Neutron diffraction data

An important modern tool for the direct observation of spin configurations is neutron diffraction. Because the neutron is a neutral particle that carries a magnetic moment, it is primarily scattered by only the atomic nucleus and electrons with unpaired spins. Halpern and Johnson (247) have shown that the differential scattering cross section of an atom, including both nuclear and magnetic scattering, is 

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In general, anions are less strongly hydrated than cations, but recent neutron diffraction data have indicated that even around the halide ions there is a well defined primary hydration shell of water molecules, which, in... 

The two major databases containing information obtained from X-ray structure analysis of small molecules are the Cambridge Structural Database (CSD) [25] and the Inorganic Crystal Structure Database (ICSD) [26] both are available as in-house versions. CSD provides access to organic and organometallic structures (mainly X-ray structures, with some structures from neutron diffraction), data which are mostly unpublished. The ICSD contains inorganic structures. 

Figure 4 Plot of time-resolved decomposition of titanium-enriched slags as extracted from neutron diffraction data collected at 1000° C.

Figure 6 Site occupancies for two of the oxygen atoms (01 and 05) in the YBa2Cu307 x superconductor as a function of temperature. The site occupancies resulted from an analysis of insitu neutron diffraction data. Reprinted by permission from Jorgensen and Hinks. ...

Figure 3.2 The geometry of iner-/ran.v-[W(CO)3-(ij--H2)(Pf 3)2l from X-ray and neutron diffraction data r(H-H) 84pm (compared with 74.14 pm for free H2), I(W-H) 175 pm. Infrared vibration spectroscopy gives v(H-H) 2690cm compared with 4159cm (Raman) for free Hj.

Pandya et al. have used extended X-ray ascription fine structure (EXAFS) to study both cathodically deposited -Ni(OH)2 and chemically prepared / -Ni(OH)2 [44], Measurements were done at both 77 and 297 K. The results for / -Ni(OH)2 are in agreement with the neutron diffraction data [22]. In the case of -Ni(OH)2 they found a contraction in the first Ni-Ni bond distance in the basal plane. The value was 3.13A for / -Ni(OH)2 and 3.08A for a-Ni(OH)2. The fact that a similar significant contraction of 0.05A was seen at both 77 and 297K when using two reference compounds (NiO and / -Ni(OH)2) led them to conclude that the contraction was a real effect and not an artifact due to structural disorder. They speculate that the contraction may be due to hydrogen bonding of OH groups in the brucite planes with intercalated water molecules. These ex-situ results on a - Ni(OH)2 were compared with in-situ results in I mol L"1 KOH. In the ex-situ experiments the a - Ni(OH)2 was prepared electrochemi-cally, washed with water and dried in vac-... 

Two-channel MaxEnt techniques have also been used in the study of magnetization and spin densities [34, 35] and to interpret unpolarised neutron diffraction data [36]. 

Papoular, R.J. and Gillon, B. (1990) Maximum entropy reconstruction of spin density maps in crystals from polarized neutron diffraction data, Europhys. Lett., 13(5), 429 134. 

Sakata, M., Uno, T., Takata, M. and Howard, C. (1993) Maximum-entropy-method analysis of neutron diffraction data, J. Appl. Cryst., 26, 159-165. 

The first and very fundamental question we had to address was how many solvent molecules coordinate to a metal ion. In the case of the Be2"1" cation, the coordination of four water molecules to form [Be(H20)4]2+ (at pH<3) is corroborated based on NMR (62-68), X-ray (69-74), or even neutron diffraction data (75). In parallel, these observations are also made by different types of computer-based simulations (76-79). In the case of Li+ one can find different values in the literature. While most X-ray structures demonstrate the existence of [Li(H20)4] + (80-82), [Li(H20)5]+ (83), and [Li(H20)6]+ (84) are also found. Even if one is doubtful and sceptical from a modern crystallographic point of view, e.g., [Li(H20)5]+ and [Li(H20)6]+ were studied at room temperature, we need to clarify the coordination number before the water exchange mechanism can be investigated. 

X-Ray/Neutron Diffraction Data for Hydrido-Bridged Complexes... 

Lengyel and Kalman have reported an electron diffraction study of water 20>. The high energy electrons used permit the structure function to be probed at (large) values of s unattainable in either X-ray or neutron diffraction. This feature is valuable in that the wider the range of s for which data are available, the more accurate is the inversion of the observed h(s) to the direct space function h(R). Typical data, for D2O at 25 °C, are shown in Fig. 5. Note the similarities, and differences, between the electron diffraction, X-ray diffraction and neutron diffraction data. 

For comparison purposes they also studied the diffraction pattern of polycrystalline ice Ih at 77 K. We shall discuss these data in detail, drawing heavily on Ref. 27>. It is convenient to begin with the case of poly crystalline ice Ih, and discuss in turn H20(as) deposited at 77 K, H20(as) deposited at 10 K, and the comparison with the neutron diffraction data. 

A very sensitive test of the model is the comparison of calculated and observed structure functions this is shown in Figs. F and G. Note that the central force model yields the characteristic double peak in sh(s) near 2.5 A-1. That the theoretical curve oscillates with greater amplitude than the experimental data indicates that the predicted distribution of near neighbor 00 separations is too narrow. The comparison with neutron diffraction data shows that the theoretical... 

G.M. Brown, M.-R. Noe-Sprilet, W.R. Busing, and H.A. Levy, Dodecatungstophosphoric Acid Hexahydrate, (HsC MPWuCLo3 ). The True Structure of Keggin s Pentahydrate from Single-Crystal X-ray and Neutron Diffraction Data, Acta Cryst., B33, 1038-1046 (1977). 

Furthermore, there is a striking parallelism between these data and the neutron diffraction data from nucleosomes in 100% D 0 (Pardon et al., 1977 Suau et al., 1977), where scattering from the histone protein dominates, and from core protein in 2 M NaCl solution (Pardon et al., 1978). The above interference phenomenon may well be the explanation for the protein-dominated scattering maximum between 35 and 37 A observed for chromatin and nucleosomes in solution (Pardon et al., 1977 Suau et al., 1977). 

A linear correlation between. obtained from X-ray and neutron-diffraction data, and 0 has been observed (Jeffrey and Yeon, 1986). The survey was based on 20 compounds having OHO hydrogen bonds with / H. . . o varying from 123.4 pm (KH malonate) to 197.9 pm (a-form of oxalic acid dihydrate). Whether based solely on those crystals for which neutron data are available, or on X-ray and neutron data, a good linear relationship is revealed, showing that the lower o more deshielded is the... 

The complexity of the solvable structure strongly depends on the spectral resolution of the diffraction method in use. Structures with about 60 atoms in the asymmetric unit were solved from powder data combining synchrotron X-ray diffraction with refinement from neutron diffraction data from the same material (Morris et al. 1994 Admans 2000). About half of that complexity can be achieved with good laboratory X-ray diffractometers (Masciocchi et al. 1996 Kariuki et al. 1999). Neutron diffraction data can better be used for structure refinement than for structure determination, for the same reason. 

Since scattering factors for neutron diffiaction are independent of the scattering angle (in contrast to the case of XRD and ED), neutron diffraction data are more frequently used for refinement than the others. 

Structure determination from X-ray and neutron diffraction data is a standard procedure. Starting with a rough model, the accurate structure is determined using a least-squares structure refinement, which is based on kinematic diffraction and in which the differences between calculated and experimental intensities are minimized. X-ray and neutron diffraction are not applicable to all crystals. To determine crystal structures of thin layers on a substrate or small precipitates in a matrix (see figure 1) only electron diffraction (ED) can lead you to the crystal structure. 

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