J.L. García-Muñoz

Structural characterization of R2BaCuO5 (R = Y, Lu, Yb, Tm, Er, Ho, Dy, Gd, Eu and Sm) oxides by X-ray and neutron diffraction

Abstract R2BaCuO5 compounds with R = rare earth atom from Sm to Lu have been studied by neutron and X-ray powder diffraction. All of them are isostructural, and belong to the space group Pnma (Z = 4). A comparative study of the influence of the rare earth size on the structural parameters is presented. The stability of the structure is estimated from the agreement between valence bond sums and formal valence states. A systematic variation of the structural stability is found depending on the size of the rare earth atom.

Selective spin-state switch and metal-insulator transition in GdBaCo2O5.5

Ultrahigh resolution synchrotron diffraction data for GdBaCo 2 O 5 . 5 throw new light on the metal-insulator transition of Co 3 + Ba-cobaltites. An anomalous expansion of CoO 6 octahedra is observed at the phase transitionon heating, while CoO 5 pyramids show the normal shrinking at the closing of the gap. The insulator-to-metal transition is attributed to a sudden excitation of some electrons in the octahedra (t 6 2 g state) into the Co e g band (final t 4 2 g e 2 g state). The t 5 2 g e 1 g state in the pyramids does not change and the structural study also rules out a d 3 x 2 r 2 /d 3 y 2 - r 2 orbital ordering at T M I .

Magnetic susceptibility and field-induced transitions in R2BaNiO5 compounds (R = Tm, Er, Ho, Dy, Tb, Gd, Sm, Nd, Pr)

Abstract Magnetic susceptibility and magnetization as a function of temperature and applied field of the series of nickelates with the general formula R 2 BaNiO 5 have been measured. At variance with the strong one-dimensional AF S = 1 correlations along the chains of NiO 6 octahedra in Y 2 BaNiO 5 , the introduction of magnetic rare earths gives rise to two separate magnetic anomalies in the magnetic susceptibility at T 1 and T 2 ( T 2 T 1 ). T 1 coincides with the setting up of magnetic ordering in Ni and R sublattices seen by neutron diffraction, and corresponds to the Neel temperature. The neutron data showed that there is no magnetic symmetry change at T 2 . A simple molecular field model roughly explains the general behaviour of the susceptibility, in particular the presence of a broad maximum below the Neel point. Most of these antiferromagnetic oxides display one or two metamagnetic-like transitions at low temperature. The magnetic field dependence of the magnetization upon temperature reveals that, except for Gd, the external field-induced transitions vanish around T 2 .

Bandwidth narrowing in bulk magnetoresistive oxides

The key structural factor governing the one-electron bandwidth (W) in (A = Ca, Sr) magnetoresistive oxides has been identified by a neutron study. Reducing the size of the lanthanide, the Mn - O bondlength in octahedra does not change () and hence does not contribute to the narrowing of the conduction bandwidth. In contrast, the Mn - O - Mn angle bends at a rate . The calculation of the bandwidth dependence on has been performed and it has been found to be proportional to the measured Curie temperature, both having almost a dependence.

Selective spin-state and metal-insulator transitions in GdBaCo 2 O 5:5

Abstract By means of ultra-high resolution synchrotron diffraction and calorimetry measurements we have studied the metal–insulator transition in GdBaCo 2 O 5.5 . The appearance of the metallic state is attributed to a sudden excitation of some electrons in the octahedra ( t 2 g 6 state) into the Co e g band (final t 2 g 4 e g 2 state). In contrast, the t 2 g 5 e g 1 state in the pyramids does not change at the transition. Calorimetry measurements show that the insulator-to-metal transition is first order and the entropy change estimated from the latent heat corroborates the pictured scheme.

Charge and orbital order in rare-earth and Bi manganites: a comparison

A comparative investigation of the charge modulation tendencies in R–M–MnO3 and Bi–Sr–MnO3 manganites reveals remarkable differences in these series of materials. By means of magnetic, magnetotransport, synchrotron X-ray and neutron diffraction, as well as pulsed high magnetic fields data, we compare the systematic electronic and magnetic properties of several rareearth-based manganites and bismuth-based Sr manganites. The differences between these types of systems are rationalized in terms of the role that the different electronic structure of bismuth and rare earth plays in these materials.

