Powder diffraction phase identification

A room temperature powder X-ray diffraction pattern for Na8[GaSi04]6(C104)2 Sodalite with Si as an internal standard for phase identification in the reaction products has been studied. X-ray powder diffraction study confirms the cubic structure of Na8[GaSi04]6(C104)2 sodalite synthesized... 

The literature abounds with countless examples that illustrate how powder diffraction has been used to distinguish between the members of a polymorphic system. It is absolutely safe to state that one could not publish the results of a phase characterization study without the inclusion of XRPD data. For example, Fig. 7.11 shows the clearly distinguishable XRPD powder patterns of two anhydrous forms of a new chemical entity. These are easily distinguishable on the overall basis of their powder patterns, and one could place the identification on a more quantitative basis through the development of criteria similar to those developed for the mandelic acid system. 

Recent developments and prospects of these methods have been discussed in a chapter by Schneider et al. (2001). It was underlined that these methods are widely applied for the characterization of crystalline materials (phase identification, quantitative analysis, determination of structure imperfections, crystal structure determination and analysis of 3D microstructural properties). Phase identification was traditionally based on a comparison of observed data with interplanar spacings and relative intensities (d and T) listed for crystalline materials. More recent search-match procedures, based on digitized patterns, and Powder Diffraction File (International Centre for Diffraction Data, USA.) containing powder data for hundreds of thousands substances may result in a fast efficient qualitative analysis. The determination of the amounts of different phases present in a multi-component sample (quantitative analysis) is based on the so-called Rietveld method. Procedures for pattern indexing, structure solution and refinement of structure model are based on the same method. 

XRD Characterization The powder x-ray diffraction of the mechano-chemically milled complex borohydride has been carried out by the Philips X pert diffractometer with Cu-Koi radiation of X= 5.4060 A. The incident and diffraction slit width used for the measurements are 1° and 2° respectively. The sample holder was covered with Polyethylene tape (foil) with an O-ring seal in an N2 filled glove box in order to avoid or at least minimize the 02/moisture pickup during the XRD measurements. The diffraction from the tape was calibrated without the actual sample and found to be occurring at 29 angles of 22° and 24°, respectively. The XRD phase identification and particle size calculation has been carried out using PANalytical X pert Highscore software, version l.Of. 

One application of powder diffraction is phase identification. Since zeolites of the same structure type give similar powder patterns, the powder pattern can be used as a fingerprint to identify the zeolite type. Furthermore, when multiple phases are present, the powder pattern is a superposition of the patterns for each of the separate phases and the relative overall intensities of the peaks is related to the amount of each phase. Thus patterns from mixtures of phases can be analyzed to determine the identity and relative amount of each phase. 

The identification of the solid phases and the evaluation of their crystallinity were performed by X-ray powder diffraction while the pore volume of the ZSM-48 materials was evaluated by isothermal (90 °C) sorption of n-hexane, followed in the thermobalance (Stanton Redcroft ST-780 combined TG-DTA-DTG thermoanalyser). 

Besides structure, X-ray diffraction gives a host of other valuable information. For example, powder diffraction, especially with counter diffractometers, has been widely used for phase identification, quantitative analysis of a mixture of phases, particle size analysis, characterization of physical imperfections (the last two being obtainable from line broadening) and in situ studies of reactions. In the case of amorphous solids, X-ray... 

The primary source of x-ray crystal diffract-tion reference data is the above-mentioned ASTM Powder Diffraction File , published by the Joint Committee on Powder Diffraction Standards. This file consists of over 38000 diffraction patterns of crystalline materials including expls and related materials. Scientists in the expls field routinely utilize this source. for the identification of expls and metastable phases by comparing the interplanar d spacings and intensities of exptl phases with those of known phases (Refs 4,10,21 22)... 

Crystallization was followed by analyzing the solid product quantitatively by x-ray powder diffraction. Prepared mixtures of a standard sample of mordenite and the amorphous substrate of mordenite composition were used to establish a calibration curve for the quantity of mordenite based on the summation of x-ray peak intensities. For zeolites A and X, the unreacted aluminosilicate gel was used to prepare mixtures with standard samples of zeolites A and X for quantitative phase identification. 

The Collection is a source of reference patterns for pure crystalline phases. The data may be helpful in identifying known zeolitic materials and indexing their diffraction patterns. Because so many factors related to both the zeolite crystal and the diffraction instrument affect powder diffraction data, phase identification is not always straightforward and frequently requires additional data. Considerable care should be exercised in comparing calculated diffraction patterns to experimental patterns. For example, the use of fixed versus variable incident slits on a powder diffractometer can drastically change the relative intensities of a diffraction pattern, and it should be emphasized that calculated patterns are only as accurate as the structure refinements on which they are based. 

