Rietveld Analysis of Complex Mixtures. What Can We Do?

Rietveld Quantitative X-Ray Diffraction (RQXRD) has become commonplace for the analysis of everything from industrial materials, such as concrete, to geologic materials, such as bauxite ores or igneous rocks. The recent report on the IUCR Round Robin study on quantitative analysis indicates that accurate results may not always be obtained when using RQXRD, even with simple three-phase mixtures. What can we do when we have samples with many phases and a large amorphous content? And what kind of errors can we expect? Coal combustion by-products (CCBs) are a diverse set of complex materials. They can be as simple as four phases in a significant amorphous matrix (>50%) or as complex as twelve or more phases in a significant amorphous matrix (>50%). The studies and results reported here have focused primarily on this class of materials, both actual CCB materials and synthetic mixtures having compositions resembling those found in CCB materials. A refinement protocol has been developed for these types of materials and the corresponding estimated errors have been determined. In developing this protocol, a number of different effects and refined parameters have been investigated. These effects include the scan time, internal standard (both material and particle size), divergence slit type, and refined parameters (such as specimen displacement and preferred orientation). In addition, the effects of microabsorption have been examined in a qualitative fashion. The methodology has been examined both with and without the Brindley correction.