Standard X-ray Diffraction Powder Patterns

usually provided by the x-ray pattern itself, whenit was indexed by comparison with theoretical^-values. Treating the sample by appropriateannealing, recrystallization, or heating in hydro thermal bombs improved the quality of most ofthe patterns. The refractive index measurementswere made by grain-immersion methods in whitelight, using oils standardized in sodium light, andcovering the range 1.40 to 2.00.X-ray techniques. At least three patterns forintensity measurements were prepared to checkreproducibility. Samples that gave satisfactoryintensity patterns usually had an average particlesizesmaller than 10 i*. [2]. In order to avoid theorientation effects when samples are packed orpressed, a sample holder was made that had anextended rectangular cavity open at its top faceand end. To prepare the sample, a glass slidewas clamped over the top face to form a temporarycavity wall. (See fig. 1.) The powdered samplewas then drifted into the remaining end openingwhile the holder was held in a vertical position.With the sample holder returned to a horizontalposition, the glass slide was carefully removed sothat the sample surface could be exposed to thex-ray beam (as shown in fig. 2). To powdersthat did not flow readily or were prone to orientexcessively, approximately 50-volume percent offinely-ground silica-gel was added as a diluent.The intensities of the diffraction lines were measured as peak heights above background and wereexpressed in percentages of the intensity of thestrongest line. Additional patterns were obtainedfor d-value measurements. Specimens for thesepatterns were prepared by packing into a shallowholder a sample containing approximately 5-wtpercent tungsten powder that served as an internalstandard. When tungsten lines were found tointerfere, 25 percent silver was used in place oftungsten. If the internal standard correctionvaried along the length of the pattern, linearinterpolations were used for the regions betweenthe peaks of the standard. For low values of 26,the pattern peak was measured in the center, at aplace averaging about 75 percent of the peakheight. For higher values of 26, where the on and«2 peaks were separated, the ai peak was measuredin the same way. The internal standard correction appropriate to each region was then appliedto the measurement of 26. The internal standardlattice constants used were 3.1648 A for tungstenand 4.0861 A for silver at 25 C. as determined byJette and Fopte [3]. All of the NBS patterns,unless otherwise noted, are made at 25 C, usingeither filtered copper or cobalt radiation (Ken),o ohaving the wavelengths 1.5405 A, and 1.7889 A,respectively.Structural data. For cubic materials a valuefor the lattice constant was calculated for eachd-value. However, the constant reported is thatobtained by averaging the last five lines becauseof the greater accuracy of measurement in thelarge-angle region of the pattern. The unit cellvalues for each noncubic substance were determined by means of a least-squares calculationmade on the IBM 7090, using those d-values forwhich only one set of Miller indices could beassigned. The number of significant figuresreported for {/-values in the NBS pattern is limitedby the quality of each sample as indicated byresiduals obtained from least squares refinement.A portion of the indexing and cell refinementcalculation was performed on a Burroughs B 220computer at the United States Geological Surveyusing a program developed by H. T. Evans, Jr.FIGURE 1 FIGURE 2