Hk0 Planes Research Articles

Ultralow thermal conductivity of turbostratically disordered MoSe2 ultra-thin films and implications for heterostructures

Films containing 8, 16, 24, 32 and 64 MoSe2 layers were synthesized using the modulated elemental reactants method. X-ray reflectivity patterns showed that the annealed films were the targeted number of MoSe2 layers thick with atomically smooth interfaces. In-plane x-ray diffraction (XRD) scans contained only hk0 reflections for crystalline MoSe2 monolayers. Specular XRD patterns contained only 00l reflections, also indicating that the hk0 plane of the MoSe2 layers are parallel to the substrate. Both XRD and electron microscopy techniques indicated that the hk0 planes are rotationally disordered with respect to one another, with all orientations equally probable for large areas. The rotational disorder between MoSe2 layers is present even when analyzed spots are within 10 nm of one another. Cross-plane thermal conductivities of 0.07–0.09 W m−1 K−1 were measured by time domain thermoreflectance, with the thinnest films exhibiting the lowest conductivity. The structural analysis suggests that the ultralow thermal conductivity is a consequence of rotational disorder, which increases the separation between MoSe2 layers. The bonding environment of the Se atoms also becomes significantly distorted from C3v symmetry due to the rotational disorder between layers. This structural disorder efficiently reduces the group velocity of the transverse phonon modes but not that of longitudinal modes. Since rotational disorder between adjacent layers in heterostructures is expected if the constituents have incommensurate lattices, this study indicates that these heterostructures will have very low cross-plane thermal conductivity.

Anisotropic inplane spin correlation in the parent and Co-doped BaFe2As2: A neutron scattering study

Antiferromagnetic spin fluctuations were investigated in the normal states of the parent (x=0), under-doped (x=0.04) and optimally-doped (x=0.06) Ba(Fe1-xCox)2As2 single crystals using inelastic neutron scattering technique. For all the doping levels, quasi-two-dimensional antiferromagnetic fluctuations were observed as a broad peak localized at Q=(1/2,1/2,l). At lower energies, the peak shows an apparent anisotropy in the hk0 plane; longitudinal peak widths are considerably smaller than transverse widths. The anisotropy is larger for the higher doping level. These results are consistent with the random phase approximation (RPA) calculations taking account of the orbital character of the electronic bands, confirming that the anisotropic nature of the spin fluctuations in the normal states is mostly dominated by the nesting of Fermi surfaces. On the other hand, the quasi-two-dimensional spin correlations grow much rapidly for decreasing temperature in the x=0 parent compound, compared to that expected for nearly antiferromagnetic metals. This may be another sign of the unconventional nature of the antiferromagnetic transition in BaFe2As2.

Diffuse Diffraction from Parallel/Antiparallel Metallocene Pillars

X-ray diffraction analyses of dimethyl ansa-metallocenes (CH3)2C(C5H4)(C29H36)M(CH3)2 (M = Zr or Hf) have revealed diffuse diffraction bands in the 0kl and h0l single-crystal reciprocal lattice images, but not in the hk0 plane. This is consistent with metallic disorder in two dimensions and constitutes a rare example of diffuse diffraction for an organometallic compound. The metal is apparently partitioned between two sites with a 60:40 occupancy ratio. Structural interpretation of the X-ray data is consistent with a 60:40 ratio of parallel/antiparallel (+z, −z) metallocene pillars dispersed in the x−y directions. Diffuse diffractionalbeit weakeris also observed for the half-metallocene (C29H37)Mn(CO)3, which occupies parallel/antiparallel pillars in an 80:20 ratio.

Neutron scattering investigation of the spin ice state inDy2Ti2O7

Dy2Ti2O7 has been advanced as an ideal spin ice. We present a neutron scattering investigation of a sample of 162Dy2Ti2O7. The scattering intensity has been mapped in zero applied field in the hhl and hk0 planes at temperatures between 0.05 K and 20 K. The measured diffuse scattering (in the static approximation) has been compared to that predicted by the dipolar spin ice model. The comparison is good, except at the Brillouin zone boundaries where extra scattering appears in the experimental data. It is concluded that the dipolar spin ice model provides a successful basis for understanding Dy2Ti2O7, but that there are issues which remain to be clarified.

