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The possibility of synthesizing layered oxycarbonates, with nominal composition Sr4Fe2-xMxO6CO3 (with M = Sc, Co, or Ni) involving trivalent scandium, cobalt, or nickel, with 0 ≤ x ≤ 2 (M = Sc), 0 ≤ x ≤ 1.5 (M = Co), and 0 ≤ x ≤ 1.1 (M = Ni), is reported for the first time. The structural studies of Sr4Sc2O6CO3 and Sr4FeMO6CO3 (M = Co and Ni) show that these three phases are closely related to the n = 3 member of the Ruddlesden−Popper family. They derive from the latter by replacing the middle layer of transition-metal octahedra by triangular CO3 groups, with two different “flag” and “coat hanger” configurations. Magnetic behaviors that are fundamentally different from the magnetic behavior of Sr4Fe2O6CO3 are also reported.

A very large oxygen deficiency has been detected in an n = 2 member of the Ruddlesden and Popper (RP) series, Sr3FeCoO7-x, with x reaching 1.55. For any x, the compounds are tetragonal I4/mmm with a ≈ 3.85 Å and c ≈ 20.2 Å, a and c decreasing with x. The neutron powder diffraction (NPD) study of the most oxygen deficient phase, x = 1.55, shows that the oxygen vacancies are located in the common apex and in the basal plane of the double perovskite layers, leading for cobalt and iron to local tetrahedral and pyramidal coordinations. XANES spectroscopy reveals a divalent oxidation state for cobalt with strongly distorted tetrahedral coordination, whereas iron exhibits a trivalent state with a square pyramidal environment. This observation associated with the low dimensional character of the structure lead to complex magnetic properties. When the oxygen content is increased to x = 1.1, the formal charge of cobalt first increases to Co³⁺ whereas its coordination changes from tetrahedral to square pyramidal, Fe³⁺ being unchanged. For an oxygen stoichiometry close to 7 (x = 0.1), both cations are tetravalent and sit in an octahedral environment. Physical properties are also discussed.

An X-ray absorption spectroscopy study of the polycrystalline La1−xSrxCoO3(x=0, 0.25, 0.5) series at O K-edge was performed above and below the various magnetic transition temperatures. A crystal field analysis is proposed to explain the observed fine structure changes. Temperature-dependent O K-edge of LaCoO3shows evidence of smooth and continuous spin transition of Co3+from low to intermediate spin states. This result is confirmed by cobalt K-edge measurements. Upon strontium doping, O K-edges of the La1−xSrxCoO3(x=0, 0.25, 0.5) series suggested a mixed (LS–IS) Co3+/IS Co4+cobalt ion configuration. The fundamental mechanism governing the magnetotransport properties shown by the compounds is then discussed.

Domain fluctuations at low temperature are evidenced in perovskite-type rare earth manganites by two local methods: X-ray absorption and Mössbauer spectroscopies. For the CMR compound La0.7Ca0.3MnO3, the thermal variations of the prepeak at the O K-edge, obtained by XAS measurements, show that the large prepeak observed at 300 K becomes a double peak below 200 K. These shape changes are very smooth and gradual around Tcand indicate the existence of FMMdomain fluctuations around the PMI–FMMtransition temperature. For Sm0.5Ca0.5MnO3doped with 1.5% 57Fe, the Mössbauer spectra evidence charge and magnetic ordering phenomena below Tco≈270 K. The magnetic transition, spread on a large temperature domain, is attributed to fluctuations in the antiferromagnetic state.

Abstract: Hyperfine electric and magnetic interactions in Y1−z Ca z Ba2Cu3−x Fe x O6+y compounds have been studied for different rates of Fe and Ca substitution (x=0.09 and 0.24;z=0.24;y=0−1) by57Fe Mssbauer spectroscopy of powder samples obtained by different thermal treatments. Calculations of the Fe3+ EFG tensor have been performed for different coordination polyhedra in Cu1 and Cu2 sites. Variations of the direction, amplitude and of the sign of the principal component of the EFG are reported versus the iron displacement from the ideal Cu sites inside the pyramidal and tetrahedral coordination polyhedra. Calculated ΔE Q and η values are compared with the experimental ones. For the Cu2 site, faithfully probed in both oxygen-depleted and oxygen-saturated samples by fairly large fraction of residing iron atoms, the mutual orientations of the axes of the principal EFG component and that of easiest magnetization are deduced from interrelations between quadrupole parameters for paramagnetic quadrupolar and magnetic Zeeman Mssbauer spectra. In the desoxygenated samples, the coexistence of a long range 2D antiferromagnetic order and of a spin-glass order has been evidenced. For the oxygen-saturated samples, the two-dimensional spin-glass order is observed belowT f =10 K. The Mssbauer spectra under applied fields at 4.2 K show an easy polarization of the spins in the Cu2 sites.

