J Eriksen

Synthesis, structural and magnetic characterisation of the double perovskite A2MnMoO6 (A=Ba, Sr)

Abstract A study of the crystallographic structure and magnetic properties of the double perovskites Ba2MnMoO6 and Sr2MnMoO6 in polycrystalline form has been carried out by means of neutron powder diffraction (NPD) and magnetization measurements. The Rietveld analysis of room temperature data shows that the Mn2+ and Mo6+ ions are B-site ordered, i.e. the structure is a NaCl-type ordered double perovskite. Ba2MnMoO6 crystallizes in the cubic space group Fm 3 m (a=8.1680(1)) and Sr2MnMoO6 crystallizes in the space group P42/n (a=7.9575(5), c=7.9583(9)). Bond valence sum (BVS) calculation revealed that these compounds have the valency pair of {Mn2+(3d5;t32ge2g), Mo6+(4d0)}. The magnetic measurements suggest that these compounds transform to an antiferromagnetic state below 10 K.

Synthesis, crystal structure, and magnetic characterization of the double perovskite Ba2MnWO6

The double perovskite Ba2MnWO6 has been prepared as a pure powder by a conventional solid-state reaction process and studied by X-ray, neutron powder diffraction (NPD), magnetization, and AC susceptibility measurements. NPD, magnetization, and AC suscepti

Nuclear and magnetic structure of Ca2MnWO6 : A neutron powder diffraction study

Abstract The nuclear and magnetic structures of the double provskite compound Ca 2 MnWO 6 have been determined by neutron powder diffraction. Rietveld refinement shows that the compound adopts a monoclinic crystal structure with P2 1 /n symmetry. Magnetic refinement at 10 K shows that the magnetic structure is based on a unit cell related to that of the nuclear structure by a propagation vector (0 1/2 1/2), which is not very common in the case of double perovskites. The crystal contains alternating MnO 6 and WO 6 octahedra, considerably tilted due to the relative small size of the cation that occupy the A-sublattice of the perovskite. The manganese magnetic moments are coupled antiferromagnetically along the unit cell axes with the magnetic moment equal to 3.49(2) μ B (M x = 1.08(13) μ B , M y = 0.94(7) μ B , M z = 3.20(4) μ B ). Antiferromagnetic behavior was confirmed by magnetization measurements, and the antiferromagnetic to paramagnetic transition was found at 16 ± 0.1 K.

Synthesis and Crystal Structure of the Double Perovskite Ca2-xSrxMnWO6 (x = 0.0, 0.5, 1.0, 1.5, 2.0)

The double perovskites Ca2-xSrxMnWO6 (x = 0.0, 0.5, 1.0, 1.5, 2.0) have been prepared as pure phases by a conventional solid-state reaction process. Stoichiometric amounts of CaCO3,SrCO3, MnO and WO3 were mixed, ground and calcined in a nitrogen atmosphere. X-ray and neutron powder diffraction (NPD), together with magnetisation measurements were carried out in order to characterise the materials. The NPD and magnetisation measurements, performed at different temperatures, were made in order to correlate the materials structural and magnetic properties. Susceptibility- Temperature (ϰ-T) curves were measured between 2K and 200K in field cooled (FC) and zero-field-cooled (ZFC) modes with an applied field (H) up to 3T. Rietveld analysis of NPD data shows that the samples were B-site ordered perovskites with different types of lattice symmetry. Magnetisation measurements at low temperatures indicate an antiferromagnetic ordering of the magnetic moments.

Structural Studies of the Spinel System GaxCoFe1-xCrO4 (0 ≤ x ≤ 1) by Neutron Powder Diffraction

Solid solutions of GaxCoFe1-xCrO4 ( x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were obtained by solid state reactions in air at 1623K. X-ray and neutron powder diffraction measurements have been performed on the materials in order to characterize them. Rietveld refinement of neutron powder diffraction data showed cubic symmetry corresponding to the space group Fd-3m for all of the samples in the series. The results reveal that Ga has been substituted for Fe selectively. Ga exclusively occupies the octahedral site up to x = 0.8, while at x =1.0 a small amount of Ga (5%)enters into the tetrahedral site in the compound. Cr, Fe and Co cations are distributed between the tetrahedral 8 a and octahedral 16 d sites. Their proportion at each site varies with composition. The fractional coordinates of oxygen are almost the same in all the samples while the lattice parameter shows a slight decrease with increasing Ga content. The magnetic structure at room temperature was ferrimagnetic for the composition x = 0.0 and x = 0.2.

Phase Transitions and Magnetic Order in La 1−x Sr x MnO 3+δ (x≤ 0.2; 2.85≤ 2−δ≤ 3.00)

The system, La 1 m x Sr x MnO 3 m i (0.0 h x h 0.2; 2.85 h 3 m i h 3.00), has been studied by neutron powder diffraction and Raman scattering. The x= 0.0 and x= 0.1 samples with 3 m i , 3.00, both show orthorhombic lattice symmetry, space group Pnma. La 0.8 Sr 0.2 MnO 3.00 on the other hand show rhombohedral lattice symmetry (space group R-3c) irrespective of temperature. At 130 K and below all the oxidised samples investigated show ferromagnetic order. LaMnO 3 m i does at 10 K show a total magnetic moment of 3.17(4) w B , directed along the c-axes. With an increase in temperature the preferred direction of the magnetic moment become less obvious, and finally, the magnetic order is lost. Quenched samples such as LaMnO 2.85 show monoclinic lattice symmetry. Below , 150 K extra magnetic peaks appear corresponding to an antiferromagnetic order. The doped system, La 0.9 Sr 0.1 MnO 2.85 on the other hand show a sequence of transitions (paramagnetic to ferromagnetic to antiferromagnetic). Raman work on the oxidised samples show an orthorhombic and a rhombohedral phase but it also becomes clear that the rhombohedral phase show a high degree of local distortions.

La 1− x Sr x MnO 3 (0.33 ≤ x ≤ 1.0) Perovskites; A Powder Diffraction and Magnetisation Study

Powders of strontium doped lanthanum manganates (LSM) with a nominal composition La 1 m x Sr x MnO 3 (0.33 h x h 1.0) were prepared by a solid-state reaction. The samples were characterised by x-ray powder diffraction (XRPD), neutron powder diffraction (NPD), and magnetisation measurements performed with a SQUID magnetometer. NPD patterns were further studied with Rietveld analysis. XRPD, NPD and Rietveld analyses show that samples with x up to 0.45 crystallise single phase in a rhombohedral unit cell in space group R-3c (#167). The composition x = 0.55 is best described with a tetragonal unit cell with unit cell parameters a = 3.846 A and c = 3.876 A. From x = 0.60 to x = 0.950 the samples are mixtures of LaMnO 3 and SrMnO 3 phases. Pure SrMnO 3 as well as the composition where x = 0.975 crystallise in hexagonal space group P63/mmc (#194). The bulk resistivity of the as prepared samples shows that increasing strontium content results in higher resistivity in the material.