Håkan Rundlöf

Investigation of the structure of the relaxor ferroelectric Pb(Fe1/2Nb1/2)O3 by neutron powder diffraction

The complex perovskite lead iron niobate, Pb(Fe1/2 Nb1/2 )O3 (PFN), has been studied by neutron powder diffraction. Following collection of diffraction data at 300 K and at 10 K, structural refinements have been carried out by means of the Rietveld method. As expected, a straightforward unit cell of symmetry R 3m was obtained for the 300 K structure, with the same symmetry and a similar unit cell also obtained at low temperature. Furthermore, in order to obtain a good agreement with experiment at 10 K, it was necessary to assign non-zero magnetic moments to the iron ions, these being in a collinear, antiferromagnetic alignment. This magnetic structure can be described with reference to doubled unit cell axes. The factors governing the observed structures of PFN are discussed by comparison with the related system of Pb(Mg1/3 Nb2/3 )O3 .

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.

Neutron Diffraction Studies of Pb(ZrxTi1-x)O3 Ceramics

Neutron diffraction studies of lead zirconate titanate Pb(ZrxT1-x)O-3 (PZT) powders, 0.20 less than or equal to x less than or equal to 0.54, were carried out at 10 K and 297 K, The phase transitio ...

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

A neutron powder diffraction study of the ferroelectric relaxor Pb(Fe1/2Ta1/2)O3

The complex perovskite lead iron niobate, Pb(Fe1/2Ta1/2)O3 (PFT), has been studied by neutron powder diffraction. Following collection of diffraction data at 300 K and at 10 K, structural refinements have been carried out by means of the Rietveld method. A straightforward unit cell of symmetry R3m was obtained for the 300 K structure, with the same symmetry, and a similar unit cell also obtained at the low temperature. In both cases, the iron and titanium ions were found to be disordered over the perovskite B-sites. Furthermore, in order to obtain a good agreement with experiment at 10 K, it was necessary to assign non-zero magnetic moments to the iron ions, these being in a collinear, antiferromagnetic arrangement. This magnetic structure can be described with reference to doubled unit cell axes. The factors governing the observed structures of PFT are discussed by comparison with the related system of Pb(Fe1/2Nb1/2)O3.

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.

Magnetic Ordering in Polycrystalline NixZn1—xO Solid Solutions

In order to examine the influence of the dilution of magnetic ions on the crystal and magnetic structure of Ni x Zn 1-x O (x = 1, 0.90, 0.80, 0.70) solid solutions we have performed neutron diffraction experiments on each sample at seven temperatures between 10 and 295 K. To determine the Neel temperatures, the magnetic susceptibility measurements were done in the temperature region between room temperature and 600 K. Starting from the known rhombohedral distortion the crystal structure has been refined in the trigonal space group R3m. In this space group the cations occupy octahedral 3a position (with the local symmetry 3m), while the oxygen anion is placed in the 3b position (with the local symmetry 3m). The magnetic structure for all concentrations x is found to be antiferromagnetic and the magnetic cell is the doubled nuclear cell along the c-axis. The magnetic atoms are arranged in the planes (003) in respect to the nuclear cell. The magnetic moments of two Ni 2+ ions in adjacent sheets are oriented antiparallely and the magnetic vector makes an angle of 66° with the c-axis. The magnetic structure is preserved upon random site dilution of the magnetic ions. This is additionally confirmed from the Brillouin-type dependence of the magnetization on temperature as well as from the t = T N (x)/T N (1) versus x variation obtained from the magnetic susceptibility measurements. Concentration dependence of the Neel temperature also shows that Ni x Zn 1-x O behaves like a Heisenberg antiferromagnet with the dominant nearest-neighbor interaction, in agreement with the similar (Ni, Co) x Mg 1-x O solid solution.

Evolution of the atomic and magnetic structure of Sr3Fe2WO9: A temperature dependent neutron powder diffraction study

Abstract Both the nuclear and magnetic structure of a complex perovskite with composition Sr 3 Fe 2 WO 9 (SFWO) has been studied by neutron powder diffraction (NPD) at different temperatures. The most striking feature of this type of complex perovskite is the coexistence of ferrimagnetic and antiferroelectric types of ordering below T = 373 K. The NPD data at 10 and 300 K were refined with a model in the tetragonal space group I4/m. SFWO has a partially ordered perovskite structure, in which Fe and W cations distribute regularly to some degree at the B-sites in the ABO 3 perovskite lattice. For both temperatures magnetic diffraction peaks were registered, and a possible model for the magnetic structure was proposed in accordance with the ferrimagnetic properties of SFWO.

Preparation of Zn substituted Ni-Fe-Cr ferrites and study of the crystal structure by neutron diffraction

The spinel type solid solutions of ZnxNi1−xFeCrO4 (x=0.2, 0.4, 0.6 and 0.8) were obtained by solid state sintering technique in air at 1523 K. X-ray and neutron powder diffraction experiments have been carried out on the samples at 295 K in order to characterize the materials. Rietveld refinement of the neutron diffraction data reveals that all the samples of the series possess cubic symmetry corresponding to the space group Fd3m. The distribution of Zn ions on the A and B sites as affected by the variation of x and also the distribution of Cr ions on the A and B sites have been determined. The Fe ions are distributed over both the A and B sites for the whole compositional range investigated. Only a small portion of the Ni ions are found to occupy the A site for x=0.2, while all the Ni ions enter the B site for x≥0.4. The fractional coordinates of oxygen u, and the lattice parameter a, increase with increasing Zn content. The moment distributions in the two sublattices for different composition have been determined.

The study of magnetic ordering in the spinel system ZnxNi1−xFeCrO4 by neutron diffraction

Abstract Neutron diffraction measurements have been performed in the temperature range 600 K⩾ T ⩾10 K on the spinel system ZnxNi1−xFeCrO4 (x=0.2, 0.4, 0.6 and 0.8) synthesized in the ceramic sintering method. The moment distributions in the two sublattices and their ordering as affected by compositional change and temperature variation have been determined. Ferrimagnetic transition temperatures and spin compensation temperatures for all the specimens have been determined. The system shows significant deviations from the usual ferrimagnetic behavior, which increases with the increase of diamagnetic substitutions, being more prominent in the B sublattice. The ferrimagnetic ordering is perturbed due to the presence of non-collinear spins mainly in the B site. Presence of small fluctuating magnetic clusters for x⩾0.6 is evident from the diffuse nature of the scattered intensity. A qualitative explanation of the observed features is put forward in the light of competing inter and intra sublattice exchange interactions. A randomly canted ferrimagnetic ordering is suggestive for the system at x⩽0.4, while a semi-spin glass like transition is favorable for x⩾0.6 due to large reduction in moment and evolution of diffuse signal from short-range clusters at low temperatures.