Low-temperature Neutron Diffraction Experiments Research Articles

Revisiting the magnetic structure of Holmium at high pressure by using neutron diffraction

Low-temperature neutron diffraction experiments at P= 8 GPa have been conducted to investigate the magnetic structures of metallic Holmium at high pressures by employing a long d-spacing high-flux diffractometer and a Paris-Edinburgh press cell inside a cryostat. We find that at P=8 GPa and T=5 K, no nuclear symmetry change is observed, keeping therefore the hexagonal closed packed (hcp) symmetry at high pressure. Our neutron diffraction data confirm that the ferromagnetic state does not exist. The magnetic structure corresponding to the helimagnetic order, which survives down to 5 K, is fully described by the magnetic superspace group formalism. These results are consistent with those previously published using magnetization experiments.

On the properties of the novel CeMgNi2T2 (T = Co, Cu) alloys and their hydrides

In this contribution, we report the results of the studies on the novel CeMgNi2Co2 and CeMgNi2Cu2 alloys and their hydrides/deuterides. Both the parent alloys and the CeMgNi2Co2H6.1 hydride crystallize in the MgCu4Sn-type structure, while the CeMgNi2Cu2H6.0 hydride becomes almost completely amorphous. In the MgCu4Sn structure the 3d metals are distributed over the 16e site forming the pyrochlore lattice. That makes this system particularly interesting as concerns the magnetic properties. Our studies reveal that magnetic susceptibilities of the alloys are weakly temperature-dependent and therefore, the Pauli-like paramagnetism can be suggested. Additionally, the results of the low temperature neutron diffraction experiments revealed the lack of the long-range magnetic ordering.

Tuning of magnetic structure and its effect on magnetic properties in Co(Cr1–xMnx)2O4 (x = 0–0.3)

We report here the magnetic properties of Co(Cr1–xMnx)2O4 (x = 0.00–0.30) through bulk-magnetization measurements and temperature variable neutron powder diffraction experiments. We have noticed the compensation temperature (Tcomp) and then the onset of negative magnetization at T = 60 K and 83 K only for Co(Cr0.73Mn0.27)2O4, i.e., x = 0.27, and Co(Cr0.70Mn0.30)2O4, i.e., x = 0.30 samples. Low temperature neutron diffraction experiments on these polycrystalline samples (i.e., x = 0.27 and 0.30) are performed to explore the understanding of observed negative magnetization, complex magnetic properties, and correlation between chemical and magnetic structures. A broad hump has been observed around the (111) Bragg reflection which gives the signature of diffuse scattering from short-range magnetic interactions coexisting with long range magnetic order below ordering temperature. The variation of bond lengths, bond angles, and magnetic moments with temperature is correlated, which clearly indicates magneto-elastic coupling around compensation temperature in these compounds. The presence of this magneto-elastic coupling results in the sign change of the magneto-caloric effect at compensation temperature.

A neutron diffraction study of crystal and low-temperature magnetic structures within the (Na,Li)FeGe2O6 pyroxene-type solid solution series

Solid solution compounds along the Li1–xNaxFeGe2O6 clinopyroxene series have been prepared by solid state ceramic sintering and investigated by bulk magnetic and calorimetric methods; the Na-rich samples with x(Na) > 0.7 were also investigated by low-temperature neutron diffraction experiments in a temperature range of 4–20 K. For samples with x(Na) > 0.76 the crystal structure adopts the C2/c symmetry at all measuring temperatures, while the samples display P21/c symmetry for smaller Na contents. Magnetic ordering is observed for all samples below 20 K with a slight decrease of TN with increasing Na content. The magnetic spin structures change distinctly as a function of chemical composition: up to x(Na) = 0.72 the magnetic structure can be described by a commensurate arrangement of magnetic spins with propagation vector k = (½, 0 0), an antiferromagnetic (AFM) coupling within the Fe3+O6 octahedra zig-zag chains and an alternating AFM and ferromagnetic (FM) interaction between the chains, depending on the nature of the tetrahedral GeO4 chains. The magnetic structure can be described in magnetic space group Pa21/c. Close to the structural phase transition for sample with x(Na) = 0.75, magnetic ordering is observed below 15 K; however, it becomes incommensurately modulated with k = (0.344, 0, 0.063). At 4 K, the magnetic spin structure best can be described by a cycloidal arrangement within the M1 chains, the spins are within the a–c plane. Around 12 K the cycloidal structure transforms to a spin density wave (SDW) structure. For the C2/c structures, a coexistence of a simple collinear and an incommensurately modulated structure is observed down to lowest temperatures. For 0.78 ≤ x(Na) ≤ 0.82, a collinear magnetic structure with k = (0 1 0), space group PC21/c and an AFM spin structure within the M1 chains and an FM one between the spins is dominating, while the incommensurately modulated structure becomes dominating the collinear one in the samples with x(Na) = 0.88. Here the magnetic propagation vector is k = (0.28, 1, 0.07) and the spin structure corresponds again to a cycloidal structure within the M1 chains. As for the other samples, a transition from the cycloidal to a SDW structure is observed. Based on the neutron diffraction data, the appearance of two peaks in the heat capacity of Na-rich samples can now be interpreted as a transition from a cycloidal magnetic structure to a spin density wave structure of the magnetically ordered phase for the Na-rich part of the solid solution series.

