Powder pattern

Powder diffraction studies with neutrons are perfonned both at nuclear reactors and at spallation sources. In both cases a cylindrical sample is observed by multiple detectors or, in some cases, by a curved, position-sensitive detector. In a powder diffractometer at a reactor, collimators and detectors at many different 20 angles are scaimed over small angular ranges to fill in the pattern. At a spallation source, pulses of neutrons of different wavelengdis strike the sample at different times and detectors at different angles see the entire powder pattern, also at different times. These slightly displaced patterns are then time focused , either by electronic hardware or by software in the subsequent data analysis. 

MAS Si speetnim of a sample of sodium disilieate (Na Si O,) erystallized from a glass is shown as an example. Whilst the statie speetnim elearly indieates an axial ehemieal shift powder pattern, it gives no evidenee of more than one silieon site. The MAS speetnim elearly shows four resolved lines from the different polymorphs present in die material whose widths are 100 times less than the ehemieal shift anisotropy. 

Physical background. MAS will narrow the inliomogeneously broadened satellite transitions to give a series of sharp sidebands whose intensity envelopes closely follow the static powder pattern so that the quadnipole interaction can be deduced. The work of Samoson [25] gave real impetus to satellite transition spectroscopy by showing that both the second-order quadnipolar linewidths and isotropic shifts are fiinctions of / and Some combinations of / and produce smaller second-order quadnipolar effects on the satellite lines than... 

Treacy M M J, Fliggins J B and von Ballmoos R 1996 Collection of Simulated XRD Powder Patterns for Zeolites 3rd revised edn (London Elsevier)... 

Fig. 15. A typical powder pattern with three phases Calcite (—), Aragonite and Brucite ( ). The lines below the peaks are the powder lines...

Sea.rch-Ma.tch. The computer identifies which crystalline phases (components) match the unknown pattern by using a file of known powder patterns maintained by the International Center for Diffraction Data (ICDD). The Powder Diffraction File contains interplanar t5 -spacings d = A/(2sin0)] and intensities of the diffraction maxima for each crystalline powder pattern submitted to the ICDD. Currendy there are about 65,000 patterns in the file. Current search—match programs can successfully identify up to seven components in an unknown pattern. A typical diffraction pattern of an unknown sample and the components identified by the computer search-match program is shown in Figure 15. 

Indexings and Lattice Parameter Determination. From a powder pattern of a single component it is possible to determine the indices of many reflections. From this information and the 20-values for the reflections, it is possible to determine the unit cell parameters. As with single crystals this information can then be used to identify the material by searching the NIST Crystal Data File (see "SmaU Molecule Single Stmcture Determination" above). 

Structure Determination from a Powder Pattern. In many cases it is possible to determine atomic positions and atomic displacement parameters from a powder pattern. The method is called the Rietveld method. Single-crystal stmcture deterrnination gives better results, but in many situations where it is impossible to obtain a suitable single crystal, the Rietveld method can produce adequate atomic and molecular stmctures from a powder pattern. 

Position Sensitive Detectors. By replacing the scintillation detector in a conventional powder diffractometer with a Position Sensitive Detector (PSD), it is possible to speed data collection. For each x-ray photon received a PSD records the angle at which it was detected. Typically, a conventional scintillation detector records x-ray photons in a range of a few hundredths of a degree at a time. A PSD can measure many degrees (in 20) of a powder pattern simultaneously. Thus, for small samples, data collection, which could require hours with a conventional detector, could take minutes or even seconds with a PSD. 

The crystal group or Bravais lattice of an unknown crystalline material can also be obtained using SAD. This is achieved easily with polycrystalline specimens, employing the same powder pattern indexing procedures as are used in X-ray diffraction. ... 

This compound has two crystallographically distinct vanadium sites. While the static spectrum is a superposition of two powder patterns of the kind shown in Figure 3, MAS leads to well-resolved sharp resonances. Weak peaks denoted by asterisks are spinning sidebands due to the quadrupolar interaction. 

