Neutron diffraction study of polycrystalline 4He in a porous medium

The elastic (diffraction) component of the neutron scattering cross section, which carries information on the atomic structure of solid helium confined in silica aerogel, has been studied. Analysis of the crystalline structure of solid helium in a porous medium, which is determined from the existing neutron diffraction data, indicates that the superfluid phase is localized inside a hexagonal close-packed phase and is not present in a body-centered cubic crystal. It has also been revealed that the addition of the ³He isotope changes the structure of solid helium and hardly affects the formation of a superfluid phase.     

Neutron diffraction study of ammonium tartrate (NH4)2C4H4O6

A neutron diffraction study of ammonium tartrate has been carried out. Using the diffractometer in symmetrical setting, intensities of 750 reflections have been measured. The positions of all the hydrogen atoms have been determined. A good agreement is noticed between the present neutron and the earlier x-ray heavy atom parameters. The tartrate ion consists of two nearly identical planar halves, with an interplanar angle of 62°. Tre structure is stabilized by a net-work of hydrogen bonds. Details of hydrogen bonding and the ammonium ions environment are discussed.     

Neutron diffraction and magnetic measurements of polycrystalline NH4MnCl3

Neutron diffraction and magnetic susceptibility measurements show NH₄MnCl₃ to be an antiferromagnet of type G with T_N ∼ 105°K. The low temperature patterns reveal a tetragonal splitting and superlattice lines which were accounted for by assuming the space group to be P bnm.     

Neutron diffraction study of the magnetic structure of Na2 RuO 4

We present a novel model to simulate real social networks of complex interactions, based in a system of colliding particles (agents). The network is build by keeping track of the collisions and evolves in time with correlations which emerge due to the mobility of the agents. Therefore, statistical features are a consequence only of local collisions among its individual agents. Agent dynamics is realized by an event-driven algorithm of collisions where energy is gained as opposed to physical systems which have dissipation. The model reproduces empirical data from networks of sexual interactions, not previously obtained with other approaches.     

Neutron diffraction study on the hyperfine-enhanced nuclear spin order of HoVO4

A neutron diffraction experiment on the hyperfine enhanced nuclear spin system of HoVO₄ has been carried out following adiabatic demagnetization cooling. Below 4.5 mK we observed magnetic diffraction peaks due to antiferromagnetic order of the hyperfine enhanced nuclear spin system. A spin-flop transition in an applied magnetic field of about 80 Oe was also observed. The neutron diffraction results are consistent with the magnetic structural model proposed by Bleaney on the basis of dipolar energy considerations.     

Neutron diffraction study of magnetically ordered TbAsO4

TbAsO₄ undergoes a magnetic phase tranfition at T_N = 1.5 K into a helical antiferromagnetic moment configuration. The spiral is planar, with the propagation vector along [001] and the moments perpendicular to [001]. The propagation period is $\text{197 ± 12}\text{A}\text{?}$, equivalent to 31 ± 2 unit cell constants. The neutron diffraction results are discussed in connection with previously measured macroscopic data.     

Neutron diffraction study of antiferromagnetism in YbVO4

The antiferromagnetism of YbVO₄ has been studied by neutron diffraction down to 50 mK. The structure is collinear with the spins along the c-axis. The Néel point is at 93 mK and the saturated moment on the ytterbium is (3.1 ± 0.16)μB. The results are in reasonable agreement with Mössbauer work.     

Neutron diffraction study of liquid iodine

A theory of molecular diffusion in liquids that takes into account the shape of intermolecular potential wells with a cosine potential produces fine spectral detail in the far infrared. The effect of a static external electric field is to shift this panoply of peaks to higher frequency, increase the number of peaks, and shift their individual relative frequencies and intensities.     

Neutron powder diffraction study of the crystal and magnetic structures of Fe2SiS4

The orthorhombic Fe₂SiS₄ (space group no. 62, Pnma) showing olivine-type structure has been studied. Measurements using neutron powder diffraction experiments were performed at 295, 140, 120, 40, 20 and 10 K in order to determine the crystal and magnetic structures of Fe₂SiS₄ at low temperatures. No crystallographic phase transition was observed between 295 and 10 K while two magnetic transitions were found. One transition around 127 K denotes a change from the paramagnetic (P) to an antiferromagnetic (AF) state while at 30 K the appearance of a ferrimagnetic (Fi) state is observed. Different magnetic models are presented. Relations are found between indirect magnetic interactions, on the one hand, and the Fe(4a), Fe(4c) and sulfur positions, on the other hand, and relations between the crystal structure and the magnetic models are given. The neutron powder diffraction results are compared with results, already reported, of Mössbauer and magnetization measurements.     

