Low-temperature Neutron Research Articles

Hydrogen in nonstoichiometric cubic titanium monoxides: X-ray and neutron diffraction, neutron vibrational spectroscopy and NMR studies

Hydrogen-induced changes in the properties of transition-metal oxides have attracted much recent attention due to numerous applications of these materials including catalysis, H2 production, low-temperature H2 sensing, solar cells, and air purification. However, basic properties of hydrogenated titanium monoxides have not been investigated so far. In the present work, we report the results of the first studies of the crystal structure, vibrational spectra, and mobility of H atoms in TiO0.72H0.30 and TiO0.96H0.14 using X-ray diffraction (XRD), neutron powder diffraction, neutron vibrational spectroscopy, and nuclear magnetic resonance (NMR). The hydrogenated compound TiO0.72H0.30 is found to retain the disordered cubic B1-type structure of the initial titanium monoxide, where H atoms exclusively occupy vacancies in the oxygen sublattice. It has been revealed that hydrogenation of the disordered cubic TiO0.96 leads to the formation of the two-phase compound TiO0.96H0.14, where the disordered B1-type phase coexists with the monoclinic phase of Ti5O5 type with an ordered arrangement of vacancies. In both phases, H atoms are found to occupy only vacancies in the oxygen sublattice. The low-temperature inelastic neutron scattering spectra of TiO0.72H0.30 and TiO0.96H0.14 in the energy transfer range of 40–180 meV exhibit a single peak due to optical oxygen vibrations (centered on about 60 meV) and a broad structure at 90–170 meV due to optical H vibrations. The unusual width of this structure can be attributed to the broken symmetry of hydrogen sites in the titanium monoxides: because of the presence of vacancies in the titanium sublattice, the actual point symmetry of these sites appears to be lower than octahedral. Proton NMR measurements have revealed that both hydrogenated compounds are metallic; no signs of hydrogen diffusive motion in TiO0.72H0.30 and TiO0.96H0.14 at the frequency scale of about 105 s-1 have been found up to 370 K.

Effects of low-temperature neutron irradiation on the microstructure and tensile properties of duplex 2304 stainless steel and its electron-beam welds

A lean duplex stainless steel material (2304-grade) in its base metal and electron beam (e-beam) welded conditions were studied microstructurally and mechanically as a function of irradiation conditions to evaluate its use as a structural material at low temperatures (60–100 °C). Neutron irradiation up to a fluence of 1.40 × 1019 n/cm2 (E > 0.1 MeV) or ~0.011 dpa decreased the total elongation of both base metal and e-beam welded samples. Overall, radiation hardening was observed in all the samples. The transversely cut irradiated samples showed some nonuniform quasi-cleavage and shearing in their fracture surfaces, indicating the variance of ductile nature of the two-phased (deformable austenite and harder ferrite) duplex structure. The e-beam welded samples also showed quasi-cleavage fracture, which is a characteristic of radiation-induced embrittlement. These observations of the e-beam welded samples were attributed to the formation of coarse ferrites, grain boundary and intragranular phases such as γ2 and γ3, and minor impurity phases such as CrN and Cr2N in the weld pool and/or heat-affected zone of the samples. Radiation-induced elemental segregation was also identified in the post-irradiated base metal.

Hot Electron and Ion Spectra in Axial and Transverse Laser Irradiation in the GXII-LFEX Direct Fast Ignition Experiment

An electron spectrometer was used to measure the electron and ion spectra in two different irradiations of implosion laser to the deuterated spherical shell targets at the direct fast ignition experiments on the Gekko-LFEX facility. In the transverse irradiation against the LFEX laser axis, the low effective hot electron temperature (Teff) and huge neutron yield (Ny) could be obtained although high Teff and low Ny could be observed in the axial irradiation. In the transverse irradiation, the efficient core heating could be obtained because the laser-plasma interaction position is close to the core and the diffusive/ion drag heating may be effective.

Structural Evolution of a Zr–2.5% Nb Alloy during Long-Term Low-Temperature Neutron Irradiation

Structural Evolution of a Zr–2.5% Nb Alloy during Long-Term Low-Temperature Neutron Irradiation

On the molecular basis of H2O/DMSO eutectic mixtures by using phenol compounds as molecular sensors: a combined NMR and DFT study.