Density-functional theory investigation of electronic structure, elastic properties, optical properties, and lattice dynamics of Ba2ZnWO6

The electronic structures, optical dielectric functions, elastic properties, and lattice dynamics of Ba2ZnWO6 have been investigated by using the generalized gradient approximation. The density of states and distributions of charge density show that O and Ba tend toward ionic bond, but O and W or Zn display the covalent bond character. The calculated energy band structure shows that Ba2ZnWO6 is a wide indirect band gap semiconductor. The static value 2.28 of the refractive index is attained. The analysis of the elastic properties of Ba2ZnWO6 indicates a rather weak elastic anisotropy. The phonon dispersion is calculated, suggesting no structural instability, which is agreement with the recent low temperature neutron diffraction experiments. The mensurability Cv (phonon heat capacity) as the function of the temperature is also calculated to judge our results for future experiment.

Magnetic Structure and Properties of the Li-Ion Battery Materials FeSO4F and LiFeSO4F

Using magnetic susceptibility and low-temperature neutron diffraction experiments we present a thorough characterization of the magnetic properties of LiFeSO4F and FeSO4F. Temperature dependent magnetic susceptibility measurements show a transition to long-range antiferromagnetic order at 100 K in FeSO4F whereas the ordering temperature in LiFeSO4F is 25 K. We attribute the decreased ordering temperature to a structural change which decreases the strength of the magnetic interactions along the length of the chains and to the difference of supersuper-exchange interactions between Fe2+ and Fe3+ ions. Powder neutron diffraction experiments were used to determine the magnetic structures of both compounds, which are discussed in terms of exchange interactions and the anisotropy of Fe2+. The iron magnetic moments are antiparallel within the chains, whereas the coupling between the chains is different in the lithiated compound compared to the delithiated one, resulting in different spin arrangements.

Quantum dynamics of small H2 and D2 clusters in the large cage of structure II clathrate hydrate: Energetics, occupancy, and vibrationally averaged cluster structures

We report diffusion Monte Carlo (DMC) calculations of the quantum translation-rotation (T-R) dynamics of one to five para-H(2) (p-H(2)) and ortho-D(2) (o-D(2)) molecules inside the large hexakaidecahedral (5(12)6(4)) cage of the structure II clathrate hydrate, which was taken to be rigid. These calculations provide a quantitative description of the size evolution of the ground-state properties, energetics, and the vibrationally averaged geometries, of small (p-H(2))(n) and (o-D(2))(n) clusters, n=1-5, in nanoconfinement. The zero-point energy (ZPE) of the T-R motions rises steeply with the cluster size, reaching 74% of the potential well depth for the caged (p-H(2))(4). At low temperatures, the rapid increase of the cluster ZPE as a function of n is the main factor that limits the occupancy of the large cage to at most four H(2) or D(2) molecules, in agreement with experiments. Our DMC results concerning the vibrationally averaged spatial distribution of four D(2) molecules, their mean distance from the cage center, the D(2)-D(2) separation, and the specific orientation and localization of the tetrahedral (D(2))(4) cluster relative to the framework of the large cage, agree very well with the low-temperature neutron diffraction experiments involving the large cage with the quadruple D(2) occupancy.

