Powder diffractometers

X-ray powder diffraction studies are perfonned both with films and with counter diffractometers. The powder photograph was developed by P Debye and P Scherrer and, independently, by A W Hull. The Debye-Scherrer camera has a cylindrical specimen surrounded by a cylindrical film. In another commonly used powder... 

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. 

Fig. 15. Size data for a metal powder obtained by A, image analysis, and B, on a diffractometer.

Bragg-Brentano Powder Diffractometer. A powder diffraction experiment differs in several ways from a single-crystal diffraction experiment. The sample, instead of being a single crystal, usually consists of many small single crystals that have many different orientations. It may consist of one or more crystalline phases (components). The size of the crystaUites is usually about 1—50 p.m in diameter. The sample is usually prepared to have a fiat surface. If possible, the experimenter tries to produce a sample that has a random distribution of crystaUite orientations. 

Fig. 14. Focusing schemes in powder diffraction (a) conventional para-focusing Bragg-Brentano diffractometer (b) parallel-beam diffractometer using a...

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. 

Area Detectors. A two-dimensional or area detector attached to a powder diffractometer can gready decrease data collection time. Many diffraction appHcations require so much time with a conventional detector that they are only feasible if an area detector is attached to the iastmment. The Siemens General Area Detector Diffraction System (GADDS) uses a multiwire area detector (Fig. 17). This detector measures an x- and ajy-position for each x-ray photon detected. The appHcations foUow. 

Texture Analysis with GADDS. With a conventional detector, a data collection for a pole figure analysis with a powder diffractometer with a texture attachment could take 12 h or more. With an area detector, it is possible to collect enough data for several pole figures (required for an ODF analysis) ia a few minutes. 

A good diffractometer-spectrograph must function satisfactorily in each role, and it must be built so that the changeover from one to the other is easy, quick, and sure. Unless this is true, two instruments will often be needed where one might have sufficed. The diffractometer should handle either powder samples or single crystals. 

The crystal structures of four chlorinated derivatives of di-benzo-p-dioxin have been determined by x-ray diffraction from diffractometer data (MoKa radiation). The compounds, their formulae, cell dimensions, space groups, the number of molecules per unit cell, the crystallographic B.-factors, and the number of observed reflections are given. The dioxin crystal structures were performed to provide absolute standards for assignment of isomeric structures and have been of considerable practical use in combination with x-ray powder diffraction analysis. 

The diffractometer has gradually evolved in terms of maximum power of sealed X-ray tubes, rotating anodes, new X-ray optics, better detector efficiency, position-sensitive detection and, lately, real-time multiple-strip (RTMS) fast X-ray detection, which replaces a single detector by an integrated array of parallel detectors to provide an up to 100-fold increase in efficiency compared with traditional detectors without compromise on resolution. Time-resolved powder diffraction is... 

The X-ray powder diffraction pattern of miconazole was performed using a Simmons XRD-5000 diffractometer. Figure 1 shows the X-ray powder diffraction pattern of miconazole nitrate, which was obtained on a pure sample of the drug substance. Table 1 shows the values for the scattering angles (26 (°)), the interplanar <7-spacing (A), and the relative intensities (%) observed for the major diffraction peaks of miconazole. 

The X-ray powder diffraction pattern of niclosamide has been measured using a Philips PW-1050 diffractometer, equipped with a single-channel analyzer and using a copper Ka radiation. The pattern obtained is shown in Fig. 1, and the data of scattering angle (degrees 20) and the relative intensities (///max) are found in Table 1. 

The X-ray powder diffraction pattern of (o)-penicillamine was obtained using a Siemens XRD-5000 diffractometer, and the powder pattern is shown in Fig. 1. A summary of the crystallographic data deduced from the pattern of (r>)-penicilla-mine is located in Table 1. 

Powder X-ray diffraction patterns were measured on PANalytical X Pert PRO high-resolution diffractometer with Alphal configuration using CuKa radiation (1.5406 A) in the range from 5° to 35° 20 with the 0.017° step per 100 s using fully opened X Celeration detector. For the XRD measurements Ti-Beta precursor gels were dried at ambient temperature and than grinded to powder. 

Chemical analyses were performed by ICP at the Vemaison Center of Chemical Analysis of the CNRS. XRD patterns were obtained on a diffractometer with a copper anode. Scan was taken at 28 rate of 0.2°/min and structural data for reference compounds were taken from the ASTM X-ray powder data file. 

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