Study of the change induced by heavy ion irradiation in the magnetic properties of magnetite ceramics Fe3O4can be important for applications. In this work, we report on the effect of the high electronic excitation induced by lead ions on the magnetic properties of highly densified Fe3O4ceramics. The physical changes induced by irradiation have been investigated by magnetization measurements and Mössbauer spectroscopy, whereas the damage morphology has been observed by TEM. Variations of the saturation magnetization have shown original behaviour versus fluence: the magnetization starts to decrease at low fluence before recovering its initial value at higher fluence. TEM observations have shown the presence of extended spherical defects in the ion wakes. This damage morphology is very similar to that observed for krypton irradiation of the garnet Y3Fe5O12at low electronic stopping power, which is characterized by a very small amorphous volume. In agreement with this observation, Mössbauer spectroscopy, recorded below the Verwey transition, has shown the absence of a paramagnetic phase. Moreover, Mössbauer spectra exhibited an irradiation-induced anisotropy of the hyperfine magnetic field which revealed the presence of defect-induced anisotropy. Such a result led us to the hypothesis that magnetic domain walls could be pinned by the extended defects, at least for low fluences.

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Y3Fe5O12 garnets have been irradiated by Mo and Xe ion beams at 2.3 and 3.3 GeV respectively. High-resolution electron microscopy (HREM), channelling Rutherford backscattering (RBS) and Mössbauer spectrometry have been used to determine the variation of the radius of latent damage track induced along the ion path as a function of the electronic stopping powers between 34 and 56 MeV cm2 mg-1. On comparing the results obtained by different experimental methods it is found that the disorder characterized by channelling RBS measurements extends over the same cylindrical volume as the paramagnetic state observed by Mössbauer spectrometry. A direct HREM measurement of the radii shows a larger cylindrical volume than both the other measurements, indicating the existence of a distorted shell around the disordered core.

Abstract:: The XAS study at Cr, Co, Ni and Mn K-edges was performed for the doped CMR manganites Ln0.5Ca0.5Mn1-xBxO3 with Ln=La, Nd, Sm and B= Cr, Co, Ni ($$(0 \le x \le 0.10)$$), on the samples that were studied previously for their ferromagnetic-metallic to antiferromagnetic-insulator transition. We observed that the formal charges of the doping elements are Ni2+, Co2+ and Cr3+. It is also evidenced that the average formal charge of the manganese is increased after doping, in agreement with the charge compensation keeping “O3” stoichiometry. These results suggest that the doping elements participate directly to the band structure.

Using a combination of microscopy, powder X-ray diffraction, and model building, we recently reported evidence for the first misfit layer oxide, Tl0.41(Sr0.9O)1.12CoO2 (Boullay et al. Chem. Mater. 1996, 8, 1482). In this work, we present a more detailed structural characterization from an additional high-resolution electron microscopy (HREM) study as well as extended X-ray absorption fine structures (EXAFS) from the Co K edge and the Tl LIII edge. We find a structural picture largely consistent with the one originally proposed, but much more quantitative. The work has been extended over a structural family, obtained by replacing Sr by Ca to obtain the title misfit phases. The evolution of composition and lattice parameters, as well as the magnetic and electrical transport properties of representative members of the misfit oxide family, are studied.

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The magnetic ordering in the orthorhombic phases LnSr2Cu2Ga1-xFexO7(Ln= Y, Ho; 0.03 ≤ x≤ 0.95) has been investigated by 57Fe Mössbauer spectroscopy. Iron occupies the Ga site (Cu(1) site) for small doping rates (x≈ 0.03), but occupies the Cu(1) and Cu(2) sites for iron-rich compositions (0.5 ≤ x≤ 0.95). For x= 0.9, the magnetic ordering takes place below TN≈ 420 K in both the Cu(1) and Cu(2) sites. The down concave curve of the temperature dependence of the hyperfine field in the Cu(1) site suggests a rather weak coupling between the Cu(1) and Cu(2) sites. The principal EFG axis VZZis directed in both sites along the longest axis of the orthorhombic cell, i.e., perpendicular to the alternating sheets. The sign of VZZis positive for the Cu(2) sites (ΔEQ= 0.6 mm/sec) and negative for the Cu(1) sites (ΔEQ= -1.9 mm/sec). In the distorted tetrahedral site, the EFG tensor is non-axial, however, its asymmetry is small (η < 0.4). The magnetic hyperfine field is perpendicular to the VZZaxis for both sites; i.e., H lies in the plane of the shortest axes. A structural analysis by electron microscopy reveals the presence of a large number of defects in the (Fe, Ga)O chain ordering as well as in the basic structure. The presence and the abundance of these defects is in agreement with the Mössbauer observations.

Unexpected emission bands have been observed for ZnCdS: Ag high in cadmium. With high Ag, the main emission band under cathode-ray excitation at 10 kv appeared to be shifted to shorter wavelength on increase in current density. Under ultraviolet excitation at −196°C, the apparent shift was resolved into two emission bands, one of which corresponded to the new band that appeared under cathode rays; the other, to the normal band of the unactivated phosphor. With low Ag, edge emission bands appeared under high current density cathode rays and sodium bands under ultraviolet at −196°C.

Transparent coatings of a number of luminescent materials have been deposited on glass surfaces by chemical reaction of the components in the vapor state. The best of these so far obtained is zinc sulfide. Arsenic, copper, manganese, phosphorus, or zinc have been successfully introduced as activators. Under cathode ray excitation, some of these have given brightness as high as 40 foot-lamberts at 20 kv and 1 microampere per sq cm. The uv response of these films is low, but varies with the conditions of preparation. The properties of these nonscattering phosphor screens are described and their advantages over powder phosphors considered.