Mixed Valent Rare Earth Research Articles

Isosymmetric Lattice Collapse in Mixed-Valence Rare-Earth Fullerides at High Pressure─Coupling of Lattice and Electronic Degrees of Freedom

The orthorhombic-structured (Sm1/3Ca2/3)2.75C60 is a member of the family of rare-earth metal intercalated C60 fullerides with stoichiometry, RE2.75C60 (RE = rare-earth metal). At ambient conditions, it crystallizes with the formation of a 2a × 2a × 2a (a ∼ 14 Å) supercell of the face-centered cubic (fcc) A3C60 (A = alkali metal) unit cell. The superstructure is stabilized by the long-range ordering of tetrahedral-site Sm/Ca metal defects (O phase). Synchrotron X-ray powder diffraction measurements at high pressure and ambient temperature reveal that (Sm1/3Ca2/3)2.75C60 is a compressible solid with a bulk modulus, K0 = 24(1) GPa, that transforms to a more densely packed isostructural high-pressure O′ phase above ∼4 GPa. The first-order phase transition retains the formation of the superstructure and is accompanied by a discontinuous lattice size decrease (ΔV/V ∼ 2%). The O′ phase is stabilized by the release of the steric crowding that develops upon compression; the Sm/Ca metal ions, which reside in the tetrahedral and octahedral holes of the fulleride sublattice, shift from their off-centered positions at low pressure (O phase) to nest at the centers of the interstices (O′ phase). The nearly exact coincidence of the pressure response of the reversible hysteretic O ↔ O′ phase transformation with that of the samarium valence transition from +2.33(2) to +2.71(3), as established before by synchrotron X-ray absorption spectroscopy, unambiguously establishes an intimate link between the crystal and electronic structures of the (Sm1/3Ca2/3)2.75C60 fulleride.

Triple-Kagomé-Layer Slabs of Mixed-Valence Rare-Earth Ions Exhibiting Quantum Spin Liquid Behaviors: Synthesis and Characterization of Eu9MgS2B20O41.

We prepared a new rare-earth compound, Eu9MgS2B20O41 (EMSBO), and characterized its structural and physical properties. EMSBO consists of triple-Kagomé-layer slabs separated by nonmagnetic ions and groups. Within each slab, intervalence charge transfer has been found to occur between the Eu2+ and Eu3+ Kagomé layers, a new channel for quantum fluctuation of magnetic moments. The measured magnetic susceptibilities and the specific heat capacity exhibit very similar features characteristic of quantum spin liquid behaviors observed in other materials.

Structural and Transport Studies on Mixed Valent Rare Earth Manganite Ceramics

ABSTRACTLa0.5Pr0.2Ca0.3–xBaxMnO3 (LPCBMO) (x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) ceramic manganites have been successfully synthesized by conventional solid state reaction method. Structural studies have been carried out by performing X-ray diffraction measurements and Rietveld refinements. Obtained value of crystallite size varies from 76 nm in lower doped sample (x=0.05) to 13 nm in the sample doped with x=0.30. Variation in crystallite size with x has been discussed in the context of size mismatch at A-site cationic lattice and structural disorder. In addition, all the samples exhibit metal to insulator transition at TP which gets increased with increase in Ba-content (x). Effect of crystallite size and other structural parameters on the transport properties of LPCBMO samples has been discussed in detail in the light of competition between the structural disorder and tolerance factor and shown that the tolerance factor can effectively modify the transport behaviour in manganites. Below 50 K, all the R-T plots of LPCBMO system show resistivity minimum behaviour which gets modified by Ba-content. This behaviour has been discussed in detail on the basis of electronelectron interaction having the form: ρ = [1/(σ0+BT1/2)]+ρnTn.

