Published Neutron Diffraction Research Articles

Thermo-mechanical-metallurgical modeling and validation for ferritic steel weldments

This work explained the effects of solid-state phase transformation (SSPT) on welding residual stress (WRS) evolution in SA508 Gr.3 C1.1(SA508) ferritic steel single-pass and multi-pass weldments. A thermo-mechanical-metallurgical (TMM) model incorporating thermo-elastic-plastic stains and SSPT-related strains was presented. The SSPT kinetics and transformation-induced plasticity (TRIP) models of SA508 steel were established by employing user-defined subroutines developed for ABAQUS. The simulated WRS distribution in the single-pass weldment was compared with previously published neutron diffraction measurement data, and a good agreement was achieved and thus validating the developed TMM model. The microstructural and WRS distribution characteristics of the multi-pass weldment were further investigated based on the validated TMM model. Numerical simulations illustrated how the transformation and heat transfer/cumulative behavior associated with different welding process conditions resulted in the generation of triaxial compressive and tensile stresses in the upper and underlying weld beads at the multi-pass welded joint, respectively. Moreover, an additional simulation model with SSPT but without TRIP was developed to reveal the potential effects of TRIP on WRS distribution. This study numerically provided valuable understanding of microstructure and WRS evolution in air cooling bainitic steel weldments under multiple thermal cycles and constituted a fundamental basis for developing new welding materials to minimize the formation of deleterious tensile WRS by simultaneously combining bainitic and martensitic transformations.

An analysis of the magnetic behavior of olivine and garnet substitutional solid solutions

AbstractThe low-temperature magnetic properties and Néel temperature, TN, behavior of four silicate substitutional solid solutions containing paramagnetic ions are analyzed. The four systems are: fayaliteforsterite olivine [Fe22+SiO4-Mg2SiO4], and the garnet series, grossular-andradite [Ca3(Alx,Fe1−x3+)2Si3O12], grossular-spessartine [(Cax,Mn1−x2+)3Al2Si3O12], and almandine-spessartine [(Fex2+,Mn1−x2+)3Al2Si3O12]. Local magnetic behavior of the transition-metal-bearing end-members is taken from published neutron diffraction results and computational studies. TN values are from calorimetric heat capacity, CP, and magnetic susceptibility measurements. These end-members, along with more transition-metal-rich solid solutions, show a paramagnetic to antiferromagnetic phase transition. It is marked by a CP λ-anomaly that decreases in temperature and magnitude with increasing substitution of the diamagnetic component. For olivines, TN varies between 65 and 18 K and TN for the various garnets is less than 12 K. Local magnetic behavior can involve one or more superexchange interactions mediated through oxygen atoms. TN behavior shows a quasi-plateau-like effect for the systems fayalite-forsterite, grossular-andradite, and grossular-spessartine. More transition-metal-rich crystals show a stronger TN dependence compared to transition-metal-poor ones. The latter may possibly show superparamagnetic behavior. (Fex2+,Mn1−x2+)3Al2Si3O12 garnets show fundamentally different magnetic behavior. End-member almandine and spessartine have different and complex interacting local superexchange mechanisms and intermediate compositions show a double-exchange magnetic mechanism. For the latter, TN values show negative deviations from linear interpolated TN values between the end-members. Double exchange seldom occurs in oxides, and this may be the first documentation of this magnetic mechanism in a silicate. TN behavior may possibly be used to better understand the nature of macroscopic thermodynamic functions, CP and S°, of both end-member and substitutional solid-solution phases.

