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Identify and Quantitate

Cross-References and Rietveld Analyses

An attribute of ICDD's quality system are the cross-references cited in the database. The cross-references are used to match entries that are chemically and structurally similar, but were determined in different laboratories. ICDD pays particular attention to structural cross-references so that we can match patterns without atomic structures to those with atomic structures. The matching criteria used by ICDD editors are specifically designed for Rietveld refinements. There are strict limits placed on chemical and structural variations. For example, most of the data shown in Figure 4 would not be cross-linked because of the variation in chemical composition or reduced cell volume, which would provide misleading results in a Rietveld refinement unless the site occupancies were varied, as well as unit cell parameters. This means that the crossreferencing system is not suitable for searching a solid solution series. However, for a single specific composition, such as dolomite at 10 atomic % Mg, we would cross link references without atomic coordinates to those with atomic coordinates, provided that there were also matches in unit cell parameters and space groups. These tight criteria are specifically used so that software developers can actively use the cross-references for both automated phase identification and Rietveld quantitation. Several ICDD software distributors are currently using this capability. Data mining software embedded with the PDF-4 databases allows searches that identify references with atomic coordinates, as well as those that are cross-referenced so that users can identify the appropriate data to use for Rietveld refinement, independent of whether they have automated software.

Coupling the phase identification process to the Rietveld analysis is always recommended. Numerous round robins have repeatedly identified several common user errors as the main reasons for incorrect results in Rietveld analyses. The phase identification process will usually identify a room temperature determination of the appropriate polymorph, (many structures are done at low temperature), and the best fit usually results in the best choice of unit cell and space group. The primary references and cross-references link the identified material to the best structure for use in quantitative analyses.

Figure 4. Increase in data entries with atomic coordinates.

New Subfiles

Periodically, members of the ICDD will request a new subfile. A task group is formed between volunteers and editorial staff to define and then populate the subfile. For Release 2011, two new subfiles will be published for the first time, Modulated Structures and Thermoelectric Materials.

Modulated Structures

PDF-4+ 2011 will introduce a new subfile on modulated structures. For the past three years, ICDD members and scientists have been working closely with world-renowned experts, Dr. Vaclav Petricek and Ing. Jan Rohlicek at the Czech Republic Institute of Physics, Dr. Evgeny Antipov, Dr. Andrei Mironov and Dr. Vladimir Morozov at Moscow State University, to develop a system for the analysis and display of modulated structures and their diffraction patterns. The system was initially based on the well-known JANA2006 program and then modified for the ICDD for the Powder Diffraction File.

Modulated structures cannot be indexed using conventional three integer indices. Analysis on such structures reveals that they deviate from the regular structures by having perturbations in their atomic position, occupation and/or thermal motion. These fluctuations can be described as a modulation wave, leading to the name modulated structures. The symmetry of the crystal structure can be described in (3+d) super space using Superspace Groups. In recent years, several of these structure types attracted various scientists on account of their interesting physical properties, in particular, superconductors and electronic materials. The number of modulated structures being published is increasing steadily, indicating the need for a database to house them and describe them properly. The ICDD is responding to this necessity of the scientific community by incorporating modulated structures in the Powder Diffraction File starting with Release 2011.

Publication of a modulated structure reference subfile has taken three years because it has several components. First, software had to be written to analyze and analytically describe modulated materials. Second, editors needed to be trained in the analyses and then start the process of extracting and analyzing the world's literature on modulated structures. Finally, modifications were made to the ICDD editorial systems so that modulated structure data can be received, edited, standardized, and processed for the PDF. Release 2011 contains 54 modulated structure data sets. Experts estimate that there are over 1,000 modulated structures today. The ICDD will continue to collect and process this data.

Figure 5. Display from the modulated structure subfile. As shown, the pattern is indexed in four dimensions h, k, l, and m1. The boxes highlight the Superspace Group and the Cell Wave Vector that describes the modulation. All peaks in the displayed pattern (top right) are indexed, which would not be possible with a conventional crystallographic system. If the modulated structure is from a single-crystal source, additional symmetry operators and atomic parameters are shown on the structure page of the display.

Microcrystalline and Amorphous Materials

The ICDD continues to add reference data for materials, such as polymers and clays that cannot be completely described by conventional crystallography, but do have reproducible diffraction patterns. These material data sets have been primarily produced by ICDD grantees or ICDD member scientists as full digital experimental patterns to help scientists identify the state of matter and crystallinity.

Figure 6. Published in Release 2011, partially crystalline (top) and amorphous patterns (bottom) for cellulose triacetate on the left and PEN (poly (ethylene -2,6 napthalate)) on the right.

Table 2. Search options, display fields and entry counts for the last six (6) releases of database products. Searches and display fields are available with ICDD's embedded data mining software. With Release 2011, every PDF database will have self-contained data mining capability.

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