Syntheses and characterization of zero-dimensional molybdoantimonites, A2(Mo4Sb2O18) (A=Y, La, Nd, Sm, Gd and Dy)

Six new isostructural A₂(Mo₄Sb₂O₁₈) (A=Y, La, Nd, Sm, Gd and Dy) compounds have been synthesized by solid-state reactions and characterized by single crystal X-ray diffraction and spectroscopic techniques. They crystallize in C2/c space group with 4 formula units and contain A³⁺ cations and discrete centrosymmetric anionic (Mo₄Sb₂O₁₈)⁶⁻ aggregates, made of tetrahedral MoO₄ and disphenoidal SbO₄ moieties. They exhibit characteristic Sb³⁺ photoluminescence.     

Synthesis and characterization of monodisperse spherical SiO2@RE2O3 (RE=rare earth elements) and SiO2@Gd2O3:Ln (Ln=Eu, Tb, Dy, Sm, Er, Ho) particles with core-shell structure

Spherical SiO₂ particles have been coated with rare earth oxide layers by a Pechini sol-gel process, leading to the formation of core-shell structured SiO₂@RE₂O₃ (RE=rare earth elements) and SiO₂@Gd₂O₃:Ln³⁺ (Ln=Eu, Tb, Dy, Sm, Er, Ho) particles. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), photoluminescence (PL), and cathodoluminescence spectra as well as lifetimes were used to characterize the resulting SiO₂@RE₂O₃ (RE=rare earth elements) and SiO₂@Gd₂O₃:Ln³⁺ (Eu³⁺, Tb³⁺, Dy³⁺, Sm³⁺, Er³⁺, Ho³⁺) samples. The obtained core-shell phosphors have perfect spherical shape with narrow size distribution (average size ca. 380 nm), smooth surface and non-agglomeration. The thickness of shells could be easily controlled by changing the number of deposition cycles (40 nm for two deposition cycles). Under the excitation of ultraviolet, the Ln³⁺ ion mainly shows its characteristic emissions in the core-shell particles from Gd₂O₃:Ln³⁺ (Eu³⁺, Tb³⁺, Sm³⁺, Dy³⁺, Er³⁺, Ho³⁺) shells.     

Structures and magnetic properties of Ln3OsO7 (Ln=Pr,Nd,Sm)

Polycrystalline samples of Ln₃OsO₇ (Ln=Pr,Nd,Sm) have been prepared. The structures of these compounds were determined by X-ray powder diffraction. They crystallize in a superstructure of cubic fluorite (space group Cmcm, Z=4). The samples have been characterized by magnetometry. The compounds show complex magnetic behavior at low temperatures caused by competing magnetic interactions leading to frustration.     

Syntheses, structure and rare earth metal photoluminescence of new and known isostructural A2Mo4Sb2O18 (A=Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) compounds

Nine new A₂Mo₄Sb₂O₁₈ (A=Ce, Pr, Eu, Tb, Ho, Er, Tm, Yb, Lu) compounds have been synthesized by solid-state reactions. They are isostructural with six reported analogues of yttrium and other lanthanides and the monoclinic unit cell parameters of all fifteen of them vary linearly with the size of A³⁺ ion. Single crystal X-ray structures of eight A₂Mo₄Sb₂O₁₈ (A=Ce, Pr, Eu, Gd, Tb, Ho, Er, Tm) compounds have been determined. Neat A₂Mo₄Sb₂O₁₈ (A=Pr, Sm, Eu, Tb, Dy, Ho, Er, Tm) compounds exhibit characteristic rare earth metal photoluminescence.     

Synthesis and crystal structure of the isotypic rare earth thioborates Ce[BS3], Pr[BS3], and Nd[BS3]

The orthothioborates Ce[BS₃], Pr[BS₃] and Nd[BS₃] were prepared from mixtures of the rare earth (RE) metals together with amorphous boron and sulfur summing up to the compositions CeB₃S₆, PrB₅S₉ and NdB₃S₆. The following preparation routes were used: solid state reactions with maximum temperatures of 1323 K and high-pressure high-temperature syntheses at 1173 K and 3 GPa. Pr[BS₃] and Nd[BS₃] were also obtained from rare earth chlorides RECl₃ and sodium thioborate Na₂B₂S₅ by metathesis type reactions at maximum temperatures of 1073 K. The crystal structure of the title compounds was determined from X-ray powder diffraction data. The thioborates are isotypic and crystallize in the orthorhombic spacegroup Pna2₁ (No. 33; Z=4; Ce: , , ; Pr: , , ; Nd: , , ) . The crystal structures contain isolated [BS₃]^3‐ groups with boron in trigonal-planar coordination. The sulfur atoms form the vertices of undulated kagome nets, which are stacked along [100] according to the sequence ABAB. Within these nets every second triangle is occupied by boron and the large hexagons are centered by rare earth ions, which are surrounded by overall nine sulfur species.     