Mono-, Di-, Tri- and Tetranuclear Rare Earth Complexes Obtained Using a Moderately Bulky Aryloxide Ligand

Redox transmetallation ligand exchange reactions involving a rare earth metal, 2,4,6-trimethylphenol (HOmes), and a diarylmercurial afford rare earth aryloxo complexes, which are structurally characterized. Both the lanthanoid contraction and the identity of the reaction solvent are found to influence the outcome of the reactions. Using THF in the reaction affords a dinuclear species [Ln2(Omes)6(thf)4].2THF (Ln=La 1, Nd 2) for the lighter rare earth metals, while a mononuclear species [Ln(Omes)3(thf)3] (Ln=Sm 3, Tb 5, Er 6, Yb 7, Y 8) is obtained for the heavier rare earth elements. Surprisingly, there is no change in metal coordination number between the two structural motifs. A divalent trinuclear linear complex [Eu3(Omes)6(thf)6] 4 is obtained for Eu, and features solely bridging aryloxide ligands. Using DME as the reaction solvent affords [La(Omes)3(dme)2] 9 from the reaction mixture, and [Ln2(Omes)6(dme)2].PhMe (La 10, Nd 11) and [Y(Omes)3(dme)2] 14 following crystallization of the crude product from toluene. The dinuclear species [Eu2(Omes)4(dme)4] 12 contains two unidentate and two chelating DME ligands, and contrasts the linear structure of 4. Treatment of HOmes and HgPh2 with Yb metal in DME affords the mixed valent Yb(II/III) complex [Yb2(Omes)5(dme)2] 13, which is stabilized by an intramolecular pi-Ph-Yb interaction, and is a rare example of a mixed valent rare earth aryloxide. Treatment of Er metal with HOmes at elevated temperature (solvent free) affords the homoleptic [Er4(Omes)12] 15, which consists of a tetranuclear array of Er atoms arranged in a 'herringbone' fashion; the structure is stabilized by intramolecular pi-Ph-Er interactions. Reaction of La metal with HOmes under similar conditions yields toluene insoluble "La(Omes)3", which affords 1 following extraction with THF.

In vitro and in vivo investigations on the antiviral activity of a series of mixed-valence rare earth borotungstate heteropoly blues

A series of mixed-valence rare earth borotungsto-heteropoly blues, K 15H 2[Ln(BW 9W 2O 39) 2]·28H 2O (Ln(2), Ln = La, Ce, Pr, Nd, Sm, Eu, Gd), have been prepared and characterized by IR, UV, XPS, ESR and electrochemistry. The cytotoxicity and antiviral activity of these rare earth borotungstate heteropoly blues were investigated against influenza A(FluVA) strain (A/H1N1/Jingfang/1/91 and A/H3N2/Jingfang/30/95) and influenza virus B(FluVB) (B/Hufang/1/87) in MDCK cells. The results show that K 15H 2[Pr(BW 9W 2O 39) 2]·28H 2O (Pr(2)) exhibits the highest antiviral activity against FluVA with EC 50 of less than 4.0 μg/ml (A/H1N1/Jingfang/1/91) and 6.0 μg/ml (A/H3N2/Jingfang/30/95), respectively, and has the lowest toxicity with CC 50 of more than 480 μg/ml against MDCK cells. Additionally, both K 15H 2[Pr(BW 9W 2O 39) 2]·28H 2O (Pr(2)) and K 15H 2[Eu(BW 9W 2O 39) 2]·28H 2O (Eu(2)) showed excellent antiviral activities against FluVB by inhibiting FluVB replication at an EC 50 of less than 8.0 μg/ml. Furthermore, investigation on in vivo antiviral activity of Pr(2) against FluVA(FM1) in mice by giving the dosage either orally (p.o.) or intraperitoneally (i.p.), indicates that Pr(2) exhibits higher inhibitory activity than that of positive control, virazole, and that it's more effective when administered by i.p.