NMR and DFT studies of phenol compounds as molecular sensors were carried out to investigate H2O/DMSO eutectic mixtures at a molecular level. The experimental 1H NMR chemical shifts of the OH groups, δexp(OH), of phenol, paracoumaric acid, and vanillic acid show maximum deshielding and, thus, hydrogen bond interactions in the range of mole fractions 0.20 < χ(DMSO) < 0.33. In the mole fractions χ(DMSO) < 0.2, a progressive decrease in δexp(OH) was observed which demonstrates a decrease in hydrogen bond interactions at infinite dilution in H2O, despite the increase in the number of available hydrogen bond acceptor and donor sites. DFT calculated δcalc(OH) of minimum energy solvation clusters were shown to be in reasonable agreement with the pattern in experimental δexp(OH) data. The chemical shift deshielding and, thus, increased hydrogen bond interactions in the natural product + DMSO + nH2O (n = 2, 3) solvation clusters, relative to complexes in DMSO or H2O solutions, cannot be attributed to a single structural parameter of the cooperative interactions between H2O and DMSO molecules with the phenol OH groups of the natural products. The minimum energy conformers of phenol compounds + 2H2O + DMSO complexes are in excellent agreement with a recent low temperature neutron diffraction experiment of 3D2O + DMSO and demonstrate a general structural motif of solvation complexes. The combined use of 1H NMR and DFT studies with emphasis on δ(OH) of phenol compounds, as molecular sensors, can provide an effective method for the study of solute-solvent interactions at the atomic level.

Anisotropy in the hardness of single crystal tungsten before and after neutron irradiation

In this work, the hardness of single crystal tungsten (W) after low, medium and high temperature neutron irradiation, reaching up to 1200°C, is studied. Micro-hardness tests with Vickers pyramid indenter are performed on reference and neutron irradiated samples by varying the orientation of the indenter on the single crystal tungsten contact surface. Due to the availability of multiple slip systems, the anisotropy of the hardness is characterized to allow the construction of uncertainty bands as well as dependence of the absolute value of the hardness on the orientation of the indenter wedges and principal crystallographic axes of the crystal. The applied neutron fluence and flux is representative for the ITER reactor, where tungsten is selected as armour material for plasma facing components. The neutron irradiation is performed in the BR2 reactor with mixed neutron spectrum up to 1 dpa (end of life fluence for ITER) but with appropriate measures to reduce the thermal neutron flux, such that the spurious transmutation into rhenium is reduced. The hardness study is accompanied with a detailed microscale examination to deduce the true indenter surface area. As a result, the true values of the hardness accounting for the pile-up effects are obtained as a function rotation angle of the indenter. The variation of the hardness as a function of indenter orientation before and after irradiation is discussed based on the indenter-slip system geometry. A simple but efficient methodology is proposed to deduce the true hardness and its angle-dependent variation for single crystal tungsten.

Long-range magnetic ordering in Li2MnGeO4 and precursor short-range spin correlations