The extended chain compounds Ln12(C2)3I17 (Ln=Pr, Nd, Gd, Dy): Synthesis, structure and physical properties

The title compounds are obtained in high yield from stoichiometric mixtures of Ln, LnI₃ and graphite, heated at 900-950 °C in welded Ta containers. The crystal structures of new Pr and Nd phases determined by single-crystal X-ray diffraction are related to those of other Ln₁₂(C₂)₃I₁₇-type compounds (C 2/c, a=19.610(1) and 19.574(4) Å, b=12.406(2) and 12.393(3) Å, c=19.062(5) and 19.003(5) Å, β=90.45(3)° and 90.41(3)°, for Pr₁₂(C₂)₃I₁₇ and Nd₁₂(C₂)₃I₁₇, respectively). All compounds contain infinite zigzag chains of C₂-centered metal atom octahedra condensed by edge-sharing into the [tcc]_∞ sequence (c=cis, t=trans) and surrounded by edge-bridging iodine atoms as well as by apical iodine atoms that bridge between chains. The polycrystalline Gd₁₂(C₂)₃I₁₇ sample exhibits semiconducting thermal behavior which is consistent with an ionic formulation (Ln³⁺)₁₂(C₂^6-)₃(I⁻)₁₇(e⁻) under the assumption that one extra electron is localized in metal-metal bonding. The magnetization measurements on Nd₁₂(C₂)₃I₁₇, Gd₁₂(C₂)₃I₁₇ and Dy₁₂(C₂)₃I₁₇ indicate the coexistence of competing magnetic interactions leading to spin freezing at T_f=5 K for the Gd phase. The Nd and Dy compounds order antiferromagnetically at T_N=25 and 29 K, respectively. For Dy₁₂(C₂)₃I₁₇, a metamagnetic transition is observed at a critical magnetic field H≈25 kOe.     

Systematic study of photoluminescence upon band gap excitation in perovskite-type titanates R1/2Na1/2TiO3:Pr (R=La, Gd, Lu, and Y)

Pr³⁺-doped perovskites R_1/2Na_1/2TiO₃:Pr (R=La, Gd, Lu, and Y) were synthesized, and their structures, optical absorption and luminescent properties were investigated, and the relationship between structures and optical properties are discussed. Optical band gap of R_1/2Na_1/2TiO₃ increases in the order R=La, Gd, Y, and Lu, which is primarily due to a decrease in band width accompanied by a decrease in Ti-O-Ti bond angle. Intense red emission assigned to f-f transition of Pr³⁺ from the excited ¹D₂ level to the ground ³H₄ state upon the band gap photo-excitation (UV) was observed for all compounds. The wavelength of emission peaks was red-shifted in the order R=La, Gd, Y, and Lu, which originates from the increase in crystal field splitting of Pr³⁺. This is attributed to the decrease in inter-atomic distances of Pr-O together with the inter-atomic distances (R, Na)-O, i.e., increase in covalency between Pr and O. The results indicate that the luminescent properties in R_1/2Na_1/2TiO₃:Pr are governed by the relative energy level between the ground and excited state of 4f² for Pr³⁺, and the conduction and valence band, which is primarily dependent on the structure, e.g., the tilt of TiO₆ octahedra and the Pr-Ti inter-atomic distance and the site symmetry of Pr ion.     

Les oxydes A2BaCuO5 (A = Y,Sm, Eu,Gd, Dy,Ho, Er,Yb)

A series of new phases, A₂BaCuO₅ (A = Y, Sm, Eu, Gd, Dy, Ho, Er, Yb), has been isolated. These compounds are orthorhombic, with $\text{a ? 7.1, b ? 12.2}$, and $\text{c ? 5.6}$Å. The probable space groups deduced from the electron diffraction patterns are Pbnm and Pbn2₁. The structure has been resolved from X-ray powder patterns. The framework can be considered as built up from distorted monocapped trigonal prisms AO₇ which share one triangular face forming A₂O₁₁ blocks. The edge-sharing A₂O₁₁ blocks form a three-dimensional network which delimits cavities where Ba²⁺ and Cu²⁺ are located. Barium is coordinated to 11 oxygen atoms, while the coordination polyhedron of copper is a distorted tetragonal pyramid CuO₅.     