Molecular dynamics study and normal mode analysis of diffuse scattering in quartz

X-ray scattering intensities in quartz were investigated in molecular dynamics (MD) simulation at different temperatures with the aid of normal mode analyses. The MD-simulated structure produced diffuse streaks most remarkably along a hexagon joining symmetry-equivalent points of index 400 in the hk0 plane, extending in ± c*. In addition, most Bragg spots were found to be associated by radial diffuse scattering in the six a* directions. The normal mode analysis and calculations of first-order scattering intensities showed that the lowest-lying branch in Γ-M is of transversal acoustic characters, and is responsible for the diffuse scattering radiating from Bragg reflections in the a*-directions. The transversal acoustic characters of atomic displacements in these modes provide the scattering in phase at points on the fourth hexagon. Phonon dispersion relations of the MD crystal were examined in view of symmetry-adapted atomic displacement patterns, and a possibility was suggested for two curves in Γ-M, optic soft branch and transversal acoustic branch polarized in the z axis (the second-lowest branch), to exchange the eigenvectors in the middle range of q (0.2–0.3).

Verneuil corundum: an integrated PBC and white beam synchrotron radiation X-ray topography analysis

In order to give a deeper insight into the cracking of the Verneuil corundum boules, several samples of different colors and from different industrial sources have been analyzed by means of white beam synchrotron radiation X-ray diffraction topography (WB-SR-XRDT) and PBC analysis (Hartman and Perdock theory). The cracking occurs along an irregular surface containing the elongation axis of boules, and the WB-SR-XRDT analysis shows that this surface is nearly parallel to the { 1 2 ̄ 0 } crystallographic planes. Following Hartman and Perdock theory, these planes form F-planes with a high density of PBC directions: 3 PBC directions—[2 1 1], [2 1 0] and [ 4 ̄ 2 ̄ 1 ]—have been recognized as parallel to the 1 2 ̄ 0 plane. The 1 ̄ 02 planes that are normal to the cracking surface of boules are F-planes too, but they show a lower bond density per unit area with respect to the 1 2 ̄ 0 planes. Ab initio calculations show that these planes have lower surface energies with respect to other hk0 planes, such as {1 0 0} and {0 1 0}.

Diffuse X-ray Scattering from Freely Suspended Strands of a Discotic Liquid Crystal

We present the first quantitative measurements of diffuse scattering in the vicinity of the Bragg $ \{ 100 \} $ peaks of a hexagonal discotic liquid crystal. The scattering takes the form of a toroid lying in the HK0 plane. The cross-section of the diffuse scattering in the H0L plane is an ellipse elongated along the L direction, in reasonable agreement with calculations for the thermal diffuse scattering. The cross section in the HK0 plane has an unexpectedly smooth six-fold sinusoidal modulation.

Extreme covalence in the Tc–Cl bond from polarized neutron diffraction

A polarized neutron diffraction experiment on a salt containing the 4d1 TcNCl4− ion (S=1/2) yielded the flipping ratios of six reflections confined to the hk0 plane. Using nuclear structure factors, FN, extrapolated from the 123 and 295 K x-ray structures, magnetic structure factors, FM, were obtained from the flipping ratios. The values of FM were fitted to a model of spin populations in the Tc-4d and the Cl-3(sp) orbitals. It was found that 46(5)% of the spin density is located on the Cl atoms, corresponding to exceptionally high covalence in the Tc–Cl bonding. The amount is distinctly higher than found indirectly by ESR measurements, but is to some extent supported by a local density functional DV–Xα calculation. Because the data is limited to the one projection, it is not possible to separate the Tc and N spin density contributions.

