds in the system. This ΔG ⁰ _syn must be negative. A surprising result is, that the highest-melting compounds in the systems, A₃LnCl₆, are formed from ACl+A₂LnCl₅ by a loss in lattice energy. They exist as high-temperature compounds due to sufficiently high gain in entropy at temperatures whereTΔS>ΔH. This revised version was published online in August 2006 with corrections to the Cover Date.     

New Quaternary Chalcogenides BaLnMQ3 (Ln = Rare Earth or Sc; M = Cu, Ag; Q= S, Se): II. Structure and Property Variation vs Rare-Earth Element

Some new quaternary compounds of the type BaLnMQ₃ (Ln = rare earth or Sc; M = Cu, Ag; Q = S, Se) have been synthesized by the reaction of the constituent binary chalcogenides and elements at 1000°C. The crystal structures of two of these compounds have been determined by single-crystal X-ray diffraction techniques and are isostructural. Crystal data: BaErCuS₃—space group D¹⁷_2h —Cmcm, M= 464.32, Z = 4 , a = 3.987(1), b = 13.377(3), c = 10.101(2) Å (T = 115 K), V = 538.7(4) ų, R_w (F²) = 0.095 for 848 observations and 24 variables, R(F) = 0.037 for 840 observations having F²₀ > 2σ (F²₀); BaYAgSe—space group D¹⁷_2h —Cmcm, M = 571.0, Z = 4, a = 4.239(1), b = 14.030(2), c = 10.636(2) Å (T = 115 K), V = 632.6(2) ų, R_w (F²) = 0.057 for 645 observations and 24 variables, R(F) = 0.023 for 595 observations having F²₀ > 2σ(F²₀). These two compounds adopt the layered KZrCuS₃ structure type. The layers, which are separated by Ba²⁺ ions, consist of edge-sharing octahedral chains and corner-sharing tetrahedral chains. The other compounds synthesized crystallize either with this same structure or with that of β-BaLaCuSe₃, a slightly distorted variation, which is isostructural with Eu₂CuS₃. The diffuse reflective UV-visible spectra of several of these compounds have been measured. From magnetic susceptibility measurements, both BaNdCuS₃ and BaGdCuS₃ show Curie-Weiss behavior, whereas BaCeCuS₃ and BaCeCuSe₃ show in addition temperature-independent paramagnetism.     

Syntheses, crystal structures and optical properties of the first strontium selenium(IV) and tellurium(IV) oxychlorides: Sr3(SeO3)(Se2O5)Cl2 and Sr4(Te3O8)Cl4

Two new quaternary strontium selenium(IV) and tellurium(IV) oxychlorides, namely, Sr₃(SeO₃)(Se₂O₅)Cl₂ and Sr₄(Te₃O₈)Cl₄, have been prepared by solid-state reaction. Sr₃(SeO₃)(Se₂O₅)Cl₂ features a three-dimensional (3D) network structure constructed from strontium(II) interconnected by Cl⁻, SeO₃²⁻ as well as Se₂O₅²⁻ anions. The structure of Sr₄(Te₃O₈)Cl₄ features a 3D network in which the strontium tellurium oxide slabs are interconnected by bridging Cl⁻ anions. The diffuse reflectance spectrum measurements and results of the electronic band structure calculations indicate that both compounds are wide band-gap semiconductors.     

Hydrothermal alteration of BaxMIVxCe2?2 x (PO4)2 [MIV=Zr, Hf] as hosts for minor actinides

Ba_xM^IV _xCe_2−2x (PO₄)₂ [M^IV=Zr, Hf] monazite-like compounds were succesfully synthesized by solid state reaction for x≤0.2 (M^IV=Zr) and x≤0.1 (M^IV=Hf). The low miscibility of BaM^IV(PO₄)₂ (M^IV=Zr, Hf) compounds in CePO₄ was explained on the basis of the monoclinic-to-trigonal phase transition that occurs at 733 K in BaZr(PO₄)₂ and at 798 K in BaHf(PO₄)₂. The hydrothermal alteration of these compounds was tested using a modified MCC-1 static leaching test in acid (1 mol·dm⁻³ HCl) and basic (1 mol dm⁻³ KOH) solutions at 373 K, 473 K and 573 K; both the experimental fluids and the reacted solid specimens were analyzed by different analytical techniques and the reaction mechanisms were elucidated. All the tested compounds are stable in 1 mol·dm⁻³ HCl until 573 K. The stability of the monazites in 1 mol·dm⁻³ KOH is a function of the temperature.     

