Computing X-Ray Powder Diffraction Intensities and Bragg Angles Using a Microcomputer

Programs exist to compute X-ray diffraction patterns (angles and intensities) for specified atomic arrangements using mainframe computers. We report here a microcomputer version (in Basic, to n m on a standard Apple ][ computer with two disk drives and optionally a printer). Additionally, the program offers considerable flexibility in entering atomic positions and using symmetry relations to simplify the input for complex unit cells, and provides a visual display of the unit cell in any orientation.There are a variety of search-match algorithms that compare measured powder diffraction patterns to previously determined standards, such as the JCPDS powder diffraction file. These have been implemented on computers ranging from mainframes to microcomputers, with tradeoffs in the practical size of the standards file and the speed of the search.

A Hanawalt Type Phase Identification Procedure for a Minicomputer

The use of computers to aid in the identification of phases from their powder diffraction patterns was pioneered in the mid 1960's by Frevel, Nichols and Johnson (1-3). Today's most widely used Johnson algorithm conducts a reverse sequential search by comparing each reference pattern in the JCPDS powder diffraction file (PDF) to the unknown pattern. A figure of merit is computed for each match and the patterns with the best figures of merit are listed at the end of the search. The Nichols approach is a reverse search of a singly inverted reference file. An inverted file is one which stores the reference patterns according to the d value of the lowest angle 100% intensity line (d1) . This type of file is analogous to the Hanawalt search books distributed by the JCPDS for manual searching. When an inverted file is stored in a random format, along with suitable disk directory files, only reference patterns containing d1 values of interest need be read in the search.

Computer Identification Techniques for Crystalline Compounds Using the JCPDS Powder Diffraction File as a Data Reference

The mineralogy laboratory of the U.S. Geological Survey in Denver has developed a series of time-sharing oriented computer programs which aid in the identification of crystalline compounds from chemical and X-ray diffraction data. These programs operate on a data base compiled primarily from the Powder Diffraction File of the Joint Committee on Powder Diffraction Standards (JCPDS). Diagrammatic X-ray diffraction patterns and various search tables are products of these programs. Additional programs can retrieve information from the data base by chemical formula components or Powder Diffraction File number, and can search and match reflections of an unknown against reference patterns.

Computer searching of the Jcpds powder Diffraction File

Jackson Laboratory is using computer searching of the powder diffraction file on an “in-house” IBM 365/65 computer for about 1000 searches per year. This paper will discuss our experiences with the search program.A Philips Automatic Powder Diffractometer (APD), modified to communicate with an XDS Sigma 5 computer collects the data. A program in the Sigma 5 provides both hard copy output and a computer deck of punched cards for input to the JCPDS search program.The JCPDS search program is indispensable for our large volume of samples . The majority of these samples are corrosion products which generally have “broad peaks, and frequently, preferred orientation.