Magnetic susceptibility and magnetic resonance study of acceptor doped fullerenes C60(MF6)2 (M=As, Sb, P)

In order to check whether superconductivity occurs in the acceptor doped fullerenes C₆₀(MF₆)₂ (M=As, P, Sb) and to study their magnetic and structural properties, we have carried out magnetic, EPR and NMR measurements of these compounds. Temperature dependences of magnetic susceptibility down to 5 K and field dependences of magnetic moment at 5 K show no ‘bulk’ transition in superconducting state. Some reasons of the absence of superconductivity, such as insufficient charge transfer between C₆₀ and intercalated species and inhomogeneity of the compounds under study, are discussed.     

Mössbauer studies and magnetic properties of Ln2SrCu2O6 compounds

Mössbauer and magnetic susceptibility measurements were used to study the magnetic properties of Ln_1.9Sr_1.1Cu₂O₆ (Ln=Pr,Nd) and La₂SrCu₂O₆ materials. These compounds were prepared by solid-state reaction and crystallize in a tetragonal structure, space group I4/mmm with two formula units per unit cell. There is only one crystallographic site for Cu atoms, which form a double layer of CuO₅ pyramids. These compounds are not superconducting, but we show, using Mössbauer spectroscopy (MS) on iron doped samples and susceptibility measurements, that the Cu planes order antiferromagnetically. The hyperfine fields on iron nuclei at 4.2 K extend from 472 kOe for La₂SrCu₂O₆ to 501 kOe for Nd_1.9Sr_1.1Cu₂O₆. The ordering temperaturesT _N are: R20, 190, and 250 K for Ln=La, Pr and Nd, respectively.     

Multiple magnetic phase transitions,dilute Fe hyperfine fields and europium valence instabilities in RMn2Si2-xGex,R=rare earth

Mössbauer studies of dilute⁵⁷Fe and¹⁵¹Eu in RMn₂Si_2-xGe_x (R=La, Sm, Eu and Gd) at temperatures 4.2 K to 480 K have been performed. The diamagnetic iron and europium reveal the magnetic order of the Mn and rare earth sublattices through transferred hyperfine interactions. The⁵⁷Fe studies show that in LaMn₂Si₂, LaMn₂Ge₂, and SmMn₂Ge₂ the Mn is magnetically ordered above the known Curie temperatures, and the compounds are antiferromagnets up to T_N=470 K, 415 K and 385 K respectively. Studies of¹⁵¹Eu in R_1-xEu_xMn₂Si₂, (R=La, Gd) display Eu subspectra corresponding to Eu²⁺, Eu³⁺ and intermediate valant Eu. All display large magnetic hyperfine fields.     

Some new results and interpretation concerning the system FeYb2S4

Stoichiometric powdered FeYb₂S₄ having a cubic (special) spinel structure was prepared. The structure was based on X-ray diffraction and Mössbauer spectroscopy. The space group is Fd3m (No. 227) and the lattice constant a₀ = 10.828Å. All the iron is divalent but only ca. 80% fills the tetrahedral 8a site while ca. 20% fills the octahedral 16d site. The ytterbium is trivalent, most of it occupying the normal octahedral 16d spinel site, but ca. 16% occupies the normally empty octahedral 16c site, having been displaced by the iron.     

151Eu mössbauer spectroscopy studies of substituted EuBa2Cu3O7 systems

Mössbauer spectroscopy studies of¹⁵¹ Eu in EuBa₂Cu₃O₇, EuBa₂Cu₃O₆, EuBa₂Cu₃O₆S, EuBaKCu₃O₇, EuBa₂Cu_2.5Fe_0.5O₇, Eu_0.5Pr_0.5Ba₂Cu₃O₇, Eu_1.25Ba_1.75Cu₃O₇ and EuCaBaCu₃O₇ at several temperatures have been performed. The major conclusions are; All substitutions affect the Eu quadrupole interaction (10–20%). Substitution of 0 by S increases the Eu quadrupole interaction by 15% and increases η from 0.3 to 0.8 which proves that S substitutes oxygen in the immediate neighbourhood of the rare earth ions. In EuCaBaCu₃O₇ the quadrupole interaction is larger by 80% than in EuBa₂Cu₃O₇, proving that in this structure Eu occupies predominantly the Ba sites.     

Intermediate valence of europium in EuxLa1−xBe13

Mössbauer studies of the 21.6 keV transition of¹⁵¹Eu in cubic EuBe₁₃ and Eu_1/2La_1/2Be₁₃ in the temperature range 100 K to 600 K have been performed. The position of the absorption line in EuBe₁₃ moves from 0.60 mm/s at 100 K to 0.04 mm/s at 575 K. In Eu_1/2La_1/2B₁₃ the line moves from 0.50 mm/s at 100 K to –0.03 mm/s at 500 K. We conclude that Eu in Eu_xLa_1–xB₁₃ is in an intermediate valence state and we analyze the temperature dependence of the isomer shift in terms of an interconfigurational fluctuation model. The model contains temperature independent parameters E_exc, the interconfiguration excitation energy, T_f, the valence fluctuation temperature, and S₃, the isomer shift of pure Eu³⁺ in EuBe₁₃. Assuming S₃–S₂= 13 mm/s the analysis yields S₃=0.70 mm/s, T_f=400 K and E_exc=3000 K for EuBe₁₃, whereas T_f=460 K and E_exc=2750 K for Eu_1/2La_1/2B₁₃.