Synthesis and electrical properties of Li1+ x M x Ti2– x (PO4)3 (where M =Sc, Al, Fe, Y; x =0.3) superionic ceramics

Compounds of the system Li_{1+ x} M _x Ti_{2– x} (PO₄)₃ (where M=Sc, Al, Fe, Y; x=0.3) were synthesized by a solid-state reaction and studied by X-ray diffraction. The ceramic samples were sintered and investigated by complex impedance spectroscopy in the frequency range 10⁶–1.2×10⁹ Hz in the temperature range 300–600 K. Two relaxation dispersions related to the fast Li⁺ ion transport in bulk and grain boundaries were found. The activation energies of the bulk conductivity and relaxation frequency were obtained from the slops of Arrhenius plots. The values of the activation energies of the bulk ionic conductivity and relaxation frequency were found to be very similar in all the materials investigated. That can be attributed to the fact that the temperature dependences of the bulk conductivity are caused only by the mobility of the fast Li⁺ ions, while the number of charge carriers remains constant with temperature. Electronic Publication     

Heterometallic clusters with the {MoNbI8} core: The synthesis and crystal structures of (Ph4P)2[Mo5NbI8Cl6] and (4-MePyH)5[Mo5NbI8Cl6]Cl2

Heterometallic chloride complexes [Mo₅NbI₈Cl₆] ⁿ⁻ (n = 2, 3) are synthesized. The crystal structures of their salts are determined: for (Ph₄P)₂[Mo₅NbI₈Cl₆] (I), triclinic crystal system, spacegroup P [`1]\bar 1, a = 10.9886(6), b = 11.4604(5), c = 13.4343(7) ?, α = 66.124(2), β = 86.892(2), γ = 86.490(2)°, Z = 1, V = 1543.35(13) ?³; and for (4-MePyH)₅[Mo₅NbI₈Cl₆]Cl₂ (II), monoclinic crystal system, space group C2/m, a = 16.4937(4), b = 14.7335(3), c = 11.6534(3) ?, β = 99.8750(10)°, Z = 2, V = 2789.94(11) ? The geometric parameters of compounds I and II and the conditions for the formation of the complexes with the charges −2 and −3 are discussed.     

Syntheses and characterization of two oxoborates, (Pb3O)2(BO3)2MO4 (M=Cr, Mo)

Two oxoborates, (Pb₃O)₂(BO₃)₂MO₄ (M=Cr, Mo), have been prepared by solid-state reactions below 700 °C. Single-crystal XRD analyses showed that the Cr compound crystallizes in the orthorhombic group Pnma with a=6.4160(13) Å, b=11.635(2) Å, c=18.164(4) Å, Z=4 and the Mo analog in the group Cmcm with a=18.446(4) Å, b=6.3557(13) Å, c=11.657(2) Å, Z=4. Both compounds are characterized by one-dimensional chains formed by corner-sharing OPb₄ tetrahedra. BO₃ and CrO₄ (MoO₄) groups are located around the chains to hold them together via Pb–O bonds. The IR spectra further confirmed the presence of BO₃ groups in both structures and UV–vis diffuse reflectance spectra showed band gaps of about 1.8 and 2.9 eV for the Cr and Mo compounds, respectively. Band structure calculations indicated that (Pb₃O)₂(BO₃)₂MoO₄ is a direct semiconductor with the calculated energy gap of about 2.4 eV.     

Structural and magnetic properties of the solid solution () YMn1−x(Cu3/4Mo1/4)xO3

Recently, the ferroelectromagnet YMnO₃ has been the focus of interest because it exhibits both antiferromagnetism (Néel temperature 80 K) and ferroelectricity (Curie temperature 914 K). There have been no reports of complete YMn_1−xM_xO₃ solid solutions in which substitution of the foreign M cation preserves the hexagonal P6₃cm structure. In contrast there exist several homeotypic phases with the general formula, Ln_1+nCu_nMO_3+3n (n=1 (M=Ti), 2 (M=V) and 3 (M=Mo); Ln: lanthanide). Several YMn_1−x(Cu_3/4Mo_1/4)_xO₃ compounds have been synthesized. The solid solution, from YMnO₃ (x=0) to YCu_3/4Mo_1/4O₃ (x=1) has been characterized by X-ray diffraction and transmission electron microscopy study. For 0<x<0.9, the compounds are found to crystallize in the non-centrosymmetric structure, space group P6₃cm, of YMnO₃. The Mn-free end member, x=1, crystallizes in a complex multiple cell, the superstructure being associated to Cu³⁺/Mo⁶⁺ cationic ordering. Dilution of the Mn³⁺ magnetic array by the paramagnetic (Cu²⁺) and diamagnetic (Mo⁶⁺) cations is found to decrease the antiferromagnetic ordering temperature and it becomes undetectable for x0.5 compositions.     