Seven new rare-earth transition-metal oxychalcogenides: Syntheses and characterization of Ln4MnOSe6 (Ln=La, Ce, Nd), Ln4FeOSe6 (Ln=La, Ce, Sm), and La4MnOS6

The quaternary oxychalcogenides Ln₄MnOSe₆ (Ln=La, Ce, Nd), Ln₄FeOSe₆ (Ln=La, Ce, Sm), and La₄MnOS₆ have been synthesized by the reactions of Ln (Ln=La, Ce, Nd, Sm), M (M=Mn, Fe), Se, and SeO₂ at 1173 K for the selenides or by the reaction of La₂S₃ and MnO at 1173 K for the sulfide. Warning: These reactions frequently end in explosions. These isostructural compounds crystallize with two formula units in space group of the hexagonal system. The cell constants (a, c in Å) at 153 K are: La₄MnOSe₆, 9.7596(3), 7.0722(4); La₄FeOSe₆, 9.7388(4), 7.0512(5); Ce₄MnOSe₆, 9.6795(4), 7.0235(5); Ce₄FeOSe₆, 9.6405(6), 6.9888(4); Nd₄MnOSe₆, 9.5553(5), 6.9516(5); Sm₄FeOSe₆, 9.4489(5), 6.8784(5); and La₄MnOS₆, 9.4766(6), 6.8246(6). The structure of these Ln₄MOQ₆ compounds comprises a three-dimensional framework of interconnected LnOQ₇ bicapped trigonal prisms, MQ₆ octahedra, and the unusual LnOQ₆ tricapped tetrahedra.     

Influence of rare earth ions on CO oxidation over Ln2NiO4 oxides (Ln=La, Pr or Nd)

CO oxidation was studied over Ln₂NiO₄ compounds in an all-glass static recirculatory system. The reactions were performed at an initial total pressure of 50±2 Torr, in the temperature interval 340–450°C. The apparent activation energy was found to decrease from La to Nd. A correlation between activity and binding energy of oxygen in the compound as well as with the tolerance factor is observed. The role of rare earth ions in these catalysts is discussed.

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CO Ln₂NiO₄ , . 50±2 340–450°C. La Nd. , . .

     

Catalytic properties of LnSrCoO4 (Ln = La, Sm) in the oxidation of CO and C3H8

Perovskite-like LnSrCoO₄(Ln=La, Sm) mixed oxides of K₂NiF₄ structure were synthesized by the polyglycol gel method and used successfully for CO and C₃H₈ oxidation for the first time. The structure was characterized by means of XRD, TEM, BET, O₂-TPD and CO₂-TPD, respectively. An oxidation mechanism over LnSrCoO₄is proposed. This revised version was published online in August 2006 with corrections to the Cover Date.     

High-pressure high-temperature synthesis and crystal structure of the isotypic rare earth (RE)-thioborate-sulfides RE9[BS3]2[BS4]3S3, (RE=Dy-Lu)

Application of high-pressure high-temperature conditions (3.5 GPa at 1673 K for 5 h) to mixtures of the elements (RE:B:S=1:3:6) yielded crystalline samples of the isotypic rare earth-thioborate-sulfides RE₉[BS₃]₂[BS₄]₃S₃, (RE=Dy-Lu), which crystallize in space group P6₃ (Z=2/3) and adopt the Ce₆Al_3.33S₁₄ structure type. The crystal structures were refined from X-ray powder diffraction data by applying the Rietveld method. Dy: a=9.4044(2) Å, c=5.8855(3) Å; Ho: a=9.3703(1) Å, c=5.8826(1) Å; Er: a=9.3279(12) Å, c=5.8793(8) Å; Tm: a=9.2869(3) Å, c=5.8781(3) Å; Yb: a=9.2514(5) Å, c=5.8805(6) Å; Lu: a=9.2162(3) Å, c=5.8911(3) Å. The crystal structure is characterized by the presence of two isolated complex ions [BS₃]^3- and [BS₄]^5- as well as [□(S^2-)₃] units.     