IF7 has been shown to act as a weak Lewis acid towards CsF and NOF, and the compounds CsIFg and NOIFg have been characterized by X-ray powder patterns and by Raman spectroscopy they are believed to contain the IFg anion. 

Pulver-band, m. (Expl.) powder strand, -biatt-chen. n, (Expl.) powder flake, powder grain, -brennztlndung, /. powder-train ignition, -dampf, in. powder smoke, -dlagramm, n. powder pattern, -fabrik, /. powder factory, -fabrikation, /. powder manufacture, -fass, n. powder cask, powder keg. -fiascbe. /. powder bottle (wide-mouthed bottle), pulverfdrmig, a. in the form of powder, powdery, pulverulent. 

A single triplet has three resonant fields, two due to Amv= l transitions and one due to Ams 2 transitions. For amorphous or polycrystalline samples, two triplet powder patterns are formed due to contributions from all possible orientations of triplets with respect to the applied field. The full-field triplet powder pattern due to Ahia-= 1 transitions is centered about If and has the following critical points ... 

The half-field triplet powder pattern due to Anis- 2 transitions has a... 

Figure 15-6. (a) Phoioiuduccd absorption-detected magnetic resonance (ADMR) spectrum of MEH-PPV. HF and FF represents tire half field and full field powder pattern for the triplet (S=l) resonance, respectively, (b) ADMR spectrum ol MEH-PPV/CW, composite film. Both spectra were measured at probe energy 1.35 eV, T=4 K and 3 GHz resonant microwave frequency (reproduced by permission of the American Physical Society from Ref. 1191). 

A reasonable interpretation can be given to the curve of Fig. 2, with its two discontinuities in slope namely, that there exist ordered phases PbTls and PbTl,. Direct evidence from the intensities of X-ray reflections is not obtained for ordering in this case, because of the approximate equality in / values of lead and thallium. We can, however, discuss the probable structures of the ordered phases. The powder patterns given by these alloys show no splitting of lines. We estimate that the... 

X-rays. The diffraction from this polycrystalline and disoriented fiber is the sum of the diffraction from all the randomly oriented microcrystallites, and it corresponds to a series of concentric rings, each with its characteristic (/-spacing. The intensity is uniform on a ring, but it varies among rings. This type of diffraction, commonly referred to as a powder pattern, is prevalent among minerals and polymers that have a low degree of polymerization. 

The sulfide bromide AgaSBr may be obtained by annealing of stoichiometric amounts of Ag S and AgBr in closed, glass ampoules at 280°C. The reaction product is ground, and repeatedly treated in the same way (317) the end of the reaction is determined by powder patterns. In a similar way, the low-temperature modification of AgsSI,... 

Preliminary X-ray work on CrSBr crystals, probably formed via a chemical-transport reaction, shows rhombic symmetry (181). Powder patterns of the iodides can be indexed with respect to a hexagonal lattice, and are similar to those of Cris (17, 181). 

Batsanov et al. 23) reacted sulfur with PtCU and PtBr2 by heating mixtures of the reactants in evacuated, sealed ampoules. At 100 -200°C after 12-24 h, sulfide chlorides PtCljS (1.70 < x < 2 0.6 s y < 3.35) and sulfide bromides PtBr S (1.87 < x 2.06 0.84 y s 1.80) were formed. The compositions depended on the initial PtX2 S ratio, and the temperature. At 320-350°C, loss of chlorine led to the compounds PtClS (1.7 y 1.9). According to their X-ray powder patterns, all of these products retained the main structural features of the original platinum halides. From considerations of molar volumes, the authors deduced the presence of polysulfide anions. 

Structural data, except powder patterns, for the other compounds are not known. 

Table XXI shows the crystallographic data as far as they are known. The structures of the three sulfide iodides have been determined by single-crystal studies. Powder patterns of the 8n(IV) compounds not given here have been reported elsewhere (24).

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