Neutron powder diffraction and high-pressure synchrotron x-ray diffraction study of tantalum nitrides

Tantalum nitride(TaN) compact with a Vickers hardness of 26 GPa is prepared by a high-pressure and hightemperature(HPHT) method. The crystal structure and atom occupations of WC-type TaN have been investigated by neutron powder diffraction, and the compressibility of WC-type TaN has been investigated by using in-situ high-pressure synchrotron x-ray diffraction. The third-order Birch–Murnaghan equation of state fitted to the x-ray diffraction pressure–volume(P–V) sets of data, collected up to 41 GPa, yields ambient pressure isothermal bulk moduli of B'_0= 369(2) GPa with pressure derivatives of B 0= 4 for the WC-type TaN. The bulk modulus of WC-type TaN is not in good agreement with the previous result(B_0= 351 GPa), which is close to the recent theoretical calculation result(B_0= 378 GPa). An analysis of the experiment results shows that crystal structure of WC-type TaN can be viewed as alternate stacking of Ta and N layers along the c direction, and the covalent Ta–N bonds between Ta and N layers along the c axis in the crystal structure play an important role in the incompressibility and hardness of WC-type TaN.     

Neutron diffraction study of high-pressure-induced structural changes in the ammonium halogenides ND4Br and ND4Cl

Structural changes in the deuterated ammonium halogenides ND₄Br and ND₄Cl have been studied by neutron time-of-flight diffraction up to pressures of 45 and 35 kbar, respectively. Data on the equations of state and pressure dependence of the deuterium position parameter have been obtained. A comparison with the hydrogen-containing analogs showed that isotopic substitution of deuterium for hydrogen affects only slightly the compressibility of the systems under study, although the effect is noticeable for ND₄Cl. It has been established that the order-disorder transition from the phase with random deuterium distribution (CsCl cubic structure, space group Pm3m) to the ordered phase (same structure, space group ) occurs in both compounds at the same critical value of the position parameter u=0.153±0.002, which is apparently the same for all ammonium halogenides, and, possibly, for other systems of this structural type as well. Fiz. Tverd. Tela (St. Petersburg) 40, 142–146 (January 1998)     

Neutron diffraction study of sputtered and annealed Tb-Fe alloys

Sputtered alloys of composition Tb_x Fe_1?x, with x = 0.118, 0.45 and 0.75 were shown by neutron diffraction to be amorphous. Line-width measurements of an anealed sputtered sample with x = 0.333 indicate microcrystallinity with a particle size of ≈ 100 Å.     

Neutron diffraction study of the orientational disorder in Ni(ND3)6Br2

The ND₃ group is in the high temperature phase of the title compound dynamically disordered. Fourier maps derived from the analysis of neutron single crystal Bragg data with phases based on a split atom and a Frenkel model show uniquely that each D₃ group has four density maxima occupying the corners of a square. Neither one nor three dimensional models of purely rotational disorder can describe the observations adequately.     

Neutron diffraction study of hydrogen in α titanium

It has been determined, using neutron diffraction, that hydrogen dissolved in the α (h.c.p.) phase of titanium is distributed randomly among the tetrahedral sites.     

Neutron diffraction study of (NH4I)0.73(KI)0.27

Powder and single crystal neutron diffraction experiments were performed on (NH₄I)_0.73(KI)_0.27 and provide experimental evidence for the occurence of a new phase below 63K. This ε phase has trigonal symmetry (space group R3m) and was, so far, not observed in the ammonium halides. It can be characterized by an almost undistorted NaCl lattice, with two in-equivalent NH₄ sites of the four NH₄ molecules per unit cell. The NH₄ group at the trigonal axis (the former cubic <111> axis) is almos perfectly tetrahedral. The other three NH₄ molecules are 0.18 Å off center and exhibit C_3v symmetry with the polar axis along the remaining three body diagonals. Hence, this ε phase reveals a complex dipolar order with a residual moment along the trigonal axis.     

Neutron diffraction study of (NH4I)0.73(KI)0.27

Powder and single crystal neutron diffraction experiments were performed on (NH₄I)_0.73(KI)_0.27 and provide experimental evidence for the occurence of a new phase below 63K. This ε phase has trigonal symmetry (space group R3m) and was, so far, not observed in the ammonium halides. It can be characterized by an almost undistorted NaCl lattice, with two in-equivalent NH₄ sites of the four NH₄ molecules per unit cell. The NH₄ group at the trigonal axis (the former cubic <111> axis) is almos perfectly tetrahedral. The other three NH₄ molecules are 0.18 Å off center and exhibit C_3v symmetry with the polar axis along the remaining three body diagonals. Hence, this ε phase reveals a complex dipolar order with a residual moment along the trigonal axis.     

Neutron diffraction study of weak antiferromagnetism in ytterbium orthoferrite

The neutron diffraction studies were carried out on the single crystal of ytterbium orthoferrite at room temperature. The relative value of the weak antiferromagnetic component Ay/Gx (hidden canting of the iron spins) was found to be 1.59(7)· 10^?2 in agreement with the theory.     

Neutron diffraction study of the structural phase transition in SnS and SnSe

The structural phase transitions in SnS and SnSe have been investigated by neutron diffraction in the temperature range 295–1000 K using a high temperature furnace. Accurate positional and thermal parameters have been obtained as a function of temperature both in the α- (GeS-type, B16) and β-phase (TlI-type, B33). These investigations indicate that the α → β phase transitions in SnS and SnSe are of second order displacive type, and consist mainly of the continuous movement of Sn and S/Se atoms almost entirely along the [100] direction, and suggest the soft-mode behaviour of a zone-boundary phonon of the β-phase.