Magnetic and electrical properties of quadruple perovskites with 12 layer structures Ba4LnM3O12 (Ln=rare earths; M=Ru, Ir): The role of metal-metal bonding in perovskite-related oxides

Structures and magnetic and electrical properties of quadruple perovskites containing rare earths Ba₄LnM₃O₁₂ (Ln=rare earths; M=Ru, Ir) were investigated. They crystallize in the 12L-perovskite-type structure. Three MO₆ octahedra are connected to each other by face-sharing and form a M₃O₁₂ trimer. The M₃O₁₂ trimers and LnO₆ octahedra are alternately linked by corner-sharing, forming the perovskite-type structure with 12 layers. For Ln=Ce, Pr, and Tb, both the Ln and M ions are in the tetravalent state (Ba₄Ln⁴⁺M⁴⁺₃O₁₂), and for other Ln ions, Ln ions are in the trivalent state and the mean oxidation state of M ions is +4.33 (Ba₄Ln³⁺M^4.33+₃O₁₂). All the Ba₄Ln³⁺Ru^4.33+₃O₁₂ compounds show magnetic ordering at low temperatures, while any of the corresponding iridium-containing compounds Ba₄Ln³⁺Ir^4.33+₃O₁₂ is paramagnetic down to 1.8 K. Ba₄Ce⁴⁺Ir⁴⁺₃O₁₂ orders antiferromagnetically at 10.5 K, while the corresponding ruthenium-containing compound Ba₄Ce⁴⁺Ru⁴⁺₃O₁₂ is paramagnetic. These magnetic results were well understood by the magnetic behavior of M₃O₁₂. The effective magnetic moments and the entropy change for the magnetic ordering show that the trimers Ru^4.33+₃O₁₂ and Ir⁴⁺₃O₁₂ have the S= ground state, and in other cases there is no magnetic contribution from the trimers Ru⁴⁺₃O₁₂ or Ir^4.33+₃O₁₂.Measurements of the electrical resistivity of Ba₄LnM₃O₁₂ and its analysis show that these compounds demonstrate two-dimensional Mott-variable range hopping behavior.     

Role of crystal structure on the thermal expansion of Ln2W3O12 (Ln = La, Nd, Dy, Y, Er and Yb)

The negative thermal expansion material Y₂W₃O₁₂ belongs to Ln₂W₃O₁₂ family of compositions. The thermal expansion behavior of Ln₂W₃O₁₂ (Ln = La, Nd, Dy, Y, Er and Yb) members synthesized by the solid-state reaction have been studied and correlated to their crystal structure. The lighter rare earth tungstates (Ln = La, Nd and Dy) crystallize in monoclinic structure (C2/c) whereas the heavy rare earth tungstates (Ln = Y, Er and Yb) form the trihydrate orthorhombic Ln₂W₃O₁₂3H₂O at room temperature and above 400 K transforms to unhydrated orthorhombic structure (Pnca). The hot pressed (1273 K and 25 MPa) ceramic pellets have been studied for thermal expansion property by dilatometry and high temperature X-ray diffraction. The heavy rare earth tungstates show a large initial expansion up to 400 K, followed by a thermal contraction. The light rare earth tungstates, on the other hand, show thermal expansion. The difference in the thermal expansion behavior in Ln₂W₃O₁₂ series is attributed to the difference in the structural features. The heavy rare earth tungstates have corner sharing of LnO₆ octahedra with WO₄ tetrahedra, where the now well established mechanism of transverse vibrations operate. The light rare earth tungstates have edge sharing of LnO₈ polyhedra where in such a mechanism is absent.     

Synthesis and crystal structure of RMgSi2 compounds (R=La, Ce, Pr, Nd), a particular example of linear intergrowth

A new series of rare earth compounds with stoichiometry RMgSi₂ (R=La, Ce, Pr, Nd) is reported. The single crystal X-ray diffraction showed that CeMgSi₂, which melts congruently at 1200 °C, crystallizes in a new tetragonal structure type (I4₁/amd, tI32, a=4.2652(4) Å, c=36.830(4) Å, Z=8; wR₂=0.042 (19 parameters, 393F₀²), R₁=0.018 (297F₀>4σF₀). The crystal structure of CeMgSi₂ can be formally built up by alternating along the z direction four CeMg₂Si₂-type CeMg₂Si₂ slabs with four AlB₂-type CeSi₂ slabs, one after the other. The structural model obtained from a CeMgSi₂ single crystal has been confirmed for the La, Pr and Nd homologous compounds by means of Rietveld refinement. The trend of the unit-cell parameters, plotted versus the R³⁺ ionic radius, shows a linear behaviour, which strongly suggests a trivalent state for the Ce atoms. An analysis of the features of this new structure is reported, in comparison with the other known CeMg₂Si₂/AlB₂-type linear intergrowth compounds.     

