Polar and magnetic layered A-site and rock salt B-site-ordered NaLnFeWO6 (Ln = La, Nd) perovskites

We have expanded the double perovskite family of materials with the unusual combination of layered order in the A sublattice and rock salt order over the B sublattice to compounds NaLaFeWO6 and NaNdFeWO6. The materials have been synthesized and studied by powder X-ray diffraction, neutron diffraction, electron diffraction, magnetic measurements, X-ray absorption spectroscopy, dielectric measurements, and second harmonic generation. At room temperature, the crystal structures of both compounds can be defined in the noncentrosymmetric monoclinic P2(1) space group resulting from the combination of ordering both in the A and B sublattices, the distortion of the cell due to tilting of the octahedra, and the displacement of certain cations. The magnetic studies show that both compounds are ordered antiferromagnetically below T(N) ≈ 25 K for NaLaFeWO6 and at ∼21 K for NaNdFeWO6. The magnetic structure of NaNdFeWO6 has been solved with a propagation vector k = ((1/2) 0 (1/2)) as an antiferromagnetic arrangement of Fe and Nd moments. Although the samples are potential multiferroics, the dielectric measurements do not show a ferroelectric response.

Influence of the Bi3+ electron lone pair in the evolution of the crystal and magnetic structure of La(1-x)Bi(x)Mn2O5 oxides

La(1-x)Bi(x)Mn2O5 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1) oxides are members of the RMn2O5 family. The entire series has been prepared in polycrystalline form by a citrate technique. The evolution of their magnetic and crystallographic structures has been investigated by neutron powder diffraction (NPD) and magnetization measurements. All the samples crystallize in an orthorhombic structure with space group Pbam containing infinite chains of Mn(4+)O6 octahedra sharing edges, linked together by Mn(3+)O5 pyramids and (La/Bi)O8 units. These units become strongly distorted as the amount of Bi increases, due to the electron lone pair of Bi(3+). All the members of the series are magnetically ordered below TN = 25-40 K and they present different magnetic structures. For the samples with low Bi content (x = 0.2 and 0.4) the magnetic structure is characterized by the propagation vector k = (0,0,1/2). The magnetic moments of the Mn(4+) ions placed at octahedral sites are ordered according to the basis vectors (Gx, Ay, 0) whereas the Mn(3+) moments, located at pyramidal sites, are ordered according to the basis vectors (0, 0, Cz). When the content of Bi increases, two different propagation vectors are needed to explain the magnetic structure: k1 = (0,0,1/2) and k2 = (1/2,0,1/2). For x = 0.6 and 0.8, k2 is predominant over k1 and for this propagation vector (k2) the magnetic arrangement is defined by the basis vectors (Gx, Ay,0) and (Fx, Cy, 0) for Mn(4+) and Mn(3+) ions, respectively.

Ferromagnetic Cu-O-Cu coupling in CaCu3Sn4O12 probed by neutron diffraction

The A-site ordered perovskite oxide with the formula CaCu(3)Sn(4)O(12) has been synthesized in polycrystalline form under moderate pressure conditions (3.5 GPa) in combination with high temperature (1000 °C). This oxide crystallizes in the cubic space group [Formula: see text] (no. 204) with the unit-cell parameter a = 7.64535(6) Å at 300 K. The SnO(6) network is extremely tilted, giving rise to a square planar coordination for Cu(2+) cations. The non-magnetic character of Sn(4+) offers an excellent opportunity to probe the magnetism of Cu(2+) at the A sublattice in CaCu(3)Sn(4)O(12). Magnetic susceptibility shows that this compound is ferromagnetic below T(C) = 10 K, which is an unusual magnetic behaviour in cuprates. This peculiar aspect has been examined by neutron powder diffraction. The refinement of the magnetic structure at 4 K indeed indicates a parallel coupling between Cu(2+) spins with a magnetic moment of 0.5 μ(B)/Cu atom.

