Modulation of the ferromagnetic insulating phase in Pr0.8Ca0.2MnO3 by Co substitution

Ferromagnetic insulator Pr_0.8Ca_0.2Mn_1–yCo_y O₃ (0 ≤ y ≤ 0.7) thin films were epitaxially grown by pulsed laser deposition on substrates of (LaAlO₃)_0.3(SrAl_0.5Ta_0.5O₃)_0.7 (100). To probe the ferromagnetic insulator state, the Co content dependences of the structural, magnetic, and transport properties were studied. Variation of lattice constant by the Co substitution is well reproduced considering that divalent and trivalent Co ions substitute for Mn ions at the perovskite B‐sites. For 0 ≤ y ≤ 0.3, the Curie temperature, saturation magnetization, and magnetoresistance increase with increasing Co content, retaining the insulating properties. Detailed analyses of transport and magnetic properties indicate the contribution of both double exchange and superexchange interactions to the appearance of the ferromagnetic insulating phase. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Application of151Eu Mössbauer spectroscopy to studies of electric field gradients in high-temperature superconductors

The rare-earth site of samples having the compositions (Fe,Cu)Sr₂(Eu,Ce) _n Cu₂O_4+2n+z (n=2 and 3) were studied by¹⁵¹Eu Mössbauer spectroscopy. The results obtained are compared with measurements of EuBa₂Cu₃O_7– (123) and Bi₂Sr₂(Ca_1–yEu_y)Cu₂O_8+E (2212) samples. The full quadrupole Hamiltonian of the 21.5-keV -transition was applied in analyzing the measured spectra. Simultaneous fitting of spectra recorded from crystallites oriented in various directions was used to determine the hyperfine parameters of then= 2 and 3 samples. Two different electric field gradients, not discernible in simple random crystallite measurements, were obtained for then=3 series.     

29Si-NMR-Untersuchungen zur Anionenstruktur von Kristallinen Tetramethylammonium-alumosilicaten und -alumosilicatlösungen

²⁹Si NMR Investigations on the Anion Structure of Crystalline Tetramethylammoniumaluminosilicates and -aluminosilicate Solutions The ²⁹Si NMR spectra of crystalline tetramethylammonium (TMA) aluminosilicates with different Si/Al ratios exhibit up to 4 sharp signals with characteristic chemical shifts which can be assigned to the central Si atom of OSi(OSi)_3?n(OAl)_n building units of double four-ring (DFR) aluminosilicate anions. The number and distributions of the Al atoms in the DFR framework can be derived from the signal intensities in connection with the results of the trimethylsilylation method [1]. A good agreement of the results of both methods has been found. The DFR can exist as monomeric unit or can be connected to polymeric structures by SiOAl bridges, but no information can be obtained about this question by the ²⁹Si NMR spectra. The investigation of the TMA aluminosilicate solutions by ²⁹Si NMR and TMS method [1] show that stable aluminosilicate anions exist in these solutions. The structure of these aluminosilicate anions is different from the structure of the crystalline TMA aluminosilicates obtained from the solutions.     

13C, 14N, 15N and 17O NMR spectra of the charged species of nitropyrroles and nitroimidazoles

Changes in ¹³C and ¹⁴N chemical shifts of the nitro derivatives of nitrogen heterocycles upon ionization (anion or cation formation) are twofold—first a uniform paramagnetic or in the case of protonation, a uniform diamagnetic shift of all the ring resonances that parallels the changes in the respective ultraviolet spectra and must be caused by changes in the molecular excited states, and second—the influence of the conjugated nitro group. About one third of the total negative anion charge may be localized on the nitro group, which causes unusually large shifts of the ring ¹³C resonances in this case.     

Conformational and long-range low field steric deuterium isotope effects on carbon chemical shifts

Comparison of the room and low temperature (203 K) ¹³C NMR spectra of 3-cis-bicyclo[4.4.0]decanone and its 2,2,4,4-tetradeuterio isotopomer provides information about the deuterium isotope effect on the conformational equilibrium in this compound. Four-bond low field deuterium isotope effects on some carbon chemical shifts are observed. These effects can be used for unambiguous assignment of the ¹³C lines to carbon atoms in alicyclic compounds.