X-ray diffraction investigation (Co-Ka radiation) was made on the powder samples for the phase identification both the parent compound and its hydride and to determine the unit cell parameters. The magnetisation measurements were carried out with a SQUID (Quantum Design MPMS 5-S) magnetometer from 5 to 340 K in magnetic fields up to 50 kOe. 

X-ray powder diffraction investigations were made in cooperation with P. Norby (Oslo University) and I.G. and E. Krogh Andersen (Odense University). For phase identification and lattice costants determination a Guinier-Hagg camera has been used (CuKa =1.5451 (10) quartz internal standard a=4.91309 A, c=5.40426 A (11). The diagrams were indexed and lattice constants refined by least square... 

The synthesized samples were analyzed by X-ray powder diffraction for qualitative and quantitative phase identification. The unit used was a Philips Model with a vertical goniometer and a scintillation counter, utilizing Ni-filtered CuK radiation. For quantitative phase identification an external standard sample of ot-A O, was used. The percentage crystallization was calculated using thez "averaged peak intensities at 20 =35.2° and 20=47.3° of the reference sample and the peak intensity at 20=23.2° for the sample under study (31). 

The identification of the solid phases and the determination of their crystallinities were carried out by X-ray powder diffraction (XRD), using a Philips PW 1349/30 X-ray diffractometer (Cu-KOt radiation). The crystallinity of each sample was evaluated by using as standard the most crystalline as-synthesized ZSM-48 from which the residual amorphous phase was further removed by ultrasonic treatment (15.). Alkali and A1 contents... 

The more detailed description of the non-conventional symmetry goes beyond the scope of this book as it has little use in powder diffraction, because even the three-dimensional diffraction from aperiodic crystals is quite complex. When the diffraction picture is projected along one dimension, its treatment becomes too complicated and the crystal structure of aperiodic crystals is rarely, if ever, completely studied by means of powder diffraction techniques beyond simple phase identification. Nevertheless, this section has been included here for completeness, and to give the reader a flavor of recent developments in crystallography. ... 

Figure 4.1. The flowchart illustrating common steps employed in a structural characterization of materials by using the powder diffraction method. It always begins with the sample preparation as a starting point, followed by a properly executed experiment both are considered in Chapter 3. Preliminary data processing and profile fitting are discussed in this chapter in addition to common issues related to phase identification and analysis. Unit cell determination, crystal structure solution and refinement are the subjects of Chapters 5,6, and 7, respectively. The flowchart shows the most typical applications for the three types of experiments, although any or all of the data processing steps may be applied to fast, overnight and weekend experiments when justified by their quality and characterization goals.

Each powder diffraction pattern is characterized by a unique distribution of both positions and intensities of Bragg peaks, where peak positions are defined by the unit cell dimensions and reflection intensities are established by the distribution of atoms in the unit cell of every crystalline phase present in the sample (see Table 2.7 in Chapter 2). Thus, every individual crystalline compound has its own fingerprint , which enables the utilization of powder diffraction data in phase identification. ... 

A digitized representation of powder data is quite compact and is especially convenient for comparison with other patterns, provided a suitable database is available. In addition to a digitized pattern, each entry in such a database may (and usually does) contain symmetry, imit cell dimensions, and other useful information phase name, chemical composition, references, basic physical and chemical properties, etc. Powder diffraction databases find substantial use in both simple identification of compounds (qualitative analysis) and in quantitative determination of the amounts of crystalline phases present in a mixture (quantitative analysis). 

Phase identification using powder diffraction data requires a comparison of several key features present in its digitized pattern with known compounds/phases. This is usually achieved by searching powder diffraction database(s) for records, which match experimentally measured and digitized pattern. Thus, a powder diffraction database or at least its subset should be available in addition to a suitable search-and-match algorithm. 

Recently the ICDD Powder Diffraction File underwent a substantial and useful upgrade calculated patterns based on single crystal data from the ICSD file have been included into the PDF-2/PDF-4 Full File calculated patterns of structures stored in the CSD file, have been included into the PDF-4 Organics (see Table 4.3). These additions make it possible to conduct searches and find matches with computed digitized powder patterns in addition to experimentally measured powder diffraction data, thus improving automation, simplifying phase identification process and considerably expanding the applicability of the powder method for a qualitative phase analysis. 

---