Synthesis and electrical properties of perovskite oxides, LnB0.75B′0.25O3

A series of ordered perovskite oxides of the type Ln²⁺_0.75B′⁶⁺_0.25O₃ (Ln = rate earth or Y; B = Mn, Fe, Co, Ni; B′ = Mo, W, Re) has been synthesized and characterized by X-ray analysis and density measurements. Compounds with Ln = La are easily formed in all cases as single-phase materials and have either cubic or orthorhombic symmetry. When Ln = rare earth or Y, single-phase materials are formed only in the case of LnFe_0.75Mo_0.25O₃ and these possess an orthorhombic structure. All the phases tested are extrinsic semiconductors in the range of 25–350°C, with E_a ranging from 0.1 to 0.4 eV. Resistivity of the Ln(Fe, Mo)O₃ series of oxides increases from Ln = La to Ln = Lu. Ni-containing compounds are p type, while those containing Fe or Co in the B sites are n type.     

The Family of Ln2Ti2S2O5 Compounds (Ln=Nd, Sm, Gd, Tb, Dy, Ho, Er, and Y): Optical Properties

The optical properties of Ln₂Ti₂S₂O₅ compounds (Ln=Nd, Sm, Gd, Tb, Dy, Ho, Er, and Y) have been measured. Diffuse reflectance spectra revealed a strong absorption band above 2 eV, which explains the color of the compounds. Moreover, other bands, with a lower intensity, have been attributed to 4f-4f rare earth transitions. In the case of neodymium the derived energy level scheme is rich enough to determinate a set of phenomenological crystal field parameters that correctly reproduce the spectrum. These parameters were also calculated from the crystallographic structure, in a good agreement with the experiment. Finally, the paramagnetic susceptibility, well reproduced by the calculation, confirms that the rare earth is in a trivalent state.     

Studies on phase transition temperature of rare earth niobates Ln3NbO7 (Ln = Pr,Sm, Eu) with orthorhombic fluorite-related structure

The phase transition of ternary rare earth niobates Ln₃NbO₇ (Ln = Pr, Sm, Eu) was investigated by the measurements of high-temperature and low-temperature X-ray diffraction, differential scanning calorimetry (DSC) and differential thermal analysis (DTA). These compounds crystallize in an orthorhombic superstructure derived from the structure of cubic fluorite (space group Pnma for Ln = Pr; C222₁ for Ln = Sm, Eu). Sm₃NbO₇ undergoes the phase transition when the temperature is increased through ca. 1080 K and above the transition temperature, its structure is well described with space group Pnma. For Eu₃NbO₇, the phase transition was not observed up to 1273 K Pr₃NbO₇ indicates the phase transition when the temperature is increased through ca. 370 K. The change of the phase transition temperature against the Ln ionic radius for Ln₃NbO₇ is quite different from those for Ln₃MO₇ (M = Mo, Ru, Re, Os, or Ir), i.e., no systematic relationship between the phase transition temperature and the Ln ionic radius has been observed for Ln₃NbO₇ compounds.     

Magnetic properties of EuLn2O4 (Ln=rare earths)

Ternary rare earth oxides EuLn₂O₄ (Ln=Gd, Dy-Lu) were prepared. They crystallized in an orthorhombic CaFe₂O₄-type structure with space group Pnma. ¹⁵¹Eu Mössbauer spectroscopic measurements show that the Eu ions are in the divalent state. All these compounds show an antiferromagnetic transition at 4.2-6.3 K. From the positive Weiss constant and the saturation of magnetization for EuLu₂O₄, it is considered that ferromagnetic chains of Eu²⁺ are aligned along the b-axis of the orthorhombic unit cell, with neighboring Eu²⁺ chains antiparallel. When Ln=Gd-Tm, ferromagnetically aligned Eu²⁺ ions interact with the Ln³⁺ ions, which would overcome the magnetic frustration of triangularly aligned Ln³⁺ ions and the EuLn₂O₄ compounds show a simple antiferromagnetic behavior.     

Analytical laser ionization spectrometry in flame: Choice of the analytical form for the determination of rare earth elements

Based on the ionization potentials and dissociation energies of rare earth monoxides, these molecules are classified into two groups. For the molecules of the first type, the ionization potential is lower than the dissociation energy. Laser excitation of these molecules leads to their predominant ionization. For the molecules of the second type, the ionization potential is higher than the dissociation energy. Depending on the absolute values of the latter, atomic lines can be observed in the ionization spectrum. The elements that form molecules of the first type in flame should be determined as monoxide molecules. For the elements forming molecules of the second type, determination in the atomic form gives better results.     

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.     

The dehydration behaviour of rare earth complexes ofm-nitrobenzoic acid

Rare earth complexes ofm-nitrobenzoic acid (LnL₃·2H₂O,Ln=La-Lu and Y, except Pm, HL=m-nitrobenzoic acid) were synthesized and characterized by elemental analysis, chemical analysis, IR spectroscopy and X-ray diffraction analysis. The dehydration behaviour of these complexes was studied in detail by means of TG-DTA and DSC. Dehydration occurs over the temperature range 76–215°C, and the temperature of formation of the anhydrous complexes decreases with increasing atomic number of the rare earth. The activation energies and enthalpy changes for te dehydration were obtained.     

