RESUMEN
An efficient synthesis route is proposed to obtain single phase powder ceramic of CaFe5O7. This complex structure can be described as an intergrowth between one CaFe2O4 unit and n = 3 slices of FeO Wüstite-type structure. A detailed structural study has been carried out at room temperature combining transmission electron microscopy (TEM) observations (ED, HREM), scanning transmission electron microscopy (STEM-HAADF), and X-ray diffraction data. The analysis of these data has revealed an unexpected supercell with a monoclinic symmetry. From the hkl conditions deduced from the electron diffraction study and the analysis of X-ray diffraction data by simulated annealing, a structural model considering the centrosymmetric P21/m setting can be proposed. In addition the first magnetic and electrical transport measurements are reported showing a sharp peak in magnetic susceptibility and a strong localization around 360 K, associated to a structural change from monoclinic setting to orthorhombic one.
RESUMEN
Stoichiometric LnBaFe4O7.0 oxides with Ln = Dy to Lu have been synthesized and protected in order to prevent oxidation at room temperature. The structural study of these compounds, using laboratory and synchrotron X-ray as well as neutron powder diffraction, shows the extraordinary flexibility of the tetrahedral [Fe4] sublattice of these compounds, which exhibit various distortions. At room temperature they all are tetragonal (I4), and at higher temperature (T > 580 K) they exhibit a cubic symmetry (F43m). Moreover, the low-temperature structures of these oxides are dependent on the nature of the Ln(3+) cation. At 110 K, compounds with Ln = Dy and Ho adopt the same monoclinic (P12(1)1) structure as YBaFe4O7.0, whereas YbBaFe4O7.0 possesses a new centered monoclinic cell (I121), and members with Ln = Er and Lu keep the tetragonal (I4) symmetry. Neutron diffraction patterns evidence long-range magnetic ordering only for the most distorted structures (Ln = Dy and Ho), showing that the geometric frustration generated by the tetrahedral [Fe4]∞ sublattice can be lifted only with the most severe distortions. The other oxides (Ln = Er, Yb, and Lu) with weakly distorted [Fe4]∞ sublattices do not exhibit magnetic ordering down to 4 K, demonstrating the importance of magnetic frustration. The behavior of these "114" iron oxides is compared to the cobalt family, showing in both cases a striking underbonding of barium.
RESUMEN
The exploration of the systems Fe-H-P-O, Li-Fe-H-P-O, and K-Fe-H-P-O using soft chemistry methods has allowed three new hydrogenophosphates to be synthesized, whose structures have been determined by ab initio calculations. The structures of two of them--FeH(2)P(2)O(7) and LiH(3)Fe(P(2)O(7))(2)--exhibit close relationships: their 3D framework consists of [FeO(4)](infinity) and [LiFe(2)O(12)](infinity) chains of edge-sharing octahedra, respectively, interconnected through diphosphate groups. These two diphosphates exhibit a paramagnetic to antiferromagnetic transition at low temperatures. The former phase exhibits intrachain ferromagnetic interactions (theta(p) > 0) in competition with the antiferromagnetic interchain ordering, whereas for the second one, the ferromagnetic interactions have disappeared due to the presence of Li in the chains. Differently, the third phosphate, K(2)Fe(HP(2)O(7))(H(2)PO(4))(2), exhibits a chain structure, involving isolated FeO(6) octahedra, and consequently is paramagnetic in the whole temperature range (4-300 K). Among these three phosphates, only the latter exhibits ionic conductivity, which may originate from the proton mobility.
RESUMEN
We have investigated systematically the physical transport properties of layered 112-type cobaltite by means of electrical resistivity, magnetoresistance and thermopower measurements. In order to understand the complex transport mechanism of LaBaCo(2)O(5.5), the data have been analysed using different theoretical models. The compound shows an electronic transition between two semiconducting states around 326 K, which coincides with the ferromagnetic transition. Interestingly, the system also depicts a significant magnetoresistance (MR) effect near the ferro/antiferromagnetic phase boundary and the highest value of MR is close to 5% at 245 K under ± 7 T. The temperature dependence of thermopower, S(T), exhibits p-type conductivity in the 60 K≤T≤320 K range and reaches a maximum value of around 303 µV K(-1) (at 120 K). In the low temperature antiferromagnetic region the unusual S(T) behaviour, generally observed for the cobaltite series LnBaCo(2)O(5.5) (Ln = rare earth), is explained by the electron magnon scattering mechanism.
RESUMEN
A new hydrogenophosphate Mn(H 2PO 4) 2 has been synthesized from an aqueous solution. Its ab initio structure resolution shows that the original layered structure of this phase consists of PO 2(OH) 2 tetrahedra and MnO 5OH octahedra, sharing corners to form [MnP 2O 8H 4] infinity layers, whose cohesion is ensured through hydrogen bonds. The excitation and emission spectra of this phase are characteristic of Mn (2+) species. This phosphate is shown to be a good protonic conductor with a conductivity of 10 (-4.4) S/cm at 90 degrees C (363 K).