Synthesis of Co/MFe2O4 (M=Fe, Mn) core/shell nanocomposite particles

Monodispersed cobalt nanoparticles (NPs) with controllable size (8–14 nm) have been synthesized using thermal decomposition of dicobaltoctacarbonyl in organic solvent. The as-synthesized high magnetic moment (125 emu/g) Co NPs are dispersible in various organic solvents, and can be easily transferred into aqueous phase by surface modification using phospholipids. However, the modified hydrophilic Co NPs are not stable as they are quickly oxidized, agglomerated in buffer. Co NPs are stabilized by coating the MFe₂O₄ (M=Fe, Mn) ferrite shell. Core/shell structured bimagnetic Co/MFe₂O₄ nanocomposites are prepared with tunable shell thickness (1–5 nm). The Co/MFe₂O₄ nanocomposites retain the high magnetic moment density from the Co core, while gaining chemical and magnetic stability from the ferrite shell. Compared to Co NPs, the nanocomposites show much enhanced stability in buffer solution at elevated temperatures, making them promising for biomedical applications.     

Uniform AMoO4:Ln (A=Sr, Ba; Ln=Eu, Tb) submicron particles: Solvothermal synthesis and luminescent properties

Rare-earth ions (Eu³⁺, Tb³⁺) doped AMoO₄ (A=Sr, Ba) particles with uniform morphologies were successfully prepared through a facile solvothermal process using ethylene glycol (EG) as protecting agent. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), Fourier transform infrared spectroscopy (FT-IR), photoluminescence (PL) spectra and the kinetic decays were performed to characterize these samples. The XRD results reveal that all the doped samples are of high purity and crystallinity and assigned to the tetragonal scheelite-type structure of the AMoO₄ phase. It has been shown that the as-synthesized SrMoO₄:Ln and BaMoO₄:Ln samples show respective uniform peanut-like and oval morphologies with narrow size distribution. The possible growth process of the AMoO₄:Ln has been investigated in detail. The EG/H₂O volume ratio, reaction temperature and time have obvious effect on the morphologies and sizes of the as-synthesized products. Upon excitation by ultraviolet radiation, the AMoO₄:Eu³⁺ phosphors show the characteristic ⁵D₀–⁷F_1–4 emission lines of Eu³⁺, while the AMoO₄:Tb³⁺ phosphors exhibit the characteristic ⁵D₄–⁷F_3–6 emission lines of Tb³⁺. These phosphors exhibit potential applications in the fields of fluorescent lamps and light emitting diodes (LEDs).     

Solubility and Ionic Processes in the Cu X 2? MX ?H2O ( X ?=Cl?, Br?; M +=Li+, Na+, K+, NH4+, Cs+) Systems

Summary.  Solubility isotherms in the CuBr₂−MBr−H₂O (M ⁺ = Li⁺, Na⁺, Cs⁺) systems at 298.15 K were measured. The results together with other available literature data for copper chloride and bromide systems were treated by hydration analysis, and comparative discussion of ionic processes taking place in the respective saturated solutions was performed. Corresponding author. E-mail: jitka@prfdec.natur.cuni.cz Received August 6, 2002; accepted (revised) November 29, 2002 Published online April 3, 2003     

Ln2Sr2PtO7+δ (Ln=La, Pr, and Nd): Three new Pt-containing [AnBn−1O3n]-type hexagonal perovskites

Polycrystalline samples of Ln₂Sr₂PtO_7+δ (Ln=La, Pr, Nd) were prepared by conventional solid state synthesis. The three compounds are new examples for n=2 members of the [A_nB_n−1O_3n] family of hexagonal perovskites containing platinum as the B-type cation. XRD Rietveld refinements show the platinates to crystallize in space group and, in the case of Pr and Nd, revealed a complete ordering of Ln/Sr on the two distinct A-type positions, while for La a partial disorder was observed. By XANES investigations at the Pt-L_III threshold the oxidation state +4 for platinum was found. Thermogravimetry revealed a small oxygen excess for Ln=La and Pr (δ=0.13 and 0.07), pointing to the presence of peroxide ions as already observed for isostructural Ru- and Ir-based compounds. UV–Vis measurements were done for the yellow lanthanum and the green neodymium compound. They revealed two optical band gaps of 2.52 and 3.05 eV, respectively. Magnetic measurements showed La₂Sr₂PtO_7+δ to be diamagnetic as expected for Pt⁴⁺ with low-spin configuration. For Ln=Pr and Nd the observed strong paramagnetism can be explained solely by the magnetic moments of the rare earths.