Effects of external magnetic field and hydrostatic pressure on magnetic and structural phase transitions in Pr0.6Sr0.4MnO3

We investigate the effect of hydrostatic pressure on the temperature dependence of magnetization and also the influence of magnetic field on the linear thermal expansion in polycrystalline Pr0.6Sr0.4MnO3, which is ferromagnetic at room temperature (TC = 305 K) but its magnetization undergoes an abrupt decrease at TS = 89 K within the ferromagnetic state. Normal and inverse magnetocaloric effects around TC and TS, respectively, were reported earlier in this single phase compound [Repaka et al., J. Appl. Phys. 112, 123915 (2012)]. The thermal expansion shows an abrupt decrease at TS in zero magnetic field but transforms into an abrupt increase at the same temperature under 7 T, which we interpret as a consequence of magnetic field-induced structural transition from the low-temperature monoclinic (I2/a symmetry) to high-temperature orthorhombic (Pnma symmetry) phase in corroboration with a published neutron diffraction study in zero magnetic field. While the external magnetic field does not change TS, the application of a hydrostatic pressure of P = 1.16 GPa shifts the magnetic anomaly at TS towards high temperature. The pressure induced shift of the low-temperature magneto-structural anomaly (ΔTS = 27 K) is nine-times higher than that of the ferromagnetic Curie temperature (ΔTC = 3 K). Our results suggest that while the hydrostatic pressure stabilizes the low temperature monoclinic phase at the expense of the orthorhombic phase, the applied magnetic field does not affect the structural transition temperature.

Assessment of the Density Functional Tight Binding Method for Protic Ionic Liquids.

Density functional tight binding (DFTB), which is ∼100–1000 times faster than full density functional theory (DFT), has been used to simulate the structure and properties of protic ionic liquid (IL) ions, clusters of ions and the bulk liquid. Proton affinities for a wide range of IL cations and anions determined using DFTB generally reproduce G3B3 values to within 5–10 kcal/mol. The structures and thermodynamic stabilities of n-alkyl ammonium nitrate clusters (up to 450 quantum chemical atoms) predicted with DFTB are in excellent agreement with those determined using DFT. The IL bulk structure simulated using DFTB with periodic boundary conditions is in excellent agreement with published neutron diffraction data.

Medium-range correlation of Ag ions in superionic melts ofAg2Se and AgI by reverse Monte Carlo structural modelling—connectivity and voiddistribution

High-energy x-ray diffraction measurements on moltenAg2Se were performed. Partial structure factors and radial distribution functionswere deduced by reverse Monte Carlo (RMC) structural modelling on thebasis of our new x-ray and earlier published neutron diffraction data. Thesepartial functions were compared with those of molten AgI. Both AgI andAg2Se have a superionic solid phase prior to melting. New RMC structural modellingfor molten AgI was performed to revise our previous model with a bond-anglerestriction to reduce the number of unphysical Ag triangles. The refined model ofmolten AgI revealed that isolated unbranched chains formed by Ag ions are thecause of the medium-range order of Ag. In contrast with molten AgI, moltenAg2Se has ‘cage-like’ structures with approximately seven Ag ions surrounding aSe ion. Connectivity analysis revealed that most of the Ag ions in moltenAg2Se are located within 2.9 Å of each other and only small voids are found, whichis in contrast to the wide distribution of Ag-void radii in molten AgI. Itis conjectured that the collective motion of Ag ions through small voidsis required to realize the well-known fast diffusion of Ag ions in moltenAg2Se, which is comparable to that in molten AgI.

Magnetic Properties of Zr$_{9}$Ni$_{11}$ Intermetallic Compound

We report on a very interesting magnetic behavior in the intermetallic compound Zr9N11, namely, (i) spin-glass behavior and (ii) fluctuations in the magnetization versus temperature curves. The magnetic properties are consistent with structural results obtained by neutron diffraction and by scanning and transmission electron microscopies. The fluctuations are believed to arise from the Dzyaloshinskii-Moriya interactions occurring in the Zr9N11 compound. Between 7.86 kA/m and 31.85 kA/m, the temperature (Tp) of the zero-field cooled (Mzfc ) peak shifted to lower temperatures with increasing field. Tp is found to be proportional to Hb, where H is the applied field and the exponent b < 1. The value of b is found to be 0.68, which is close to the de Almeida-Thouless 2/3 power law that applies to spin glasses. Published neutron diffraction data showed that a Zr9N11 compound possessed Zr9Pt11-type tetragonal structure with columnar atomic chains parallel to the c axis and consisting of alternating Zr and Ni atoms. Small fluctuations present in the Mfc versus T plot at 31.85 kA/m are believed to be the result of atomic displacements along these chains over short distances. Another interesting featured observed in the Zr9N11 compound is the fact that at 3.98 kA/m the Mzfc values reside mostly above the corresponding Mfc values.