Long-range magnetic ordering and short-range spin correlations in layered noncentrosymmetric orthogermanate ${\mathrm{Li}}_{2}\mathrm{MnGe}{\mathrm{O}}_{4}$ were studied by means of polarized and unpolarized neutron scattering. The combined Rietveld refinement of synchrotron and neutron powder diffraction data at room temperature within the $Pmn{2}_{1}$ space group allowed us to specify the details of the crystal structure. According to the additional Bragg peaks in low-temperature neutron diffraction patterns a long-range antiferromagnetic ordering with the propagation vector $k=\phantom{\rule{0.16em}{0ex}}(1/2\phantom{\rule{0.16em}{0ex}}1/2\phantom{\rule{0.16em}{0ex}}1/2)$ has been found below ${T}_{N}\ensuremath{\approx}8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Symmetry analysis revealed the model of the ground state spin structure within the ${C}_{a}c$ (no. 9.41) magnetic space group. It is represented by the noncollinear ordering of manganese atoms with a refined magnetic moment of $4.9\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/{\mathrm{Mn}}^{2+}$ at 1.7 K, which corresponds to the saturated value for the high-spin configuration $S=5/2$. Diffuse magnetic scattering was detected on the neutron diffraction patterns at temperatures just above ${T}_{N}$. Its temperature evolution was investigated in detail by polarized neutron scattering with the following XYZ-polarization analysis. Reverse Monte Carlo simulation of diffuse scattering data showed the development of short-range ordering in ${\mathrm{Li}}_{2}\mathrm{MnGe}{\mathrm{O}}_{4}$, which is symmetry consistent on a small scale with the long-range magnetic state below ${T}_{N}$. The reconstructed radial spin-pair correlation function $S(0)\phantom{\rule{0.16em}{0ex}}S(r)$ displayed the predominant role of antiferromagnetic correlations. It was found that spin correlations are significant only for the nearest magnetic neighbors and almost disappear at $r\ensuremath{\approx}12\phantom{\rule{0.16em}{0ex}}\AA{}$ at 10 K. Temperature dependence of the diffuse scattering implies short-range ordering long before the magnetic phase transition. Besides, the spin arrangement was found to be similar in both cases above and below ${T}_{N}$. As a result, an exhaustive picture of the gradual formation of magnetic ordering in ${\mathrm{Li}}_{2}\mathrm{MnGe}{\mathrm{O}}_{4}$ is presented.

Incommensurate magnet iron monophosphide FeP: Crystal growth and characterization

We report an optimized chemical vapor transport method, which allows growing FeP single crystals up to 500 mg in mass and 80 $mm^{3}$ in volume. The high quality of the crystals obtained by this method was confirmed by means of EDX, high-resolution TEM, low-temperature single crystal XRD and neutron diffraction experiments. We investigated the transport and magnetic properties of the single crystals and calculated the electronic band structure of FeP. We show both theoretically and experimentally, that the ground state of FeP is metallic. The examination of the magnetic data reveals antiferromagnetic order below T$_{N}$ =119 K while transport remains metallic in both the paramagnetic and the antiferromagnetic phase. The analysis of the neutron diffraction data shows an incommensurate magnetic structure with the propagation vector Q=(0, 0, $\pm{\delta}$), where ${\delta}$ $\sim$ 0.2. For the full understanding of the magnetic state, further experiments are needed. The successful growth of large high-quality single crystals opens the opportunity for further investigations of itinerant magnets with incommensurate spin structures using a wide range of experimental tools.

Magnetization spin reversal and neutron diffraction study of polycrystalline Tb0.55Sr0.45MnO3

We have investigated the structural and magnetic phase transitions in Sr doped polycrystalline Tb0.55Sr0.45MnO3, using temperature-dependent high-resolution neutron powder diffraction in the HRPT diffractometer at PSI to address the origin of magnetization reversal at low temperature. The solid solution Tb0.55Sr0.45MnO3 crystallizes in O′ type orthorhombic structure having the Pnma symmetry. The substitution of the divalent cation of Sr2+ at the site of Tb3+ dilutes the Mn–Tb interaction and affects the magnetic structure, which was observed by the field and temperature-dependent dc magnetization and neutron diffraction study. The temperature-dependent zero field-cooled and field cooled dc magnetization study reveals the indication of spin reversal phenomena, which is the nature of canted antiferromagnetic or ferrimagnetic transition at 100 Oe because of Mn–Mn sub-lattices interaction below 65 K. By increasing the applied magnetic field up to 20 kOe, a weak ferromagnetic type of behaviour is observed. The field-dependent magnetization shows weak coercivity due to the canted spin structure at low temperatures. The low-temperature neutron diffraction study of polycrystalline Tb0.55Sr0.45MnO3 reveals the non-collinear canted antiferromagnetic structure due to Tb ordering at 1.5 K, which turns into a non-collinear ferromagnetic structure along with spin canting followed by the spin reversal phenomena at 25 K.

