Absence of long-range magnetic order in the La1.4Sr0.8Ca0.8Mn2O7 bilayered manganite.

This paper presents the results of a high-resolution neutron diffraction and magnetometry investigation on the optimally doped (x = 0.3) La(1.4)Sr(0.8)Ca(0.8)Mn(2)O(7) bilayered manganite. In particular, two samples with different oxygen contents have been studied to put in prominence the role of the Mn average valence states at fixed cation concentration. The results show, for the first time, the absence of long-range magnetic order in this optimally doped manganite when the A-site of the structure is doped with equal proportions of isovalent Ca and Sr. This holds for both samples, which present different lattice effects with T, thus suggesting the primary role of cation disorder as the source of the lack of long-range order. The presence, for both samples, of defined insulating- to metallic-like transitions suggests that the transport properties are not linked to the evolution of long-range order and that two-dimensional spin ordering in the layers of the perovskite blocks may be sufficient to "assist" the hole hopping. A possible reason for the suppression of magnetic order induced by the Ca doping is a size effect coupled to the cation size mismatch between the Sr and Ca ions.

Doping effects in single-layered La0.5Sr1.5MnO4 manganite.

In this paper we report the results of the synthesis and structural, transport, and magnetic characterization of pure La(0.5)Sr(1.5)MnO(4) and B-site lightly doped samples, i.e. La(0.5)Sr(1.5)Mn(0.95)B(0.05)O(4), where B = Ru, Co, and Ni. The choice was made in order to probe the charge ordering/orbital ordering ground state of the monolayered La(0.5)Sr(1.5)MnO(4) manganite as a consequence of the cation doping. It is shown that even a light doping is successful in suppressing the charge and orbital order found in pure La(0.5)Sr(1.5)MnO(4). No long-range magnetic order has been detected in any of the doped samples but the setup of a spin-glass state with a common freezing temperature ( approximately 22 K). Structural parameters show an anisotropy in the lattice constant variation, with the tetragonal distortion increasing as the cell volume reduces, which may suggest a variation in the orbital character of the e(g) electrons along with the overall cation size.

Local structure and electronic properties of the rhombohedral and orthorhombic colossally magnetoresistive manganites La1-xNaxMnO3 by Mn-K edge EXAFS and XANES.

The local structure and electronic properties of Mn in Na doped lanthanum manganites have been investigated by means of Mn-K edge XAFS, at various temperatures ranging from RT to 30 K. The hole content depends on the amount of doping, both with Na which injects two holes per doping atom and with oxygen nonstoichiometry. The holes are annihilated by oxygen under-stoichiometry via the formation of oxygen vacancies; the presence of a greater static disorder in the orthorhombic manganites is evidenced by both EXAFS and XANES analysis. In the rombohedral samples, a single Mn-O distance is found. In addition, the trend of the EXAFS Debye-Waller factors for the Mn-O distance can be fitted with a simple Einstein model, with no static disorder at low temperature. This evidence suggests small disorder of polaronic and of Jahn-Teller origin in the rombohedral samples at low temperatures, which is in contrast with what was found for the orthorhombic samples below T(c).

Influence of Ru doping on the structure, defect chemistry, magnetic interaction, and carrier motion of the La(1-x)Na(x)MnO(3+delta) manganite.

In this work, we report a structural, electrical, and magnetic characterization of the La(1-x)Na(x)Mn(1-y)Ru(y)O(3+delta) (LNMRO) system with x = 0.05 and 0.15 and y = 0, 0.05, and 0.15, also comprising an investigation of the role of the oxygen content on the related redox properties. The experimental investigation has been realized with the aid of X-ray powder diffraction, electron microprobe analysis, thermogravimetry, electrical resistivity and magnetization measurements, and electron paramagnetic resonance. We demonstrate that the effect of ruthenium doping on the studied LNMRO compounds is not only directly related to the Ru/Mn substitution and to the Ru oxidation state but also indirectly connected to the oxygen content in the sample. Our data show that ruthenium addition can "improve" electrical and magnetic properties of nonoptimally (low) cation-doped manganites, causing an increase of the T(C) value and the insurgence of magnetoresistance effect, as observed for the x = 0.05 and y = 0.05 sample (MR congruent with 60% at 7 T and at approximately 260 K).

Unexpected effect of Ru-substitution in lightly doped manganites.

In this communication we report the unexpected effect of ruthenium doping in sodium lightly doped manganites. This effect seems to be in contrast with the usual model applied to describe the effect of this magnetic ion into the manganite structure. We propose a possible compensation mechanism which seems also able to describe other peculiar features encountered in these materials.

Nature of the monoclinic to cubic phase transition in the fast oxygen ion conductor La2Mo2O9 (LAMOX).

La2Mo2O9 (LAMOX) is a fast oxygen ion conductor which shows high oxygen ion conductivities comparable to those of yttria-sabilized zirconia (YSZ). LAMOX is subject to a structural phase transition from the nonconductive monoclinic form to the highly conductive cubic form at about 580 degrees C. The origin of the conductivity in cubic LAMOX has been suggested to be due to a "disorder" in the O sublattice without any insight into the real distribution of the oxygen ion. In this paper, thanks to the application of the neutron atomic pair distribution function (PDF) analysis, we provide evidence that the local structure of the cubic polymorph of LAMOX is exactly the same of that of the monoclinic phase, thus indicating that the structural phase transition is actually a transition from a static to a dynamic distribution of the oxygen defects. This work represents the first application of the atomic-pair distribution function analysis to the study of an oxygen fast-oxide ion conductor and clearly indicates that a more reliable and detailed description of their local structure, particularly in the highly conductive phases, can lead to a better comprehension of the structure-property correlation, which is the starting point for the design of new and optimized functional materials.