Mossbauer Investigation of Fe-Doped Ni(III) Perovskites ANi0.98Fe0.02O3 (A=Pr, Nd, Sm, Y, Lu, Tl) versus Temperature

Nickelates ANiO₃ (A=Pr, Nd, Sm, Lu, Y, Tl) containing Mössbauer probe ⁵⁷Fe atoms were synthesized. In the case of nickelates with larger rare earth (A=Pr, Nd, Sm) the Mössbauer spectra confirm that ferric ions are located in single type of crystallographic positions. On the contrary, the spectra of ANi_0.98Fe_0.02O₃ with small cations (A=Lu, Y, Tl) can be described as a superposition of two sub-spectra which indicate that ⁵⁷Fe probe atoms are simultaneously stabilized in two non-equivalent crystallographic positions. These results have been interpreted in terms of partial charge disproportionation of Ni³⁺ cations associated with the electronic localization in monoclinic distorted Lu, Y, Tl nickelates. The modification of ⁵⁷Fe spectra for TlNi_0.98Fe_0.02O₃ as a function of temperature has shown that this charge disproportionation occurs in varying degrees, corresponding to the charge states Fe^(3+σ)+ and Fe^(3−σ′)+. On the contrary, the spectra for Lu and Y nickelates show that charge variation (σ,σ′) for dopant Fe(1) and Fe(2) cations does not depend on temperature.     

New perovskite-related oxides having high dielectric constant: Ln2Ba2CaZn2Ti3O14 (Ln = La and Pr)

Two new oxides, La₂Ba₂CaZn₂Ti₃O₁₄ and Pr₂Ba₂CaZn₂Ti₃O₁₄, have been synthesized by the ceramic route at 1100°C. These oxides crystallize in the disordered cubic structure with an ‘a’ lattice parameter of 3.9728 (2) and 3.9448 (5) respectively. These oxides show high dielectric constant (70 and 57) and low loss (0.003 and 0.013 at 100 kHz) for La₂Ba₂CaZn₂Ti₃O₁₄ and Pr₂Ba₂CaZn₂Ti₃O₁₄ respectively. The dielectric constant is highly stable with frequency and temperature. Dedicated to Professor C N R Rao on his 70th birthday     

Effect of rare earth ions on structural and optical properties of specific perovskite orthochromates; RCrO3 (R = La,Nd, Eu,Gd, Dy,and Y)

We present here, a systematic investigation on orthorhombic perovskite type rare earth chromates; RCrO₃ (R = La, Nd, Eu, Gd, Dy, and Y) powder samples via X-ray diffraction, Raman and UV–Visible spectroscopy. The Rietveld fitted X-ray diffraction patterns confirm the formation of single phase orthorhombic structure with Pnma space group for all the samples. A comprehensive analysis of Rietveld fitted data has been performed to further see the effect of change in the size of rare earth (R³⁺) ions on bond length and structural distortions. It has been noticed that bond length (RO) decreases with decrease in radius of R-site ions, consequently an increase in the distortion or the octahedral tilting. Raman spectroscopy results reveal the blue shift in these samples with decrease in the size of the rare-earth ion, owing to change in their bond lengths. Optical properties have been also noticed via UV–visible absorption spectroscopy technique. These results indicate that the RCrO₃ materials are transparent in visible range with band gap varying from 2.19 to 3.20 eV.     

Acid-base properties and catalytic activity of ABO3 (perovskite-type) oxides consisting of rare earth and 3d transition metals

The surface acidity/basicity of perovskite-type mixed oxides (LaCrO₃, PrCrO₃, SmCrO₃, LaMnO₃, PrMnO₃, SmMnO₃, LaFeO₃, PrFeO₃, SmFeO₃, LaCoO₃, PrCoO₃, SmCoO₃, LaNiO₃, PrNiO₃ and SmNiO₃) are reported. These properties have been correlated with the catalytic activity of these oxides towards esterification of acetic acid usingn-butanol.