Solution-grown crystals of isotactic polystyrene: neutron scattering investigation of the variation of chain trajectory with crystallization temperature

Previous neutron scattering investigations carried out on isotactic polystyrene single crystals grown at 130°C from dilute solutions have revealed a sheet-like conformation and hence very regular chain folding. Here we report the same type of experiments performed on samples crystallized at T c = 75, 85, 105 and 115°C. From the results in both the Guinier and the intermediate ranges, we come to the following conclusions: (i) At 115°C, we reproduce the results obtained at 130°C (regular folding in a single sheet). (ii) Decreasing T c leads to a change of conformation for relatively high molecular weights (7 × 10 5 and 5 × 10 5). While crystallization still occurs along an hk0 plane, the regular folding is interrupted, leading both to superfolding and to crystallization in at least two different lamellae. (iii) For lower molecular weights (3 × 10 5 and 1.5 × 10 5) significant deviations from the sheet-like model appear only at T c = 85°C. We conclude that above a critical molecular weight, which is temperature-dependent, irregularities in the folding habit take place. In this respect, polystyrene resembles polyethylene.

Structural control of young basaltic fissure eruptions in the plateau basalt area of the Arabian Plate, northeastern Jordan

Abstract In the El Hamadieh region, part of the large intracontinental basalt plateau extending NW-SE from Syria to Jordan and Saudi-Arabia in the northern part of the Arabian Plate, young volcanic edifices formed by fissure eruptions have been investigated; all geometrical dimensions and parameters of the ridges have been measured, the field relationships documented, the data evaluated and the structural patterns analysed. Using the attitude of the fissures as palaeostress indicators, a stress-strain-model is introduced, showing the superimposition of stress fields. Magma was extruded at the intersection of two regional structures: the NE-SW-striking Amman-Hallabat fold belt and the NW-SE-trending Azraq-Sirhan graben. A first effusion with a relatively fluid magma took place in reactivated fold-related conjugated hk 0-shear fractures with a compressional σ 1 -stress trending NW-SE. A second phase of fissure effusions produced a more viscous lava. These fissures represent (a) hk 0-planes with an acute angle about the fold axis, and (b) planes of normal faults along the southwestern shoulder of the Azraq-Sirhan graben in the position of fold-related reactivated conjugated 0 hl -shear planes. The geometrical relationships between the Amman-Hallabat folds, the Azraq-Sirham faults and the volcanic fissures indicate a rotation of the local stress field with the originally NW-SE-trending compressional σ 1 - and the NE-SW-trending, σ 3 -stress changing into a NE-SW-acting tensional σ 1 -stress.

Delindeite titanosilicates and lourenswalsite, two new from the Magnet Cove region, Arkansas

AbstractDelindeite and lourenswalsite are two new barium titanosilicate minerals found as microscopic crystals in miarolitic cavities in nepheline syenite in the Diamond Jo quarry, Hot Spring County, Arkansas. Delindeite is found as aggregates of flake-like crystallites in compact spherules, light pinkish grey in colour, with a resinous, pearly lustre. The flakes are biaxial positive with average n ∼ 1.813; the measured density is 3.3 g/cm3. Electron diffraction revealed a monoclinic unit cell in space group C2/m or subgroup, with a = 21.617(13), b = 6.816(5), c = 5.383(3) Å, β = 94.03(5)° (refined from X-ray powder data). The strongest X-ray lines are (hkl, dobs, Irel): (200, 10.80, 100); (311, 3.54, 24); (6̄01, 3.083, 28); (601, 2.888, 31); (2̄21, 2.806, 20); (910, 2.262,18). The crystals are submicroscopically twinned on (100) and also produce additional continuous diffraction streaks parallel to a*, which double the b and c axes. The formula derived from electron and ion probe analyses (H2O by difference), as constrained by density and molar volume data, is approximately (Na,K)2.7(Ba,Ca)4(Ti,Fe,Al)6Si8O26(OH)14, with Na > K, Ba ≫ Ca, Ti ≫ Fe,Al; Z = 1. Lourenswalsite occurs as very thin hexagonal plates in rosettes, silver grey to light brownish grey in colour. The crystals are biaxial negative with very low 2V angle. Indices of refraction are nα = 1.815, nβ ≈ nγ = 1.840; the measured density is 3.17 g/cm = 1.840; the measured density is 3.17 g/cm3. X-ray and electron diffraction show a sharp pseudohexagonal lattice with a = 5.244 Å, but extremely diffuse diffraction streaks normal to the hk0 plane. In these streaks a period of 20.5 Å can be discerned. A hexagonal unit cell with a = 5.244(2) Å, c = 20.49(3) Å can be refined from the powder diffraction data but does not account for some lines, probably because of extreme layer disorder as shown by precession single-crystal patterns. The strong X-ray powder lines are (002, 10.22, 20); (-, 3.93, 20); (111, 2.608, 100); (300, 1.5145, 80); (220, 1.3111, 25). The formula given by microprobe analyses, constrained by density and molar volume data, is approximately (K,Ba)2(Ti,Mg,Ca,Fe)4(Si,Al,Fe)6O14(OH)12 with K > Ba, Ti ≫ (Mg,Ca,Fe), Si > Al > Fe; Z = 1. These minerals are formed under oxidizing weathering conditions, and iron is assumed to be in the Fe3+ state.