RESUMEN
Cerium for strontium substitution allows an oxygen deficient perovskite Sr(1-x)Ce(x)CoO(3-δ) to be stabilized with a cerium solubility limited to x≤0.15 (Trofimenko et al 1997 Solid State Ion. 100 183). For these samples, the magnetic properties depend clearly upon the oxygen content: Sr(0.9)Ce(0.1)CoO(2.74) and Sr(0.9)Ce(0.1)CoO(2.83) are weak and strong ferromagnets (T(C) = 160 K), respectively, the maximum ac-magnetic susceptibility of the latter being larger by two orders of magnitude than that of the former. In contrast to other Sr(1-x)L(x)CoO(3-δ) series (L = lanthanide, Y(3) or Th(4+)), the electrical resistivity (ρ) behaviour does not simply reflect the magnetic behaviour. For x = 0.10 the re-entrant ρ feature becomes more pronounced whereas the ferromagnetic fraction and cobalt oxidation state increase. This unexpected behaviour could be related to the Ce(3+)/Ce(4+) mixed valency, the 4f localized moment of the Ce(3+) cations interacting with the conduction electrons through a Kondo-like mechanism. It is also found that the increase of oxygen vacancies favours the appearance of magnetoresistance at low T, reaching -50% at 5 K in 7 T for the Sr(0.95)Ce(0.05)CoO(2.61) sample prepared in a sealed tube.
RESUMEN
Phosphonate esters react with gamma-alumina under microwave (MW) irradiation; this reaction is a simple preparative method to graft organic pendant groups onto the surface of alumina; the efficiency of the grafting was readily checked by solid-state NMR techniques (31P and 27Al).
RESUMEN
The orthorhombic perovskite, (La(1-x/2)Bi(x/2))(Fe(0.5)Cr(0.5))O(3) was investigated for 0≤x≤1. Its space group, Pnma, compatible with the disordering of iron and chromium in the B sites, confirms previous observations. More importantly this compound is found to be an uncompensated weak ferromagnet, with a very peculiar zero magnetization behaviour, generally observed for ordered magnetic cations in the B sites. It exhibits a magnetic transition at high temperature (T(C)) above 450 K, while the zero magnetization occurs between 100 and 160 K depending on the x-value. The AC magnetic susceptibility study shows that this compound does not exhibit a spin glass or cluster glass behaviour, in contrast to what was suggested for the x = 0 compound. This zero magnetization phenomenon can be interpreted by the fact that this perovskite is an uncompensated weak ferromagnet, which consists of canted weak ferromagnetic domains and clusters of pure chromium and pure iron composition, antiferromagnetically coupled through Cr-O-Fe interactions.
RESUMEN
A new layered titanoniobate, LiTiNbO5, an n = 2 member of the A(x)M(2n)O(4n+2) family, has been synthesized using a molten salt reaction between HTiNbO5 and an eutectic "LiOH/LiNO3". This compound crystallizes in the P2(1)/m space group with a = 6.41 A, b = 3.77 A, c = 8.08 A, and beta = 92 degrees . It exhibits |TiNbO5|(infinity) layers similar to HTiNbO5, but differs from the latter by a "parallel configuration" of its |TiNbO6|(infinity) ribbons between the two successive layers. The topotactic character of the reaction suggests that exfoliation plays a prominent role in the synthesis of this new form. This new phase intercalates reversibly 0.8 lithium through a first-order transformation leading to a capacity of 94 mAh/g at a potential of 1.67 V vs Li/Li+.
RESUMEN
Thermoelectric power measurements have been performed for an ordered oxygen-deficient perovskite, HoBaCo2O5.5, in which the alternative layers of CoO6 octahedra and of [CoO(5)](2) bipyramids are occupied by Co3+ species. The T-dependent Seebeck coefficient S shows a clear change of the conduction regime at the metal-insulator (MI) transition (T(MI) approximately 285 K). The sign change of S from S<0 to S>0 can be explained assuming that a spin state transition occurs at T(MI). In the metallic state, Co2+ e(g) electrons are moving in a broad band on the background of high or intermediate spin Co3+ species. In contrast, the insulating behavior may result from the Co3+ spin state transition to a low-spin Co3+ occurring in the octahedra. In this phase the transport would occur by hopping of the low-spin Co(4+)t(2g) holes, whereas the high-spin Co2+ electrons become immobilized due to a spin blockade.
RESUMEN
The structure of the insulating manganites Nd(1-x)Ca(x)MnO(3) (0.3