The first layered analogue of Sr2FeMoO6; the structure and electronic properties of Sr4FeMoO8

The n = 1 Ruddlesden–Popper phase Sr4FeMoO8 has been synthesised by high temperature ceramic methods under reducing conditions. Rietveld refinements of the structure against X-ray and neutron powder diffraction data shows that this phase adopts the space group I4/mmm (a = 3.92962(5), c = 12.6707(2) A) and contains a crystallographically disordered arrangement of Fe and Mo on the single octahedral site in the structure. Mossbauer spectroscopy data show that iron is in the trivalent state and that the transition metals are fully disordered at a local scale. SQUID magnetometry measurements and low temperature neutron diffraction experiments have been used to examine the magnetic properties of this material. The magnetic susceptibility shows Curie–Weiss paramagnetism above a magnetic transition at 16(1) K. Below this temperature the sample shows magnetic hysteresis, but the neutron diffraction data show no evidence of the additional Bragg intensity associated with the formation of a magnetically ordered phase. The magnetic transition at 16 K is therefore assigned to the formation of a spin-glass phase. The absence of magnetic order in this material is due to the chemical disorder in the Fe/Mo occupancy of the octahedral site and thus the randomisation of the magnetic exchange interactions.

Magnetism and structural chemistry of the n = 1 Ruddlesden-Popper phases La4LiMnO8 and La3SrLiMnO8.

Polycrystalline samples of La4LiMnO8 and La3SrLiMnO8 have been studied by a combination of X-ray diffraction (XRD), neutron diffraction (ND), 6Li MAS NMR, electron microscopy (EM), and magnetometry. Room-temperature XRD and ND measurements suggest that both compounds have the K2NiF4 structure, with a disordered arrangement of Li and Mn over the six-coordinate sites. However, MAS NMR and EM demonstrate the presence of local 1:1 Li:Mn order on these sites, and EM shows that although cation order is well-developed in each xy sheet of corner-sharing octahedra, the sheets are stacked randomly along z. The structures are best described as paracrystalline, and many of the concepts of conventional crystallography are inapplicable. Magnetometry and low-temperature ND experiments show that, despite their paracrystallinity, the two compounds are ordered antiferromagnetically with susceptibility maxima at 26 and 18 K, respectively, and with ordered magnetic moments of 3.61(6) and 2.3(1) muB per Mn cation at 2 K. Anisotropic peak broadening reveals a 2D character in the magnetic behavior of both compounds, and La3SrLiMnO8 is well-modeled as a quadratic layer S = 3/2 Heisenberg antiferromagnet.

Li4MgReO6: An S=1/2 antiferromagnet exhibiting spin-glass behavior

Polycrystalline ${\mathrm{Li}}_{4}{\mathrm{MgReO}}_{6}$ was prepared by means of solid-state synthesis. The crystallographic structure was determined from high-resolution powder-neutron-diffraction data. It was found that ${\mathrm{Li}}_{4}{\mathrm{MgReO}}_{6}$ crystallizes in space group $C2/m$ with lattice parameters, $a=5.0978(3) \AA{},$ $b=8.8163(5) \AA{},$ $c=5.0815(3) \AA{},$ and $\ensuremath{\beta}=109.835(2)\ifmmode^\circ\else\textdegree\fi{}.$ This structure is an ordered variant of ${\mathrm{Li}}_{2}{\mathrm{SnO}}_{3}$ in which ${\mathrm{Li}}^{+},$ ${\mathrm{Mg}}^{2+},$ and ${\mathrm{Re}}^{6+}$ ions occupy the ${\mathrm{Sn}}^{4+}$ sites, which can be desribed as honeycomb layers. Due to the large differences in formal charge, the ${\mathrm{Re}}^{6+}$ ions order on half of the Sn sites forming a distorted face-shared tetrahedral sublattice. As ${\mathrm{Re}}^{6+}$ is an $S=1/2$ ion this material is an attractive candidate for the study of the effects of geometric frustration on a quantum spin system. The investigation of bulk magnetic susceptibilities and magnetization studies as well as low-temperature neutron-diffraction experiments reveals a spin-glass behavior with a glass transition temperature of 12 K. Both heat-capacity and muon spin-relaxation measurements at low temperatures are consistent with a spin-glass interpretation with no long-range order detectable down to $T=0.5 \mathrm{K}.$

Magnetic Properties of the LaNi1–xWxO3(0≤x≤0.25) Perovskites

Magnetic properties of the LaNi1−xWxOx(0≤x≤0.25) mixed oxides have been studied from the temperature and magnetic field (H) dependence of the magnetization (M) and low-temperature neutron diffraction experiments. The variation ofMwith temperature shows the presence of weak ferromagnetic interactions with Curie temperatures ranging from 200 to 250 K. These interactions become stronger asxincreases and this fact has been interpreted in relation to the progressive electronic localization that takes place in this perovskite-like system. The observed ferromagnetic behavior is confirmed by the hysteresis loops obtained in theMvsHcurves at different temperatures. The saturation values are however very low in comparison with the expected ones for Ni2+or Ni3+. On the other hand, an extra peak in the neutron diffraction experiments for thex=0.20 phase shows the appearance of an antiferromagnetic ordering, which has been interpreted on the basis of a G-type magnetic mode. Both results can be understood considering antiferromagnetic in character interactions with an important canting of the spin arrangement, giving therefore rise to a weak ferromagnetic component, which has been characterized from magnetization measurements. These results have been summarized in a phase diagram in which the most relevant electronic features of the title system can be visualized.