The Yttria-Alumina Glasses - MD and TD Models

The ab-initio molecular dynamics (MD) calculations of the yttrium - aluminate binary system 23Y2O3·77Al2O3 were performed with help of the Vienna ab-initio simulation package VASP. The thermodynamic model (TD) of glasses of the studied binary system was constructed. Both the Y/O and Al/O coordination numbers calculated from the partial pair radial distribution function (PP RDF) obtained for MD simulated melt at the temperature of 2500 K were significantly lower than the corresponding coordination numbers obtained from the TD model. The published neutron diffraction and X-ray diffraction studies give the results close to the results of the present ab-initio MD simulation. The TD model resulted in higher Al/O and Y/O coordination when compared with experimental results.

Neutron, synchrotron X-ray diffraction and X-ray absorption studies of CaLaFeMnO 6 double perovskite

High-resolution neutron and synchrotron X-ray powder diffraction data of CaLaFeMnO 6 double perovskite have been used in a joint Rietveld refinement method to obtain accurate room temperature structure which includes a precise set of atomic positions and thermal motion parameters. The valence states of Mn and Fe in the octahedra were also determined by X-ray absorption spectroscopy by measuring Mn and Fe K edges at room temperature. The simultaneous refinement of both synchrotron X-ray and neutron diffraction data reaffirmed that the system indeed possesses orthorhombic structure with space group Pbnm. X-ray absorption spectroscopic measurements reveal that both Mn and Fe occupy the octahedral sites with valence state of +4 and +3, respectively. No cationic ordering arrangement was found and Fe/Mn was found to be randomly distributed over the octahedral sites. The results agree well with the earlier published neutron diffraction data.

Ionic diffusion within the α* and β phases of Ag3SI

The ionic diffusion mechanism of mobile ions within an underlying body-centred cubic(bcc) sublattice of immobile counterions is discussed. In particular, the case ofequal numbers of two ionic species forming long-range ordered and disorderedbcc arrays is considered, since these form the basis of the cubic perovskite andα-AgI-type crystal structures, respectively. Their structural behaviour, and its influence on thedynamic ionic disorder which characterizes superionic conduction, is illustrated for the case ofAg+ diffusionwithin the β and α* phases of Ag3SI. The calculated behaviour obtained by molecular dynamics (MD) computer simulations isvalidated with reference to published neutron diffraction and ionic conductivity measurements ofAg3SI, and used to examine the preferred diffusion pathways. The relevance of these findings forthe anion conduction mechanisms within perovskite structured compounds is brieflydiscussed.

Magnetic properties of NpNiGa5

We have investigated a polycrystalline sample ofNpNiGa5 by magnetization,specific heat and 237Np Mössbauer spectroscopy. We confirm that a ferromagnetic order sets in atTC≈30 K and thatNpNiGa5 undergoes a secondtransition at TN≈18 K. Mössbauer data indicate that the transition atTN is accompanied by a steep increase of the Np magnetic moment, which saturates to a value of0.94 µB, in goodagreement with the recently published neutron diffraction results. The Sommerfeld specific heat coefficientγ = 114 mJ mol−1 K−2 is the highestamong the NpTGa5 compounds. It compares to the ones observed in thePuTGa5 superconductors. The trend of the isomer shift along theNpTGa5 (T = Fe,Co, Ni, Rh) series points to a tendency towards localization of the 5f electronswhen increasing the number of d electrons of the transition metal T. Neither fullylocalized nor fully itinerant models are able to reproduce the whole experimentaldata. Realistic models should include the dual nature of the 5f electrons,i.e. consider the difference in the degree of itinerancy among 5f electrons in theNpTGa5 compounds.