Evolution of the crystal and magnetic structure of the R2MnRuO7 (R = Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y) family of pyrochlore oxides

The members of the family of pyrochlore oxides with the formula R(2)MnRuO(7) (R = Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) have been synthesized and characterized. Polycrystalline samples were prepared by a soft chemistry procedure involving citrates of the different metal ions, followed by thermal treatments in air or O(2) pressure. The crystallographic and magnetic structures have been analysed from X-ray diffraction (XRD) and neutron powder diffraction (NPD) data, in complement with magnetic measurements; the evolution along the series of the crystallographic parameters (unit-cell parameters, bond distances and angles) is discussed. In R(2)MnRuO(7) pyrochlores, Mn and Ru ions statistically occupy the 16c sites in a cubic unit cell with space group Fd ̅3m, which defines an intrinsic frustrated three-dimensional system. In all the cases, the low-temperature NPD data unveils an antiferromagnetic coupling of two subsets of Mn(4+)/Ru(4+) spins, indicating that the magnetic frustration is partially relieved by the random distribution of Mn and Ru over the 16c sites. At lower temperatures there is a polarization of the R(3+) magnetic moments, which also participate in the magnetic structure, when a magnetic rare earth is present.

Evolution of cobalt spin states and magnetic coupling along the SrCo(1-x)Sb(x)O(3-δ) system: correlation with the crystal structure

The oxides of the SrCo(1-x)Sb(x)O(3-δ) perovskite family have been recently designed, characterized and described as cathode materials for solid-oxide fuel cells with competitive power performance in the temperature range 750-850 °C. They feature a number of interesting properties including a good electronic conductivity, low electrode polarization resistance and adequate thermal expansion; the crystal structure adopts a 3C corner-linked perovskite network with a considerable number of oxygen vacancies. This paper reports on the effects of Sb-doping on the crystal structure features, the Co oxidation state and magnetic properties related to the presence of spin-state transitions in the Co cations. A phase transition was observed from the tetragonal P4/mmm space group for x≤ 0.15 to the cubic Pm ̅3m space group in the x = 0.2 composition from neutron powder diffraction data. For the tetragonal phases the oxygen vacancies were found to be ordered and localized in the axial O2 and equatorial O3 atoms surrounding the Co2 positions. A noticeable distortion of CoO(6) octahedra is observed for x = 0.05 and 0.1, exhibiting a charge-ordering with a mixed oxidation state of Co(3+/4+) at Co1 sites and Co(3+) at Co2 positions: the Jahn-Teller Co(3+) in an intermediate-spin configuration is responsible for the octahedral distortion. Increasing Sb contents promotes a higher average oxidation state of cobalt, from a valence of 3.2+ for x = 0.05 to 3.4+ for x = 0.2, inducing a decrease of the oxygen vacancies and favouring a random distribution over a Pm ̅3m cubic symmetry. All the samples present an antiferromagnetic behaviour with a G-type (k = 0) magnetic structure. The increase of the Sb content induces the weakening of the crystal field (Δ(cf)) in the octahedral environment promoting the Co spin-transition from the intermediate-spin to the high-spin configuration, as evidenced by the decrease of the octahedral distortion, increment of the unit-cell volume and enhancement of the ordered magnetic moment.

High-pressure synthesis of the double perovskite Sr(2)FeMoO(6): increment of the cationic ordering and enhanced magnetic properties

The double perovskite Sr(2)FeMoO(6) has been prepared in polycrystalline form by high-pressure methods, starting from a precursor developed via a citrate technique, containing an elevated degree of anti-site disordering. The application of high external pressure (2 GPa) to Sr(2)FeMoO(6) promotes the long distance Fe/Mo cationic order, due to the smaller lattice volume of the ordered sample. Both the disordered perovskite obtained at ambient pressure and the sample synthesized under high-pressure methods have been characterized by means of x-ray diffraction, neutron powder diffraction and magnetic measurements. The magnetic properties of the two oxides have been compared; the specimen prepared under high pressure not only presents an improved cationic ordering, but also displays a superior saturation magnetization and a sharpener ferromagnetic transition at a significantly high temperature of 430 K.