Extraordinary thermopower in magnetoresistive (La1−xYx)0.67Ca0.33MnO3 oxides

We report that the thermopower Q(T) of the (La1−xYx)0.67Ca0.33MnO3 (x=0–0.25) magnetoresistive oxides, displays a pronounced peak at temperatures Ts close to the ferromagnetic ordering temperature. The temperature Ts decreases monotonously when increasing x whereas the maximal Q(Ts) is found to increase with the Y contents. The huge enhancement of Q at Ts is found to be as high as 460% for x=0.25. It is shown that the extraordinary thermopower observed at Ts results from the reduced charge carrier mobility when approaching Ts and the variation of Q(T) with the Y contents reflects the accompanying band narrowing.

Structural and magnetic characterization of RCrO4 oxides (R=Nd, Er and Tm)

The crystal and magnetic structure of RCrO 4 oxides (R = Nd, Er and Tm) has been studied by owder neutron diffraction. These compounds crystallize with the zircon-type structure, showing tetragonal symmetry, space group I4 1 /amd. In the case of NdCrO 4 , magnetic susceptibility measurements reveal the existence of an antiferromagnetic ordering in which both Cr 5+ and Nd 3+ sublattices are involved. This ordering has been explained on the basis of a propagation vector k = 0 and a collinear structure, described by the symmetry mode A x , the ordered magnetic moments being 0.62 and 0.66 μ B at 2 K for Nd 3+ and Cr 5+ , respectively. Magnetic susceptibility and magnetization measurements reveal that both ErCrO 4 and TmCrO 4 behave as ferromagnetic compounds with a Curie temperature of 15 and 18 K, respectively. Rietveld refinement of the neutron diffraction data for ErCrO 4 yields a collinear magnetic structure described with an F x mode. In the case of the TmCrO 4 oxide, the ferromagnetic sublattices of Tm 3+ and Cr 5+ are aligned antiparallel in the a-b plane, while along the c-axis the magnetic moments point to the same direction. In both compounds, the rather small values obtained for the Er 3+ and Tm 3+ ordered moments compared with the theoretical ones have been attributed to crystal field effects. The differences in the ferromagnetic structure of these compounds have been explained as the result of the higher rare-earth anisotropy of Tm 3+ when compared with Er 3+ , for which no magnetic component is present along the c-direction.

Magnetic and structural features of the NdNi1 − xMnxO3 perovskite series investigated by neutron diffraction

Selected members of the perovskite series NdNi(1 - x)Mn(x)O(3) (0 ≤ x ≤ 1) have been prepared by a soft chemistry technique, followed by thermal treatments either under high oxygen pressure (x ≤ 0.5) or in air (x > 0.5). The crystal and magnetic structures have been studied by means of neutron diffraction, complemented with magnetic susceptibility measurements. For x = 0.25, 0.75, the crystal structure of the perovskites can be defined in the orthorhombic Pbnm space group, with Ni and Mn distributed at random over the octahedral sites of the structure. In contrast, the x = 0.5 compound crystallizes in a monoclinic P 2(1)/n structure containing two different octahedral positions, occupied by Ni and Mn, respectively. This is a result of the charge disproportionation of Ni(3+) + Mn(3+) to give Ni(2+) + Mn(4+) cations. The Ni(2+)O(6) octahedra are considerably larger than the Mn(4+)O(6) octahedra. This compound can be considered as a double perovskite of composition Nd(2)NiMnO(6). Unlike NdNiO(3) and NdMnO(3), which exhibit an antiferromagnetic ordering at low temperatures, the intermediate samples for x = 0.25, 0.50, 0.75 exhibit a ferromagnetic arrangement of (Ni, Mn) spins, with the moments aligned along the z axis, as probed using neutron diffraction. A maximum T(C) of 200 K is observed for x = 0.5, whereas T(C) = 150 K and 130 K are observed for x = 0.25 and 0.75, respectively. While NdNiO(3) is metallic above 200 K, a semiconducting behavior is determined between 120-300 K for the intermediate compositions.

Spin state transition: the origin of structural, magnetic and metal–insulator transitions in GdBaCo2O5+δ (δ≈0.5)

The metal–insulator transition in GdBaCo2O5:5 at TMIE360 K has been investigated by synchrotron X-ray powder diffraction and magnetic measurements. We have found that,at TMI remarkable structural distortions and magnetic changes take place. These changes evidence a spin state transition of Co ions at this temperature that is the origin of the metal–insulator transition. r 2002 Elsevier Science B.V. All rights reserved.