Tight binding prediction of the α-Gd 2S 3 magnetic structure

Spin-dependent extended Hückel tight binding (EHTB) calculations were carried out for the magnetic solid Gd 2S 3 by considering 20 different variations in the ordering of the 4 f 7 moments. The tight-binding calculations are used to interpolate the band structure of a nonmagnetic congener (Y 2S 3) and the 4 f/5 d,6 s exchange interactions are introduced as perturbations via the introduction of spin-dependent H dd and H ss parameters. The calculations predict that Gd 2S 3 adopts an antiferromagnetic ordering of the 4 f 7 moments that is consistent with published neutron diffraction results. Our attempt to account for the calculated energies of the spin patterns using an Ising model was unsuccessful.

A Tight-Binding Method for Predicting Magnetic Ordering in Gd-Containing Solids: Application to GdB2C2

Herein we present a method to compute d-f mediated exchange coupling in Gd-containing systems with a spin-dependent extended Hückel-tight binding (EHTB) method. EHTB parameters were chosen to exactly reproduce the spin density functional calculation (SDFT) energy gap of the S = 45/2 and 39/2 spin patterns for a model compound, Gd6CoI12(OPH3)6. Comparison between SDFT and EHTB results shows a good match between the spin-pattern energy distribution for the two methods. We applied our EHTB method to the solid-state compound GdB2C2 by considering 6 different variations in the ordering of the 4f7 moments. Calculations indicate that this metallic system should exhibit antiferromagnetic ordering of the 4f7 moments with a magnetic structure consistent with published neutron diffraction results.

Intermediate-range order in vitreousSiO2 and GeO2

The total structure factors, S(Q), obtained from high-energy x-ray and neutron diffraction measurements on vitreousSiO2 (v-SiO2) andvitreous GeO2 (v-GeO2) have been analysed by the reverse Monte Carlo (RMC) modelling technique to generate athree-dimensional structural model. The bond angle distributions and the ring size distributionsfrom the model indicated that the sixfold ring and six- and sevenfold rings are dominant inv-SiO2 and v-GeO2, respectively. However, the fraction of threefold rings of Ge inv-GeO2 is larger thanthat of Si in v-SiO2 glass. These features are consistent with the published neutron diffraction and Ramanscattering studies.

D-Electron mediated [formula omitted]– [formula omitted] exchange in Gd-rich compounds; spin density functional study of Gd 2Cl 3

Spin-density functional theory (SDFT) calculations of the d– f exchange coupling for the pseudo-one-dimensional (pseudo-1-D) chain compound Gd 2Cl 3 has been carried out using the 1-D model, Gd 8Cl 12(OPH 3) 4, by considering seven variations in the ordering of the 4 f 7 moments. The calculations indicate that this semiconducting system should exhibit antiferromagnetic ordering of the 4 f 7 moments in a pattern consistent with published neutron diffraction data. An attempt to account for the calculated magnetic energies of spin patterns using an Ising model was unsuccessful, indicating that the latter model is inappropriate. The qualitative features can be interpreted using a perturbative molecular orbital model that focuses on the influence of the 4 f 7 – d exchange interaction on the d-based molecular orbitals. Fundamental to the d-electron-mediated exchange mechanism is the intra-atomic 4 f 7 – d exchange interaction. The essence of this interaction is present in the Gd atom [4 f 75d 16s 2] , which is computationally investigated within SDFT. In Gd 2Cl 3, the d-electron-mediated f– f exchange interaction was interpreted using basic perturbation theory. Computed density of states and spin polarization information was used to support the perturbation-theoretic analysis.

Orbital and spin magnetism in US - comparison with USe and UTe

We report the results of two experiments performed on uraniummonosulphide: measuring the magnetic Compton scattering (MCS) andx-ray magnetic circular dichroism (XMCD) at the M4,5 uraniumabsorption edges. From the MCS experiment we get the spin moment ofboth the localized (5f electrons) and the diffuse (mainly 6delectrons) contributions. Combining these results with bulkmagnetization and published neutron diffraction data, we canseparate the orbital and spin contributions to the localized anddiffuse moments. Using the XMCD measurements, we deduce theexpectation value of the magnetic dipole operator. We finallycompare our results on US with published measurements made on USeand UTe.