Fatigue properties of ferritic/martensitic steel after neutron irradiation and helium implantation

• Effects of displacement damage and helium on the fatigue life of F82H-IEA. • Degradation of fatigue life after the low temperature neutron irradiation. • Degradation of fatigue life after the high temperature helium implantation. The effects of displacement damage and helium on the fatigue life of the reduced activation ferritic/martensitic steel, F82H-IEA, were evaluated by fatigue tests at room temperature after neutron irradiation and helium implantation. Those effects at various test conditions, e.g., irradiation and implantation temperatures and applied strain in fatigue tests, were discussed using data from both literature and the present study. As for the fatigue life under plastic strain dominant conditions, the neutron-irradiated specimens, which were irradiated at 389–423 K, showed shorter fatigue life compared to the as-received ones. In contrast, the specimens neutron-irradiated at 573 K showed no degradation. As well as the specimens neutron-irradiated at relatively low temperature, the helium-implanted specimens, which were implanted at 743 K, showed shorter fatigue life. The decrease in fatigue life of specimens neutron-irradiated at relatively low temperatures could be caused by the irradiation hardening, while that of the specimen helium-implanted at relatively high temperatures could be caused by an embrittlement with no/small hardening.

Structural disorder, phase stability and compressibility of refractory body-centered cubic solid-solution alloys

The Debye-Waller factors of refractory bcc solid-solution alloys TiVNbTa and TiVNbMoTa measured by low temperature neutron diffraction are obviously larger than that of the face-centered cubic (fcc) solid-solution alloy NiCoFeCrMn. The bcc lattice of the studied solid-solution alloys is stable up to ∼50 GPa, but a structural distortion is clearly observed at pressures higher than ∼20 GPa. The bulk modulus of bcc high entropy alloys (HEAs) is closely relevant to both the size mismatch in the lattice and the number of elements in the systems.

Synthesis, Crystal Structure, and Cooperative 3d–5d Magnetism in Rock Salt Type Li4NiOsO6 and Li3Ni2OsO6

Two new transition metal oxides with the nominal chemical compositions of Li4NiOsO6 and Li3Ni2OsO6 were successfully synthesized. Both compounds crystallize in an ordered rock salt structure type in the monoclinic C2/m space group. The crystal structures were determined using both synchrotron X-ray and time-of-flight neutron, powder diffraction data. In both phases, Ni2+ ions are present while oxidation states of osmium are +6 and +5 in Li4NiOsO6 and Li3Ni2OsO6, respectively. Ni2+ ions in the hypothetical fully ordered phase form a honeycomb arrangement in the ab crystallographic plane and these hexagons are centered by osmium ions. The magnetic layers are separated along the c axis by the octahedra, which are centered by Li+ (or Li+/Ni2+, depending on the chemical compositions). Crystal structure refinements reveal that there is some degree of mixed occupancy in cationic positions. Temperature dependent magnetic susceptibility data for both phases show ferrimagnetic transitions with predominant antiferromagnetic (AFM) interactions among 3d electrons of nickel and 5d electrons of osmium. Iso-thermal magnetization loops as a function of the applied magnetic field below the transition temperatures confirm the ferrimagnetic nature in magnetic transitions. Temperature dependent heat capacity data, however, did not exhibit any anomaly in either phase, indicating the absence of long-range magnetic ordering. The lack of long-range order for both Os5+ and Os6+-based compounds was also confirmed by low temperature neutron diffraction data down to 10 K. Temperature dependent AC magnetic susceptibility data in various frequencies for both samples indicate that Li4NiOsO6 exhibits spin-glass-like behavior, while the transition temperature for Li3Ni2OsO6 is nearly frequency independent.

Preparation of the noncentrosymmetric ferrimagnetic phase La0.9Ba0.1Mn0.96O2.43 by topochemical reduction

Topochemical reduction of La0.9Ba0.1MnO3 with NaH at 225 ​°C yields the brownmillerite phase La0.9Ba0.1MnO2.5. However, reduction with CaH2 at 435 ​°C results in the formation of La0.9Ba0.1Mn0.96O2.43 via the deintercalation of both oxide anions and manganese cations from the parent perovskite phase. Electron and neutron diffraction data reveal La0.9Ba0.1Mn0.96O2.43 adopts a complex noncentrosymmetric structure, described in space group I23, confirmed by SHG measurements. Low-temperature neutron diffraction data reveal La0.9Ba0.1Mn0.96O2.43 adopts an ordered magnetic structure in which all the nearest neighbor interactions are antiferromagnetic. However, the presence of ordered manganese cation-vacancies results in a net ferrimagnetic structure with net saturated moment of 0.157(2) μB per manganese center.