Spectroscopic properties of Ln2MoO6:Eu3+

The spectroscopic properties of Ln₂MoO₆:Eu³⁺ (Ln = La, Gd, Y) compounds were investigated. The differences in the recorded fluorescence spectra are in accord with the different structures. For the La₂MoO₆:Eu³⁺ case, the spectrum is compatible with a C₂ point site symmetry. It appears that the energy level scheme is connected with the rare earth oxychloride one, so it is possible to determine accurately sets of crystal field parameters simulating the spectrum. For the other compounds, the Eu³⁺ ions occupy three different point sites. By using the site-selective excitation on the ⁵D₀ level it is possible to identify the energy level scheme characterizing each point site.     

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.     

Rare-earth cobalticyanides LnCo(CN)6·nH2O

Crystalline cobalticyanides LnCo^III(CN)₆·nH₂O with Ln = La,…, Lu, Y have been synthesized by a double-infusion technique. In analogy to the Cr and Fe compounds, the large rare-earth ions form a hexagonal modification while the smaller ions lead to the orthorhombic structure with 4H₂O. Experiments show that no magnetic ordering occurs down to 1°K. The Stark splitting of the J ground state due to the crystalline field is analyzed for the Ce and Sm compounds.     

Raman scattering study and electrical properties characterization of elpasolite perovskites Ln2(BB′)O6 (Ln=La, Sm…Gd and B,B′=Ni, Co, Mn)

Two series of elpasolite perovskites Ln₂CoMnO₆ and Ln₂NiMnO₆ (Ln=La, Pr, Nd, Sm, Gd) have been prepared. The electronic band gap and magnetic Curie temperature vary systematically as a function of the rare earth cation size within both series. Here we used Raman scattering spectroscopy along with the results of previous structural studies to show that there is little change in octahedral distortion but significant changes in the octahedral tilting angle upon decreasing lanthanide ionic radius. The data indicate differences in the orbital overlap and bond strengths between the two series of materials that allow us to understand variations in the magnetic and electrical properties within and between the two perovskite series.     

Etude par rayons X à haute temperature des transformations polymorphiques des pérovskites LnAlO3 (Ln = élément lanthanidique)

The temperatures of phase transitions of the rare earth aluminates have been determined by high temperature X-ray diffractometry. All of the transitions are reversible and occur respectively for Rh ? C at 500°C (LaAlO₃), 1330°C (PrAlO₃), 1550°C (NdAlO₃), and 1950°C (SmAlO₃) and for O ? Rh at 770°C (SmAlO₃), 1330°C (EuAlO₃), and 1700°C (GdAlO₃). LnAlO₃ perovskites from TbAlO₃ up to LuAlO₃ are orthorhombic without any phase transition.     

The preparation and characterization of CdVO3 prepared at ambient and high pressure

The solid state reaction of VO₂ and CdO yielded a phase of unknown structure, which transforms to CdVO₃(I) after treatment under 60–65 kbar pressure at 1200°C. The high-pressure product was characterized by crystallographic, electrical and magnetic properties. CdVO₃(I) is an orthorhombic perovskite-type compound and a metallic conductor, exhibiting Pauli paramagnetic behavior. In contrast, the ambient pressure phase displays Curie-Weiss magnetic behavior above 77°K.     

Synthesis of mercury rare earth ternary sulfides

Three new mercury rare earth sulfides have been synthesized by heating a mixture of the sesquisulfide of rare earth and cinnabar at 800°c for 25 h in an evacuated (10^?4 Torr) sealed quartz tube. Their general formula, Ln₄HgS₇ (Ln = Tm, Yb and Lu), was determined by chemical analysis. These sulfides are tetragonal. Lattice parameters for Ln = Tm, Yb and Lu are: a = 11.09(2), 11.07(3) and 11.03(2)Å, c = 8.38(5), 8.35(2) and 8.33(2)Å respectively. These compounds are stable toward air and moisture at room temperature, but are oxidized slowly at elevated temperature. Thermogravimetric curves show that Tm₄HgS₇, Yb₄HgS, and Lu₄HgS₇ are decomposed and oxidized at 500°, 650° and 450°C respectively.     

A study on mixed halide compounds MFX (M = Ca,Sr, Eu,Ba; X = Cl,Br, I)

MFX (M = Ca, Sr, Eu, Ba; X = Cl, Br, I) compounds have been prepared by solid-state reaction. Lattice parameters and X-ray diffraction patterns are presented for these compounds, which are all isostructural with tetragonal PbFCl. Attempts to synthesize solid solutions of Sr(Eu)FCl and of MFCl compounds with several rare earth oxychlorides are reported. The crystal chemistry of MFX, MHX, and LnOX compounds is briefly discussed in comparison, and the observed $\text{c}\text{a}$ ratios are interpreted on the basis of electrostatic calculations.     