Observation of magnetization jump associated with a spin-state transition in oxygen-deficientY0.33Sr0.67CoO3−δ

dc magnetization, electronic transport, and structural properties are studied on the mixed valence rare earth Sr-doped cobaltites ${\mathrm{Y}}_{0.33}{\mathrm{Sr}}_{0.67}\mathrm{Co}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$. Oxygen deficiency and alternative layered structure with $\mathrm{Co}{\mathrm{O}}_{6}$ octahedron and $\mathrm{Co}{\mathrm{O}}_{4}$ tetrahedral polyhedron lead to the mixed valence states of Co ions. We found a Curie temperature at $304\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and a magnetization jump around $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ under $0.01\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ with a thermal hysteresis indicating a kind of magnetic memory effect. Temperature dependence of the electrical resistance exhibits nonmetallic with a thermal activation type. The observed magnetization jump is interpreted in term of a spin state transition of ${\mathrm{Co}}^{3+}$ ions in the octahedral site from the intermediate to low spin state. The intermediate spin configuration of ${\mathrm{Co}}^{3+}$ with ${t}_{2g}^{5}{e}_{g}^{1}$ may develop an orbital ordering which accounts for the nonmetallic transport. With increasing magnetic field, the magnetization jump gradually disappears and its onset temperature shifts to lower temperature. The oxygen rich compound ${\mathrm{Y}}_{0.33}{\mathrm{Sr}}_{0.67}\mathrm{Co}{\mathrm{O}}_{2.704}$ with 6% rich ${\mathrm{Co}}^{3+}$ population compared to the less oxygen compound ${\mathrm{Y}}_{0.33}{\mathrm{Sr}}_{0.67}\mathrm{Co}{\mathrm{O}}_{2.614}$ shows a relatively large magnetization jump as well as the enhancement of the ${e}_{g}$ electron localization leading to higher magnetic transition temperature from the intermediate to low spin state. Contrary, the less oxygen ${\mathrm{Y}}_{0.33}{\mathrm{Sr}}_{0.67}\mathrm{Co}{\mathrm{O}}_{2.614}$ shows the substantial low oxidation state as ${\mathrm{Co}}^{2+}$ which guides to a possible intermediate spin state stabilization.

Negative Thermal Expansion in the Mixed Valence Ytterbium Fulleride, Yb2.75C60

The powder synchrotron X-ray diffraction technique was used to study the temperature evolution of the structural properties of the mixed valence rare earth fulleride, Yb2.75C60, in the temperature range 5−295 K. The large lattice expansion (negative thermal expansion) on cooling below 60 K provides an unambiguous signature of a temperature-induced valence transition of the Yb atoms. The transformation is of electronic origin and is driven by the coupling of the Yb 4f band and the t1u band of C60. It is analogous to that observed at lower temperatures in Sm2.75C60, but it is absent when the electronically active 4f sublattice is missing in the related alkaline earth fulleride, Ca2.75C60.

Antiviral activity of mixed-valence rare earth borotungstate heteropoly blues against influenza virus in mice.

The acute and accumulated toxicity of the rare earth borotungstate heteropoly blues, HPB-2, Ce2H3[BW9(VI)W2(V)Mn(H2O)O39].12H2O, which is active against influenza virus in Kunming mice, were investigated in Kunming mice following oral and intraperitoneal administration. The activity of HPB-2 against influenza virus (FM1) in the mice was then investigated. HPB-2, given either orally (p.o.) or intraperitoneally (i.p.), was shown to have activity. HPB-2 was shown to be more effective than the positive control, ribavirin, and it was also found that i.p. administration was more effective than p.o. administration.

On the Origin of an Energy Gap in Semiconducting Mixed-Valent Rare-Earth Compounds

Results of recent band calculations on SmB 6 , YbB 12 , Ce 3 Pt 3 Sb 4 and CeNiSn support our previous prediction that the band gap appearing in their non-f reference systems correlates strongly with the origin of an energy gap in the mixed-valent rare-earth compounds. Based on these results, a more detailed model on the mechanism of the appearance of an energy gap is discussed.

Dynamic symmetry breaking in mixed-valence systems.