Effect of Mn substitution on the crystal and magnetic structure of Bi1−xCaxFeO3−x/2 multiferroics

The room- and low-temperature neutron diffraction measurements of the Bi0.9Ca0.1Fe0.6Mn0.4O3+δ compound have been carried out to disclose the influence of Mn substitution on the multiferroic properties of the low-doped Bi1−xCaxFeO3−x/2 perovskites combining ferroelectric and weak ferromagnetic behavior. It has been proven that the material under study retains a polar R3c structure specific to the parent Bi0.9Ca0.1FeO2.95. The Mn doping results in the elimination of oxygen vacancies giving rise to the increase in spontaneous electric polarization. The chemical modification stabilizes the collinear antiferromagnetic structure at room temperature. The reorientation of the antiferromagnetic vector from the c to a axis takes place with decreasing temperature. Reflecting the competitive character of the superexchange interactions between Fe3+, Mn3+ and Mn4+, the coexistence of ferromagnetic glassy and antiferromagnetic long-range-ordered phases is observed at low temperatures.

A fast-cooling sample-positioning probe for low-temperature neutron scattering experiments

Top-loading exchange gas cryostats have for a long time been a widely used sample environment device for a variety of neutron scattering experiments. In particular, they allow for simpler and faster changes of samples mounted on sample-positioning probes than is possible with bottom-loading cryostats under vacuum. Here, a new design for sample probes using composite materials is investigated, which significantly decreases the time required to cool a newly installed sample in two different types of closed-cycle exchange gas cryostat.

Low-temperature crystal structure of the unconventional spin-triplet superconductor UTe2 from single-crystal neutron diffraction.

The crystal structure of a new superconductor UTe2 has been investigated using single-crystal neutron diffraction for the first time at the low temperature (LT) of 2.7 K, just above the superconducting transition temperature of ∼1.6 K, in order to clarify whether the orthorhombic structure of type Immm (No. 71), reported for the room-temperature (RT) structure persists down to the superconducting phase and can be considered as a parent symmetry for the development of spin-triplet superconductivity. In contrast to the previously reported phase transition at about 100 K [Stöwe (1996). J. Solid State Chem. 127, 202-210], our high-precision LT neutron diffraction data show that the body-centred RT symmetry is indeed maintained down to 2.7 K. No sign of a structural change from RT down to 2.7 K was observed. The most significant change depending on temperature was observed for the U ion position and the U-U distance along the c direction, implying its potential importance as a magnetic interaction path. No magnetic order could be deduced from the neutron diffraction data refinement at 2.7 K, consistent with bulk magnetometry. Assuming normal thermal evolution of the lattice parameters, moderately large linear thermal expansion coefficients of about α = 2.8 (7) × 10-5 K-1 are estimated.

Absence of magnetic ordering in the spin liquid candidate Ca3Cu2GeV2O12

Typically, quantum spin liquid candidates can be found in materials with a combination of geometrical frustration along with low spin. Due to its spin of S = 1/2 the copper (II) ion is often present in the discussion on spin liquid candidates. The solid state compound Ca3Cu2GeV2O12 is a material that crystallizes in the garnet structure (s.g. #230, Ia-3d), where 3D frustration is known to occur. Heat capacity has shown a lack of magnetic ordering down to 0.35 K, confirmed with low temperature neutron diffraction to 0.07 K. This system displays a Weiss temperature of −0.93(1) K indicating net antiferromagnetic interactions and significant J1–J2 competition causing frustration. Using both neutron and x-ray diffraction along with heat capacity and magnetometry, the work presented here shows Ca3Cu2GeV2O12 has potential as a new spin liquid candidate.