Donnees cristallographiques sur une nouvelle serie de manganites mixtes de terre rare et d'alcalino-terreux

The synthesis of two new series of oxides: BaLn₂Mn₂O₇ and SrLn₂Fe₂O₇, where Ln is a rare earth, was performed. The experimental conditions are given along with some crystal data. The diffraction patterns show a close resemblance with those of the double-perovskite-block compounds BaLn₂Fe₂O₇ and SrLn₂Fe₂O₇. However, the $\text{c}\text{a}$ ratio of the tetragonal cell is somewhat larger for the manganese compounds than for the iron compounds, due to a Jahn-Teller distortion of the Mn³⁺O₆ octahedral sites.     

Oxidation resistance of LnSiON powders (Ln=Y,Gd and La)

For application of LnSiON (Ln=Y, Gd and La) oxynitride materials, e.g. as host-lattices for lamp phosphors, oxidation resistance up to about 600 °C in air is a prerequisite. In this study we prepared LnSiON (Ln=Y, Gd and La) powders by solid state reaction and observed via TGA/DTA-experiments that most compounds are oxidation resistant up to 600 °C in air. The stability in air at high temperatures increases going from Ln₅(SiO₄)₃N, Ln₄Si₂O₇N₂, LnSiO₂N, Ln₂Si₃O₃N₄ to Ln₃Si₈O₄N₁₁. This is explained by an increasing cross-linking between the siliconoxygennitrogen tetrahedra in this sequence. For the lattices with less cross-linking between the siliconoxygennitrogen tetrahedra we observed that the oxidation resistance decreases slightly going from Y and Gd to La. For these lattices, also, an additional weight gain was observed during the oxidation reaction, which was higher than expected for complete oxidation. The additional weight gain was ascribed to an intermediate phase in which nitrogen retention takes place.     

Rare earth ions in a hexagonal field IV

Energy levels and eigenfunctions of rare earth ions in a crystal field of hexagonal symmetry have been obtained using a Hamiltonian of the form $\text{H}$ = B°₂O°₂ + B°₄O°₄. Results are presented for all J values appearing in the rare earth series. The order of the energy levels has been determined for all relative values of the second and fourth order crystal field intensity parameters, B°₂ and B°₄. This, of course, includes information for the commercially significant RCo₅ compounds, for which the second order term is dominant. The eigenfunctions are pure M states with permanent magnetic moments ± Mg μ_B. The moments are unchanged by a field applied along the c axis.     

Synthesis,Structure, Magnetic and Optical Properties of Ternary Thio‐germanates: Ln4(GeS4)3 (Ln = Ce,Nd)

Single crystals of two ternary thio‐germanates containing rare‐earth metals, Ln₄(GeS₄)₃ (Ln = Ce ( I ), Nd ( II )), have been isolated from the reaction of anhydrous rare‐earth trichloride (LnCl₃) and ternary sodium thio‐germanate (Na₂GeS₃) in evacuated quartz ampoules. We have determined the crystal structure of the compounds, which are isostructural to La₄(GeS₄)₃ and crystallize in trigonal system in the space group R3c with the cell dimensions: I , a = b = 19.375(3) Å, c = 8.028(2) Å, Z = 6; II , a = b = 19.250(3) Å, c = 7.949(2) Å, Z = 6. The structure is built with the complex network of two independent tricapped trigonal prisms of CeS₉, in which Ge atoms occupy tetrahedral holes of sulfur atoms. The bulk synthesis of the two compounds has also been achieved by the stoichiometric combination of the elements. The magnetic and optical properties of the compounds have been investigated. The magnetic moments of 2.32 and 3.49 μ_B for I and II , respectively, are in good agreement with the theoretical magnetic moments of Ce and Nd in the +3 oxidation state. The optical band gap of I is found to be located around 2.3 eV, while the optical band gap of II lies around 2.62 eV. In addition, Raman spectroscopic characterizations have also been performed for I , II , and La₄(GeS₄)₃.     

High-throughput and microwave investigation of rare earth phosphonatoethanesulfonates—Ln(O3P-C2H4-SO3) (Ln=Ho, Er, Tm, Yb, Lu, Y)

Following the strategy of using bifunctional phosphonic acids for the synthesis of new metal phosphonates, the flexible ligand 2-phosphonoethanesulfonic acid, H₂O₃P-C₂H₄-SO₃H (H₃L), was used in a high-throughput (HT) and microwave investigation of rare earth phosphonatoethanesulfonates. The HT-investigation led to six isotypic compounds Ln(O₃P-C₂H₄-SO₃) with Ln=Ho (1), Er (2), Tm (3), Yb (4), Lu (5) and Y (6). The syntheses were scaled-up in glass reactor tubes in order to obtain larger amounts for a detailed characterization. Based on these results all compounds could be also synthesized by microwave-assisted heating and the influence of reaction time and stirring rate during the synthesis was established. For compound 2 the crystal structure was determined by single-crystal X-ray diffraction. The compounds contain isolated slightly distorted LnO₆ octahedra that are connected by the phosphonate and sulfonate groups into a three-dimensional framework. Thermogravimetric investigations demonstrate the high thermal stability of the compounds up to 460 °C.