Neutron diffraction study of the magnetic structure of U 0.1Np 0.9Ru 2Si 2

The magnetic structure of the U 0.1Np 0.9Ru 2Si 2 solid solution was studied using neutron diffraction techniques on a single crystal. At T N=25 K this compound orders magnetically into an incommensurate longitudinally modulated structure which is very similar to the one reported for pure NpRu 2Si 2. At T com≈13 K, however, an incommensurate to commensurate transition takes place and the low temperature magnetic structures is the same as the one observed in URu 2Si 2. The calculated value of the magnetic moment is consistent with the published neutron diffraction and Mössbauer results.

Structural and computational analysis of published neutron diffraction data show that crystalline vitamin B12 coenzyme contains a strong intramolecular N-H...Ph hydrogen bond

The neutron diffraction crystal structure of vitamin B12 coenzyme [Bouquiere et al. (1993). Acta Cryst. B49, 79–89] contains a yet unnoticed intramolecular aromatic hydrogen bond donated by a propionamide side chain and accepted by the benzimidazole group. The distance from the H-atom to the aromatic midpoint is only 2.58 Å and the bond energy is calculated to be 16.7 kJ mol−1.

The Structures of Higher Rare Earth Oxides: Role of the Coordination Defect

A milestone in our understanding of the chemistry of the defect solid state has been achieved through new insights provided by recently published neutron diffraction studies of five rare earth oxides with compositions in the range M2O3-MO2. For more than four decades the exact way by which gross concentrations (i.e., up to 25%) of vacant sites can be accommodated in the fluorite MO2 lattice has remained largely unresolved. There is now compelling evidence that the vacancies do not exist in isolation but as octahedral 'coordination defects' of composition {MIII/2MIV/1.5□O6}. It is now clear that the intricate planar patterns formed by anion vacancies in the fluorite lattice arise from the unique and structure-determining topology of these coordination defects. The decisive role played by the coordination defect in generating extended defects in these fluorite-related oxides is highlighted in this paper.

The structure of molten K-Pb, Rb-Pb and Cs-Pb alloys

The authors have considered previously published neutron diffraction data, obtained at two different sources, on the molten alloys KxPb1-x, RbPb and CsPb. The difference between results for the same system obtained at the two sources has been shown to be attributable to instrumental resolution. The reverse Monte Carlo method has been used to produce structural models of all these melts. The authors have demonstrated that there is a range of structures consistent with the experimental data, all of which have in common a tendency for the lead to form clusters, typically of three or four atoms, with 60 degrees Pb-Pb-Pb bond angles. The results appear to be generally consistent with the existence of some tetrahedral Pb44- Zintl ions; however it seems unlikely that all Pb atoms occur in such well defined species since in this case the tetrahedra must approach each other too closely in order to accurately reproduce the experimental data. The authors have shown that for unconstrained (i.e. not completely tetrahedral) models the peak at a momentum transfer of approximately 1 AA-1 observed in the diffraction patterns occurs in the partial structure factor APbPb(Q) and is related to correlations between Pb clusters. The height of this peak varies with Pb concentration in KxPb1-x alloys in the same way as electrical resistivity, and the authors have postulated that this is due to electron localization associated with the cluster formation. For KPb the unconstrained model contains 25% tetrahedra, in good agreement with thermodynamic data.

Ground state properties of the AnFe 2 systems (An = U, Np and Pu)

We report on spin polarized energy band calculations for the series AnFe 2 (An = U, Np and Pu) in which both spin-orbit coupling and splitting of the energies of the 5f orbital polarization are included self-consistently. The new feature of the magnetism is the existence of large orbital moments on the actinide sites. The calculated moments agree with experiment. Finally we compare with published neutron diffraction results for the actinide 5f form factor of UFe 2 and PuFe 2, obtaining agreement with experiment and predict the form factor of NpFe 2.