Long-range and short-range ordering in 2D honeycomb-lattice magnet Na2Ni2TeO6

The magnetic structure of quasi-two-dimensional (2D) honeycomb lattice Na2Ni2TeO6 has been determined by low-temperature neutron diffraction and the crystal structure fine details at room temperature have been established by a combination of synchrotron and neutron powder diffraction. The atomic structure is described by the P63/mcm space group, but the strong presence of stacking faults defects in layered ordering is found for the first time. Both magnetization and specific heat data indicates an establishment of a long-range antiferromagnetic order with TN = 25 ± 1 K, preceded by a short-range order on 2D honeycomb lattice at about 34 K. Determined effective magnetic moment μeff = 4.35 μB is in excellent agreement with numerical estimation using effective g-factor g = 2.19 directly measured by electron spin resonance. The ground magnetic state at T = 1.5 K is represented by the commensurate zigzag-type magnetic order. Magnetic moments of Ni are almost perpendicular to the honeycomb layers, which antiferromagnetically coupled along the c-direction. Besides, the coherent magnetic scattering area has a disk shape that is homogeneous over the ab plane and compressed along the c-axis that indicates 2D nature of magnetic correlations in the compound. The magnetic diffuse neutron scattering related to the presence of strong short-range spin-spin correlations above TN was observed. Based on our experimental measurements we propose magnetic phase diagram for Na2Ni2TeO6.

Effect of Tb substitution in naturally layered LaMn2Si2: magnetic, magnetocaloric, magnetoresistance and neutron diffraction study

Evolution of physical and magnetic properties of La rich La1−xTbxMn2Si2 (x = 0, 0.1 and 0.225) compounds is studied using temperature and field dependent dc magnetization, electrical transport, and heat capacity measurements. LaMn2Si2 undergoes a paramagnetic to antiferromagnetic ordering at TN ~ 400 K and a long range ferromagnetic ordering below TC ~ 308 K. However, the substitution of Tb at La site results in the contraction of unit cell as well as decrease in the Mn–Mn spacing, which leads to an additional antiferromagnetic phase below TN ~ 87 K and 257 K for La0.9Tb0.1Mn2Si2 and La0.775Tb0.225Mn2Si2, respectively. Using magnetization isotherm results, we have constructed an H–T phase diagram for x = 0.225 and have found the coexistence of this additional antiferromagnetic phase with ferromagnetic phase. The phase coexistence at x ~ 0.225 is further analysed using magnetoresistance (MR) and magnetocaloric effect (MCE) studies and a reasonable correlation is established between MCE and MR. In order to analyse the nature of the magnetic transition at TC, a universal master curve is constructed by rescaling the magnetic entropy curves. Low temperature neutron diffraction measurements performed on the polycrystalline samples revealed a canted ferromagnetic structure for x ~ 0.1 and a canted antiferromagnetic structure for x ~ 0.225.

Magnetic ordering in rare earth metal dysprosium revealed by neutron diffraction studies in a large-volume diamond anvil cell

ABSTRACTA high pressure low-temperature neutron diffraction study has been carried out on rare earth metal Dysprosium (Dy) to 10.8 GPa and 8 K in a large-volume diamond anvil cell using a spallation neutron source. Dy crystallizes in an ambient pressure hexagonal close packed phase and forms an incommensurate helical antiferromagnetic phase, marked by the appearance of superlattice magnetic peaks below 176 K with a turn-angle between the hexagonal layers that increases both with increasing temperature and pressure. The ferromagnetic transition below 87 K is marked by an increase in intensity of nuclear peaks with disappearance of magnetic superlattice reflections. Dy transforms to an alpha-Samarium (α-Sm) phase at 7 GPa and only ferromagnetic ordering is observed for this high pressure phase. The ferromagnetic transition is observed at 59 K in the α-Sm phase at 10.8 GPa in close agreement with the magnetic ordering temperature obtained from electrical transport measurements. In the entire pressure-temperature range of this study, Dy shows a negative thermal expansion coefficient of as much as −2.5% in the magnetically ordered phases between 200 and 8 K.