W. Kockelmann

Nitrogen ordering and ferromagnetic properties of ε-Fe3N1+x(0.10 0.39) and ε-Fe3(N0.80C0.20)1.38

Abstract The ϵ-phase iron nitrides ϵ-Fe3N1+x with x=0.10, 0.22, 0.30, 0.33, 0.39 and a carbonitride ϵ-Fe3(N0.80C0.20)1.38 were investigated by neutron diffraction. In these phases the non-metals occupy octahedral interstices in an hcp arrangement of Fe. Type and degree of occupational order of N and N+C, respectively were determined as a function of composition and temperature. The ordering of N is closely related to that of the ϵ-Fe3N ‘ideal’ structure (space group P6322). Neutron diffraction data on ϵ-Fe3N1.22 revealed lowered symmetry (P312). An earlier proposed ϵ-Fe2N type ordering (P31m) was not observed for the iron nitride samples but it was found for ϵ-Fe3(N0.80C0.20)1.38. Magnetisation data show that the nitride and carbonitride phases are soft ferromagnetic materials and confirmed magnetic structure data as derived from neutron diffraction.

ε-Fe3N : magnetic structure, magnetization and temperature dependent disorder of nitrogen

Abstract e-Fe 3 N has been investigated by time-of-flight neutron diffraction (temperature range 4.2–618 K) and SQUID magnetometry (2–700 K, B ≤5 T). A ferromagnetic spin structure is observed with magnetic moments oriented perpendicular to the c -axis of the hexagonal nuclear structure. The magnetic saturation moment of iron is 2.2 μ B at 4.2 K from neutron diffraction and 2.0 μ B from magnetic measurements and decreases in a Brillouin-like manner on heating to T C =575 K. Above 450 K an increasing but reversible disorder of the nitrogen partial structure is observed.

Possible deuterium positions in the high-temperature deuterated proton conductor Ba3Ca1+yNb2−yO9−δ studied by neutron and X-ray powder diffraction

Abstract High-temperature proton conductors with perovskite structures are a class of well-known systems with high protonic conductivity, which is of high technological interest in view of the possible applications in solid oxide fuel cells. We present neutron and X-ray diffraction studies of the crystal structure of the Ba 3 Ca 1.18 Nb 1.82 O 9− δ + z D 2 O (BCN18) system that exhibits high proton conductivity. It is assumed that the mechanism of proton conductivity in BCN18 is connected with trapping of protons at some interstitial positions in the crystal lattice as deduced from earlier quasielastic neutron scattering and muon spin rotation studies published in the literature. In order to get more information on the location of deuterium, systematic high-resolution neutron diffraction studies of the stoichiometric Ba 3 Ca 1 Nb 2 O 9 and non-stoichiometric BCN18 compounds with and without D 2 O were performed. The ROTAX time-of-flight (TOF) diffractometer at the ISIS neutron spallation source and the SV7 double-axis diffractometer at the DIDO reactor at Julich were used. Refinements of the crystal structures were performed by using the FULLPROF and GSAS programs. The possible deuterium positions in the crystallographic unit cell are discussed.

Setup and use of the ROTAX instrument at ISIS as angle-dispersive neutron powder and texture diffractometer

The ROTAX instrument at ISIS is run as an angle-dispersive time-of-flight instrument using the JULIOS scintillation detector in forward-, 90°- and backscattering geometry; d-spacing coverage is from more than 50 A to less than 0.2 A. Δd/d-resolution is calculated to 3.5 × 10−3 in backscattering. Tests and examples of a multipurpose use of the neutron instrument as powder diffractometer for crystallographic and magnetic structure analysis are presented; the additional use as texture diffractometer is discussed.

Neutron and synchrotron radiation studies of archaeological objects

This chapter focuses on the neutron and synchrotron radiation studies of archaeological objects. Time-of-flight neutron and synchrotron X-ray diffraction are used for the fingerprint determinations and quantitative mineral phase analysis of archaeological objects. Both neutron and X-ray diffraction techniques have their advantages and drawbacks when used in archaeological research. Neutron diffraction allows non-destructive analysis of complete and unprepared objects. Synchrotron X-ray diffraction can be used for fast and high-resolution data collection on small amounts of powder samples, surfaces, or thin sections. This chapter concentrates on the introduction of the white-beam neutron diffraction technique applied on archaeological pottery. X-ray diffraction results from both laboratory and synchrotron sources are given for comparison. X-ray and neutron diffraction are well known experimental methods for investigating minerals, or other inorganic and organic materials wherever their crystal structures are the matter of interest in the wide field of material science and industrial applications. The knowledge of the crystal structure of a material or the abundance of different known phases in a multi-phase mixture of minerals are not only fundamental for the understanding of the physical and chemical properties of the material, but can also help to fingerprint the origin of objects of archaeological interest. It concludes that neutron and SR X-ray radiation is capable of providing high quality diffraction fingerprints of archaeological ceramics.