Methode d'etude des reflexions hkl de monocristaux de grande taille a l'aide d'un diffractometre a compteur

Resume This method is the generalization of the application of an experimental technique allowing the study of the zero layer of the reciprocal lattice of large size single crystals set up in our laboratory [1]. It is mainly designed for the study of single crystal samples whose one axis is easily distinguishible. It was, up to now, employed for the study of graphite and its intercalation compounds [2–7]. This method allows one, with 3 rotations around two perpendicular axes, to bring all the hkl reflexions of a single crystal into the goniometer plane. Then, one can determine, for each reflexion, defined by the three angles θ, γ and α (Figs. 1–4): • —the reticular distance (θ). • —the angular shift, in the hkl plane, with respect to a given reference (γy), • —the angular shift of the hkl planes with respect to the corresponding hk0 plane (α). • —the multiplicity factor, • —the intensity. The knowledge of these values allows one to construct the whole reciprocal lattice of the crystal and to assign its space group. As as example, the 210 reflexions of graphite are shown in Fig. 5. In addition, the method can be applied to imperfect crystals (polysynthetic character) whereas a study with an automatic diffractometer can be disturbed by the systematic twining of the crystal. The experimental device is shown in Fig. 6. Our method was applied to the scanning along the hk rods of lamellar compounds[8].

The crystal structure and one-dimensional disorder of the orange modification of HgI2

The metastable orange modification of HgI2, described earlier by several authors, was obtained by recrystallization of HgI2 from 2-chloroethanol in the form of square, platelike crystals ranging in color from red-orange to yellow-orange. The x-ray photographs of the yellow-orange crystals indicated one-dimensional disorder along c, whereas the reflections from the red-orange form were sharp. The Friedel symmetry is 4/mmm and the lattice constants are twice those of the red form, i.e., a = 8.776, c = 24.723 Å, c/a≃;2√2. Both the hk0 and hhl reciprocal-net planes are identical with the hk0 plane of the red form and there are many systematic extinctions. The structure was solved from these observations. The structural motif is a Hg4I10 group, consisting of four corner-linked HgI4 tetrahedra in a tetrahedral arrangement. These groups are connected into HgI2 layers which are stacked with I–I contacts in the same way as the HgI4 tetrahedra in the red form. In contrast to the red form, there exist two kinds of stacking, leading to a cubic close packing of the iodine atoms. One results in a four-layer structure (red-orange crystals), the other in a two-layer structure. The disorder of the yellow-orange crystals is caused by an irregular succession of the two kinds of stacking. The intensity of the diffuse scattering arising from the disorder was calculated, and is in qualitative agreement with the observations.

Approximate Elastic Spectrum of Acoustic Waves in AgCl from X-Ray Scattering

The temperature diffuse scattering of x-rays from a large single crystal of AgCl has been measured at room temperature using a Geiger counter spectrometer and crystal monochromated CuK-α radiation. Measurements were made throughout the hk0 plane in reciprocal space, and in certain other regions. The data are analyzed on the basis of the theory developed by Laval and James, in which temperature vibrations are expressed in terms of traveling elastic waves. An assumed set of vibrational frequencies is used for the optical branch. Dispersion curves for waves in the acoustical branch traveling in the [100], [110], and [111] directions are obtained. The velocities at infinite wavelength are shown to agree fairly well with those given by the static elastic constants. An approximate frequency spectrum is then calculated, and suggestions are made as to how the effects of the optical and acoustical branches may be separated.