M.T. Fernández-Díaz

Neutron diffraction study on structural and magnetic properties of La2NiO4

An overall survey of the structural and magnetic features of the La2NiO4+ delta system is presented as a result of neutron diffraction experiments. The stoichiometric compound ( delta =0) presents two structural phase transitions. At T0 approximately=770 K, La2NiO4 transforms from tetragonal (I4/mmm) to orthorhombic (Bmab); at T1 approximately=80 K, from orthorhombic to a new tetragonal (P42/ncm) phase. Associated with this second phase transition a strong microstrain produces anisotropic broadening of Bragg reflections. La2NiO4 is three-dimensional (3D) antiferromagnetically ordered at room temperature (TN=330 K). A weak ferromagnetic component appears below T1. Oxygen excess suppress the 3D magnetic ordering and the structural phase transformations, giving rise to a non-stoichiometric compound with interstitial oxygens. A tentative phase diagram is proposed.

The magnetic structure of YMnO3 perovskite revisited

The magnetic structure of the orthorhombic perovskite YMnO3 has been investigated. A study on a polycrystalline sample based on neutron diffraction data and magnetization measurements has shown that YMnO3 becomes magnetically ordered below TN = 42?K. In the space group Pnma, the sinusoidal magnetic structure is defined by a (Cx,0,0) mode and characterized by the propagation vector k = (kx,0,0). The kx-component increases from 0.420(4), immediately below the ordering temperature, to 0.435(2) at T = 28.7?K. Below 28 K the kx-component remains unchanged. The sinusoidal spin arrangement remains stable down to 1.7?K; at this temperature the amplitude of the sinusoid is Ak = 3.89(6)??B. YMnO3 is the most distorted perovskite of the RMnO3 series (R?=?rare earths); the observed sinusoidal magnetic structure is in contrast with those exhibited by the less-distorted members (i.e.?LaMnO3), which are commensurate-type antiferromagnetic structures.

A structural study from neutron diffraction data and magnetic properties of (R = La, rare earth)

The title compounds (R = La, Pr, Nd, Sm, Eu, Tb, Ho, Er) have been prepared in polycrystalline form by a citrate technique and, excepting the Sm and Eu phases, structurally studied by high-resolution neutron powder diffraction. All the materials are isostructural (space group Pbam, Z = 4) and contain infinite chains of octahedra sharing edges, linked together by and units. The size of the three kinds of coordination polyhedron regularly decreases as R cations become smaller. A bond-valence study allowed us to detect the presence of important tensile and compressive stresses in the crystal structure of , which are progressively released along the series as the rare-earth size decreases. The magnetic properties strongly depend on the nature of R, going from the spin-glass behaviour observed at low temperature in to the field-induced transitions exhibited by . A cusp in the susceptibility curves suggests an antiferromagnetic ordering at low temperatures, which is masked in the compounds containing strongly paramagnetic rare earths (Tb, Ho, Er). At high temperatures the paramagnetic moments are consistent in all cases with the presence of high-spin and cations.

Magnetic structure evolution of NdMnO3 derived from neutron diffraction data

The orthorhombic NdMnO3 perovskite (space group Pnma ) has been studied on the basis of magnetization and neutron powder diffraction (NPD) data. Magnetization measurements suggest the coexistence of ferromagnetic and antiferromagnetic interactions: magnetization versus magnetic field curves present a remnant magnetization in the ordered region, which is around 1 µB at T = 6 K. The thermal evolution of the magnetic structure has been followed down to 1.5 K from the NPD data. These measurements show that the Mn sub-lattice becomes ordered below T N 78 K with a spin arrangement (C x ,F y ,0), in such a way that a ferromagnetic component appears along the y -direction. The Nd sub-lattice becomes ordered below T 13 K according to a ferromagnetic arrangement with the moments parallel to the y -direction. At T = 1.5 K the magnetic moment values are 3.22(9) µB for Mn atoms and 1.2(2) µB for Nd atoms.

Crystal and magnetic structure of the complex oxides Sr2MnMoO6, Sr2MnWO6 and Ca2MnWO6: a neutron diffraction study

A study of the crystallographic and magnetic structure of the double perovskites Sr2MnMoO6, Sr2MnWO6 and Ca2MnWO6 has been carried out on polycrystalline samples using neutron powder diffraction (NPD) data. A room temperature analysis of high-resolution NPD patterns has shown that these compounds crystallize, at room temperature, in the monoclinic space group P 21 /n. The three perovskites contain divalent Mn cations. Ca2MnWO6 presents the strongest distortion with respect to the ideal cubic perovskite structure. The low-temperature antiferromagnetic ordering has been followed from sequential NPD data. The magnetic structures are defined by the propagation vectors k = (1/2, 0, 1/2) for Sr2MnMoO6 and Sr2MnWO6, and k = (0, 1/2, 1/2) for Ca2MnWO6. The possible arrangements for the Mn2+ magnetic moments have been derived from a group theory analysis.

