Neutron refinement

Hawthorne F. C. and Grundy H. D. (1976). The crystal chemistry of the amphiboles, IV X-ray and neutron refinement of the crystal structure of tremolite. Canadian Mineral, 14 334-345. 

Figure 4.7 Molecular structure of monohydride HMn(CO)4PPh3, shown with 60% probability ellipsoids based on neutron refinement. (Reproduced with permission from ref. 14.)...

The numerical data comprises 20 values for CuKo radiation (A = 1.5418 A), d-spacings, relative intensities, hkl Miller indices and multiplicity, Mhki- Data representing 133 framework topologies have been included in this Collection. In most cases, X-ray or neutron refinements of hydrated or as-synthesized forms are used. 

An example of a system in which both cations and oxygen can vary is Ba3La3Cu6014+2/ (34). This phase was reported to exist for 0.05 < y < 0.43. The phase progressively becomes more conductive going from semiconducting to semimetallic with increasing y. A recent powder neutron refinement (35) indicates that the structure of this compound is like that of Ba2YCu307 with several small differences. First, the Y site is occupied only... 

Hanson, J. C., Sieker, L. C., Jensen, L. H. (1973). Sucrose X-ray refinement and comparison with neutron refinement, Acta Cryst, B29 797. 

Some refinement can be done from powder diffractometer data, but even then the provision of a specimen of sufficient size may be a problem. By combining the results of X-ray and neutron refinements we can increase by one the number of species that can be assigned unambigously to any given number of sites, because the species have different effects on the structure factors for the different radiations. 

In all these examples, the importance of good simulation and modeling cannot be stressed enough. A variety of methods have been used in this field to simulate the data in the cases studies described above. Blander et al. [4], for example, used a semi-empirical molecular orbital method, MNDO, to calculate the geometries of the free haloaluminate ions and used these as a basis for the modeling of the data by the RPSU model [12]. Badyal et al. [6] used reverse Monte Carlo simulations, whereas Bowron et al. [11] simulated the neutron data from [MMIM]C1 with the Empirical Potential Structure Refinement (EPSR) model [13]. 

The observation that atoms of a single element can have different masses helped scientists refine the nuclear model still further. They realized that an atomic nucleus must contain subatomic particles other than protons and proposed that it also contains electrically neutral particles called neutrons (denoted n). Because neutrons have no electric charge, their presence does not affect the nuclear charge or the number of electrons in the atom. However, they do add substantially to the mass of the nucleus, so different numbers of neutrons in a nucleus give rise to atoms of different masses, even though the atoms belong to the same element. As we can see from Table B.l, neutrons and protons are very similar apart from their charge they are jointly known as nucleons. 

The close-packed-spheron theory of nuclear structure may be described as a refinement of the shell model and the liquid-drop model in which the geometric consequences of the effectively constant volumes of nucleons (aggregated into spherons) are taken into consideration. The spherons are assigned to concentric layers (mantle, outer core, inner core, innermost core) with use of a packing equation (Eq. I), and the assignment is related to the principal quantum number of the shell model. The theory has been applied in the discussion of the sequence of subsubshells, magic numbers, the proton-neutron ratio, prolate deformation of nuclei, and symmetric and asymmetric fission. 

C22-0108. Free neutrons are unstable. Since they cannot be collected and weighed, it is difficult to measure their half-life accurately. Early estimates gave 1-1 s, but more refined experiments give t f2 — 876 21 s. Suppose a neutron source generates 10 neutrons per second for 30 seconds. Assuming no neutrons are captured by nuclei, how many will be left after 5 hours, according to each of these half-lives ... 

P21212i Z = 4 D = 1.663 R = 0.024 for 1,863 neutron intensities at 123 K. This is a neutron-diffraction, low-temperature refinement of a partially deuterated molecule. The undeuterated molecule had previously been studied by neutron diffraction at room temperature.10 The experiment was performed in order to seek evidence of favored replacement of hydrogen by deuterium at the anomeric hydroxyl group, but no evidence therefor was observed. 

As at room temperature Bragg reflections contain both nuclear and magnetic structure factors, the nuclear structure was refined from a combination of polarized and unpolarized neutron data. Contrary to the ideal structure where only three atomic sites are present, it has been shown [11, 12] that some Y atoms were substituted by pairs of cobalt. These pairs, parallel to the c-axis are responsible for a structure deformation which shrinks the cobalt hexagons surrounding the substitutions. The amount of these substituted Y was refined to be 0.046 0.008. Furthermore, the thermal vibration parameter of Coi site appeared to be very anisotropic. The nuclear structure factors Fn were calculated from this refined structure and were introduced in the polarized neutron data to get the magnetic structure factors Fu. 

In order to figure out the FWs, the nuclear structure was refined from unpolarized neutron data taken at 30 K, in the paramagnetic state, on a 4-circle diffractometer. Furthermore, a set of 248 flipping ratios was measured with polarized neutrons at 1.6 K, with the spin density long range ordered by a 4.65 T applied magnetic field. 

Positional parameters of the non-hydrogen atoms obtained from refinements I and II are in good agreement with those of SC (1980) or Dam, Harkema and Feil (hereafter DHF) [16] from X-ray data as well as those from neutron data [13, 17]. 

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