Magnetic characterization and structural study of new ternary carbides: ThFe11Cx (1.5

The authors report on the crystal and magnetic structure determination of ThFe11Cx. The positions of the metal atoms correspond to the BaCd11 type of structure. The carbon atoms occupy octahedral sites formed by four iron and two thorium atoms. Compounds having carbon concentrations x=1.5 and 2 were found in order magnetically between 400 and 500 K. Low-temperature neutron diffraction experiments provide evidence of a ferromagnetic structure with moments oriented perpendicular to the c axis whereas at room temperature the magnetic moments appear to be aligned parallel to the c axis of the tetragonal structure.

Temperature-dependent structure and anisotropic debye-waller factors of the dimer compound Cs 3Ho 2Br 9

The temperature-dependent structure of the dimer compound Cs 3Ho 2Br 9 was investigated in great detail with use of the high-resolution neutron powder diffractometer D2B at ILL (Grenoble). The refinement of around 2350 nuclear reflections within the trigonal space group R3 c yields largely anisotropic thermal vibrations of the Br atoms in contrast to Cs and Ho. The orientations of the thermal ellipsoids are reasonable when considering the reduced distances within the dimeric units Ho 2Br 3− 9. Additional low temperature neutron diffraction experiments performed down to T = 7mK did not give evidence for long-range magnetic ordering.

The crystal and magnetic structures of Ca 2YRuO 6 and the electronic properties of the series M2+2X3+Ru 5+O 6 ( M = Ca, Sr, Ba; X = La, Y)

Profile analysis of constant-wavelength powder neutron diffraction data has been used to refine the crystal structure of the ordered perovskite Ca 2YRuO 6. The material is monoclinic (space group P2 1 n ) with a disordered arrangement of calcium and yttrium on the A site and one of the B sites, such that the formula is best written as Ca 1.43Y 0.57[(Ca 0.57Y 0.43)Ru]O 6. Low-temperature neutron diffraction experiments showed that the material is a Type I antiferromagnet at 2.5 K with an ordered magnetic moment of 1.2(1)μ B per Ru 5+. It is suggested that the dominant factor in determining the electronic properties of the series M 2+ 2 X 3+Ru 5+O 6 ( M = Ca, Sr, Ba; X = La, Y) is the Ru-Ru separation distance.

The crystal structures and magnetic properties of Ba 2LaRuO 6 and Ca 2LaRuO 6

Profile analysis of high-resolution, powder neutron-diffraction data was used to refine the previously reported structures of the ordered, distorted perovskites Ba 2LaRuO 6 and Ca 2LaRuO 6. Low-temperature neutron diffraction experiments showed that, at 2K, the former is a Type IIIa antiferromagnet while the latter is Type I. Both compounds have an ordered magnetic moment of μ Ru ⋍ 1.95μ B per Ru 5+ ion. The Néel temperature of Ba 2LaRuO 6 was determined to be 29.5K, and the covalent mixing between the ruthenium and nearest-neighbor anions is described by A 2 π = 8.2 ± 1% for Ba 2LaRuO 6 and 8.6 ± 1% for Ca 2LaRuO 6. The ionic radius of a Ru 5+ ion is 0.56 Å. These data are consistently interpreted within the framework of a strongly correlated, half-filled π∗ band. Extension of this interpretation to the magnetic data for the perovskites CaRuO 3 and SrRuO 3 leads to a fundamental theoretical prediction.

Extremely short non-bonded H ? H distance in two derivatives of exo, exo-tetracyclo[6.2.1.13,6.02,7]dodecane

Non-bonded H ⋯ H distances of 1.754(4) and 1.713(3)A have been measured by low-temperature neutron diffraction experiments on the crystals of exo,exo-tetracyclo[6.2.1.13,6.02,7]dodecan-4-yl 3′,5′-dinitrobenzoate (2) and exo,exo-tetracyclo[6.2.1.13,6.02,7]dodecane-2,7-dicarboxylic anhydride (3); empirical force field calculations for parent gaseous hydrocarbons give larger values by about 0.14 A.