The x-ray diffraction method is a very powerful technique in modern research. Besides providing the best means for the identification of a crystalline compound, it is invaluable in all materials research, relating physical and chemical properties to the crystalline state.
To effectively utilize the powder method, one often requires a standard reference pattern for direct comparison. Such standard patterns are obtained by using the best material available, carefully preparing it into a special sample holder, and recording the diffraction effects by means of film or counting devices. The most common recording techniques currently in use are the Debye-Scherrer camera and the counter diffractometer. Because the Guinier camera is increasing in usage, it should be considered among the standard methods. Even with modern precision equipment the experimental pattern is usually less than perfect, suffering from sample problems, geometric aberrations of the recording apparatus, and limitations of the detection system.
The basic requirement for a standard pattern is that it be truly representative of both the material and the recording method. The pattern must be reproducible for different mounts of the same sample both by different experimentalists and on different units of similar apparatus. In practice, these conditions are not easy to achieve. Samples are rarely ideal and suffer many defects, including nonrandom orientation of crystallites, inhomogeneity of chemical or structural composition, small or imperfect crystallites, primary and secondary extinction, strain, absorption (or the lack of it), microabsorption, and admixtures with other phases. Apparatus may not be precisely aligned, or the detection. . . .