Evolution of the crystal structure of RVO3 (R = La, Ce, Pr, Nd, Tb, Ho, Er, Tm, Yb, Lu, Y) perovskites from neutron powder diffraction data.

RVO3 perovskites have been prepared in the widest range of R (3+) ionic size, from LaVO3 to LuVO3. Pure polycrystalline samples have been obtained by a citrate technique leading to reactive RVO4 precursors, followed by thermal treatments in a reducing H2/N2 (15/85%) flow to stabilize V(3+) cations. These oxides have been studied at room temperature by high-resolution neutron powder diffraction to follow the evolution of the crystal structures along the series. The distortion of the orthorhombic perovskite (space group Pbnm), characterized by the tilting angle of the VO6 octahedra, progressively increases from La to Lu due to simple steric factors. Additionally, all of the perovskites show a subtle distortion of the VO6 octahedra which significantly increases from La to Tb, and then slightly decreases for the last terms of the series. The stability of the crystal structure is also discussed in light of bond-valence arguments.

A structural and magnetic study of the defect perovskite from high-resolution neutron diffraction data

The fine details of the crystal structure of have been determined from a high-resolution neutron diffraction study at room temperature. The powder sample, showing an excellent crystallinity, was prepared by topotactic reduction of in the presence of Zr. The unit cell can be described as a monoclinic superstructure of perovskite ( is the edge of the ideal perovskite), and contains one-dimensionally infinite chains of flattened octahedra running parallel to the c axis, with square planar units connecting the octahedra chains. Bond valence calculations are consistent with divalent Ni cations in both coordination polyhedra. Neutron diffraction experiments reveal 3D magnetic ordering below K. A magnetic structure with S = 1 moments antiferromagnetically coupled along the chains parallel to the c axis is proposed, and the absence of magnetic moments at the units is also discussed.

Enhancement of the curie temperature along the perovskite series RCu3Mn4O12 driven by chemical pressure of R3+ cations (R = Rare Earths)

The compounds of the title series have been prepared from citrate precursors under moderate pressure conditions (P = 2 GPa) and 1000 degrees C in the presence of KClO(4) as oxidizing agent. The crystal structures are cubic, space group Im3 (No. 204); the unit cell parameters linearly vary from a = 7.3272(4) A (R = La) to a = 7.2409(1) A (R = Lu) at room temperature. A neutron or synchrotron X-ray diffraction study of all the members of the series reveals an interesting correlation between some structural parameters and the magnetic properties. The electron injection effect upon replacement of Ca(2+) with R(3+) cations in the parent CaCu(3)Mn(4)O(12) oxide leads to a substantial increment of the ferrimagnetic Curie temperature (T(C)). An essential ingredient is supplied by the internal pressure of the R(3+) cations upon a decrease in size along the rare-earth series, from La to Lu: the concomitant compression of the MnO(6) octahedral units for the small rare earths provides progressively shorter Mn-O distances and improves the overlapping between Mn and O orbitals, thereby promoting superexchange and enhancing T(C) by 50 K along the series. This interaction is also reinforced by a ferromagnetic component that depends on the local distortion of the MnO(6) octahedra, which also increases along the series, constituting an additional factor, via intersite virtual charge transfer t-e orbital hybridization, for the observed increment of T(C).

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.

Evaluation of Sr2CoMoO6−δ as anode material in solid-oxide fuel cells: A neutron diffraction study

The oxygen-deficient Sr2CoMoO6−δ double perovskite has been proposed as an anode material in solid-oxide fuel cells (SOFC). The evolution of its crystal structure has been followed by “in situ” temperature-dependent neutron powder diffraction from 23u2009°C (RT) to 867u2009°C in the heating and cooling runs in ultrahigh vacuum (PO2≈10−6u2002Torr) in order to simulate the reducing atmosphere corresponding to the working conditions of an anode in a SOFC. At RT the sample is described as tetragonal in the I4/m space group. When this oxide is heated above Tt=262u2009°C it undergoes a tetragonal I4/m to cubic Fm-3m phase transition. This phase transition takes place at a temperature around 25u2009°C lower than that previously described for the oxidized sample, and it is affected by a significant hysteresis (Tt=174u2009°C in the cooling run). The absence of tilting of the CoO6 and MoO6 octahedra in the high-temperature cubic phase favors the orbital overlap and the electronic conductivity; a high mobility of the oxygen atoms is derive...