The precise mechanism which allows the hybridization between nearly degenerate localized 4f and (6s,5d)-conduction-band states of a mixed-valent rare earth (RE) is investigated in detail. We propose that a dynamic symmetry breaking, generated by coupling the displacement of the RE ion from its equilibrium position to the mixing term of the Anderson Hamiltonian, induces the hybridization of the opposite-parity f and sd states. The model is solved under the assumption that the degeneracy of the 4f level, N/sub f/, is large, which yields exact results in the N/sub f/..-->..infinity limit. Also, only one Einstein phonon mode is retained. Both a semiclassical and a full quantum-mechanical treatment are presented. Finally, we apply our model to the metal-insulator transition of SmS.

Xanes study of trivalent and homogeneous mixed valent rare earth systems

We report for the first time a systematic study of X-ray absorption structures 20–30eV above the L III edge (XANES) of homogeneous mixed valent (SmB 6, YbAl 2, CePd 3...) and trivalent reference Rare-Earth compounds (e.g. GdB 6, LaAl 2, NdPd 3). The XANES of integral valent compounds in simple structures (NaCl, AuCu 3...), which is due to multiple scattering of the photoelectron on the neighbouring atomic shells, is characterized by a single peak. On the contrary the XANES of mixed-valent materials presents a splitted structure which replicates the edge structures. We interpret this splitting as a direct evidence for the existence of two threshold energies and corresponding presumably to two different Rare-Earth metal distances in agreement with 4 f fluctuations. Finally, we shall propose a consistent picture for core hole spectroscopies (L III edge, XANES, EXAFS and XPS) in mixed-valent materials.

On the static susceptibilities of mixed valence systems

Magnetic and charge susceptibilities of mixed valence systems are investigated at temperatures above their fluctuation temperature. It is pointed out that an important contribution to their “paramagnetic Curie temperatures” is of single ion origin. This contribution is calculated for the various mixed valent rare earth ions. Special attention is paid to Tm-systems.

Electrical properties of mixed valence rare earth sulfides, (Sm1−xGdx)3S4

Abstract A new type of the valence transition was found in the mixed valence rare earth sulfides, (Sm1−xGdx)3S4. The conductivity of the materials gradually decreased up to x = 0.80 with increasing Gd concentration, in spite of mixing the metallic Gd3S4 with the semiconducting Sm3S4. A drastic change from the semiconducting to metallic conduction, however, was observed at the range between x = 0.80 and x = 0.85. These phenomena were explained by the valence change of the samarium ion in the solid solution.

Lattice dynamics in mixed valent Rare Earth compounds

In the frame of a single coherent picture for the electron lattice interactions in mixed valent Rare Earth compounds, the phase diagrams and phonon dispersions are studied. Our model is introduced on a microscopic level and discussed in detail. The mechanism of the phase transition and the role of the lattice is illuminated and various properties are considered in a model calculation. Anomalies in the phonon dispersions of Sm1−xYxS, which have recently been found, find a satisfying explanation.

Renormalization of transversal phonons in mixed valent rare earth compounds

The possibility of a softening of transversal acoustic phonons in mixed valent rare earth compounds is demonstrated. The effect should be largest at the zone boundary.

Spontaneous Magnetization in a Mixed-Valent Rare-Earth Compound

Spontaneous Magnetization in a Mixed-Valent Rare-Earth Compound

Susceptibility, electronic specific heat and magnetization of mixed-valent rare earth compounds

Applying our previous theory for mixed-valent materials, we report calculations of susceptibility, specific heat and magnetization of α-Ce, CePd 3, YbAl 3 and YbCuAl. The results are compared with experiment.

Electronic structure of divalent-, trivalent- and mixed valence rare earth compounds

The electronic structure of divalent, semiconducting EuS and SmS, trivalent, metallic GdS and SmS (high pressure phase), and some rare earth pnictides GdN, GdP, GdSb, GdBi, DyP, and HoP has been derived by a Kramers Kronig analysis of optical reflectivity over the photon energy range 0.03 eV–12 eV.