Neutron diffraction study of the ferrimagnetic structures of RFe5Al7 compounds with R=Tb, Dy, Ho, Er, Tm

Abstract Long range order of ternary RFe 5 Al 7 (R = Tb, Dy, Ho, Er, Tm) intermetallics of ThMn 12 -type structure is analysed by neutron powder diffraction. Ferromagnetic order is found for both the heavy rare earth and the iron sublattices. Rare earth and iron moments are oriented antiparallel to each other, resulting in ferrimagnetic structures. The ordering temperatures are about 235 K for the Tb, Dy and Tm compounds and about 60 K for the Ho and Er compounds. Magnitudes and orientations of ordered magnetic rare earth and iron moments are determined; a moment modulation is observed in TbFe 5 Al 7 . The formation of the ferrimagnetic structures is preceded by short range or frozen magnetic states. The temperature behaviour of the magnetization is discussed.

The two-component non-collinear antiferromagnetic structures of DyNiC2 and HoNiC2

Abstract DyNiC 2 and HoNiC 2 , which crystallize in the orthorhombic space group Amm2, have been studied by neutron powder diffraction in the temperature range from 300 to 1.9 K. Both compounds exhibit modulated antiferromagnetic configurations of the rare earth magnetic moments. The magnetic transition temperatures are 10 and 4 K for DyNiC 2 and HoNiC 2 , respectively. The magnetic structures are described by two propagation vectors: the commensurate k 1 =[0, 0, 1] and the incommensurate k 2 =[ 1 2 −τ, 1 3 +τ, k z ] with τ=0.03 and 0 and k z =0.935 and 0.86 for DyNiC 2 and HoNiC 2 respectively. The magnitude and direction of the magnetic moments are determined. The resultant non-collinear antiferromagnetic spin configurations are discussed with respect to crystal field and magnetic exchange interactions.

Magnetic structure of LaFe10.8Al2.2 and LaFe10.8Al2.2N3 cluster compounds

The magnetic structure at 15 K of LaFe10.8Al2.2 and its ternary nitride LaFe10.8Al2.2N3 have been determined by high resolution time-of-flight neutron powder diffraction. Magnetism in these materials is determined by clusters formed by a central Fe atom surrounded by Fe atoms with icosahedral symmetry. This icosahedral symmetry is not affected by N uptake since both compounds are confirmed to crystallize in the cubic NaZn13-type structure. The Al atoms are statistically distributed over the 96i cluster site only, while N atoms are found to fully occupy the highly symmetrical 24d interstitial hole site. Charging with N gives rise to an 8% increase in the unit cell volume of the parent compound. The magnetic structures are overall ferromagnetic for both compounds, with each of the two Fe sublattices aligned parallel to each other. The influence of N uptake on the magnitudes of the cluster Fe moments will be discussed.

Neutron diffraction on YFe5Al7 as reference of the f-magnetism of isostructural rare earth-iron-aluminium compounds

Abstract YFe5Al7 crystallizes isostructurally with the rare earth(R)-iron-aluminium compounds RFe5Al7 in the tetragonal ThMn12-type structure in space group I4/mmm. Lattice parameters are refined from X-ray diffraction lines and atomic parameters and site occupancies from room and low temperature neutron diffraction intensities. Ordered cation distributions are found with Y on 2a, Fe on 8f and Al on 8i sites. 8j sites are occupied by both Fe and Al according to stoichiometry. A broad diffuse neutron peak indicates short-range structural order originating from iron clusters on 8j positions. Below 100 K, YFe5Al7 exhibits an incommensurate magnetic structure of small Fe moments (〈μ〉 = 0.5 μB) with a propagation vector close to [ 1 7 , 1 7 , 0]. Contrary to the findings in the other rare earth compounds, no long-range ordered ferromagnetic iron sublattice comes off in YFe5Al7.

Structural and magnetic properties of RCoC2 (R = Pr, Dy, Ho) compounds studied by neutron diffraction

Abstract PrCoC 2 , DyCoC 2 and HoCoC 2 have been studied by neutron powder diffraction in the temperature range from 300 to 1.6 K. PrCoC 2 crystallizes in monoclinic space group Cc, DyCoC 2 and HoCoC 2 is paramagnetic down Refined lattice and atomic positional parameters and interatomic distances are given. PrCoC 2 is paramagnetic down to 1.6 K. DyCoC 2 and HoCoC 2 order ferromagnetically at T C = 9 K . The ordered Dy and Ho moments are 6.0(5) and 6.6(1) μ B at 1.7 K respectively. The moments are oriented along the a -axis. No moment is observed on Co.