Use for X-Ray Spectrometry of the (200) Reflection of Ammonium Dihydrogen Phosphate William L. Baun Materials Physics Division ( M A Y A ) , Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio
MANYof the X-ray analyzers and the cuts of single crystals that are used for X-ray spectrometry have come into use not by design, but rather because they were available for other purposes. Some large crystals have been grown and cut along certain directions to take advantage of electronic properties. For instance, ammonium dihydrogen phosphate (ADP) was grown during World War 11 for use in submarine detection devices. Crystals which were cut for this purpose were then used for X-ray spectrometry. ADP grows from H,O solution with the outer morphology shown in Figure 1. Conventionally ADP is cut parallel t o the { 101 face, yielding an X-ray analyzer having a 2d of 10.64 A. The other growth face (100) is also useful as a dispersing element for X-rays. This cut is easy to obtain because a slice may be taken from the crystal using the natural growth face as a reference. Care must be taken to grow ADP from pure solution. If foreign ions are present, such as Fe”, the faces may not be flat and probably will taper toward the ends of the crystal. Because the structure factor is zero for (loo), the (100) reflection cannot be used, but the (200) reflection results in a crystal having a 2d of 7.50 A. Tbis spacing makes a good intermediate crystal for the 7 A region. A number of crystals such as silicon and germanium are available for use to
/‘00(200’
I
ADP
Figure 1. Outer growth morphology in ammonium dihydrogen phosphate
about 6 A, but then there is a large jump to a 2d of 8.80 A (EDDT) or 8.75 A (PET). Inasmuch as the dispersion of a flat crystal spectrometer increases with an increase in diffraction angle, it is advantageous to use a crystal spacing just larger than the wayelength to beoanalyzed. Using the EDDT spacing of 8.80 A, SiKa (7.12 A) is dispersed at only about 108’ 26, while the (200) reflection from A D P moves the reflection to 143’ 26, which is just within the operating range of most commercial spectrometers. The SiK satellite ~ , ~ occur just to the short wavelength side of lines K L Y which the parent K a line are shown in Figure 2. In the inset is shown these same two lines using a very high resolution EDDT
Figure 2. SiK satellite lines K L Yfrom ~ , ~ silicon using ADP (200) (inset shows same lines using EDDT) 830
ANALYTICAL CHEMISTRY
crystal. These spectra are computer plots, linear in energy, obtained by step scanning and point by point counting techniques. The ADP crystal provides much greater dispersion which would be useful for separating SiK lines from multiple orders of K and L series lines and for obtaining better results on shift and shape measurements in conjunction with chemical combination and bonding studies in silicon compounds. In addition to these advantages, ADP (200) also gave a lower minimum detectable limit (mdl) for silicon
in aluminum when compared to EDDT and ADP (101). However, this comparison was made using only one crystal of each. Because individual crystals can vary considerably, a more comprehensive study would have to be done to prove that ADP (200) provides better sensitivity for silicon in aluminum. RECEIVED for review January 13, 1969. Accepted February 25, 1969.
Clinochlore: A Versatile New Analyzing Crystal for the X-Ray Region 5.271 W. L. Baun Materials Physics Division ( M A YA), Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio 45433 E. W. White Materials Research Laboratory, The Pennsylvania State University, University Park, Pa. 16802 DEVELOPMENTS of electron microprobe analysis of the light elements and basic X-ray spectral shift studies have been impeded by the lack of good analyzing crystals for the X-ray region 10 to 30 A. It is relatively easy to find natural minerals in the form of large crystals, or to grow crystals for use !s X-ray analyzers in the conventional region 0.1 to 10 A. However, in the “soft” X-ray region it is difficult to find materials having the desired combination of large spacing, crystal size, and diffraction intensity. In the early days of X-ray spectrometry, experiment$ists were generally limited to the use of beryl (2d = 15.2 A) and mica (2d = 19.9 A). More recently, salts of phthalic aci$--particularly potassium acid phthalate (KAP, 2d = 26.6 A)-have been grown ( I ) . KAP has been used widely but has some disadvantages. Its diffraction efficiency appears to degrade over a period of several months; and when it is used for oxygen K , an anomalous second peak is observed. Both the intensity and the position of the anomalous peak depend o n the composition of the crystal and the nature of the sample (2). Because of these difficulties, several mineral groups were surveyed in an :ttempt to find a stable mineral having a 2d of around 30 A which also occurs as large single crystals. The chlorite group contains two members that are particularly promising. Clinochlore and penninite both appear to be suitable analyzer crystals but to date most work has been done with clinochlore. Both members are relatively common in nature but it is extremely rare to find specimens having useful physical dimensions and perfection. The only suitable source for large flat sheets has been from the Tilly Foster mine near Brewster, N. Y . , which is now abandoned and filled. At least one commercial source of analyzer grade material has been established recently, thus making clinochlore of more than just academic interest (LeMont Scientific, Inc., Pike St., Lemont, Pa. 16851). (1) W. Ruderman, E. Losin, and D. Uphoff, ASD TDR 63-311, April 1963. (2) R. A. Mattson and R. C. Ehlert, Advances in X-ray Analysis,
9,471 (1966).
Table I. X-ray Dispersion Characteristics of Clinochlore Optimum elements K series 2d, A L series Order 0 Ti 1 28.392 2 14.196 Na Cu A1 Se 3 9.464 P, SWP) Sr 4 7.098
The chlorites are a group of minerals within the large classification of sheet silicates having properties similar to mica. They are classified generally according to the content of silicon and iron. Clinochlore and penninite are monoclinic with unit cell parameters on the order of: a = 5.3 A, b = 9.2 A, c = 14.3 A and /3 = 97” (3). The repeat distance of interest in X-ray dispersion applications is csinb. The perfect { 001 1 cleavage eliminates problems of crystal orientation and cutting. The single crystals are mechanically quite strong and may be bent to a small radius. Clinochlore is stable in air to at least 550 “C. Additional details of chemistry, structure, properties, and occurrences have been summarized recently by Deer, et al. (3). Clinochlore has a repeat distance of 14.196 A at 25 “C, giving a first order 2d of 28.392 A. Several orders are very strong, allowing its use to quite short wavelengths when necessary. The high intensity of diffraction and excellent resolution of clinochlore and penninite in the 16th order for Cu Ka,,? is illustrated in Figure 1. The 0.125’20 difference in pe!k position is the result of a difference in 2d of only 0.02 A, Using clinochlore is equivalent to having four or more crystals on the spectrometer at one time. However, as there are so many strong orders of diffraction, the spectrum may become too cluttered if many elements are present in a given (3) W. A. Deer, R. A. Howie, and J. Zussman, “Rock Forming Minerals Volume 3: Sheet Silicates,” Wiley, New York, 1962, pp 131-163. VOL. 41, NO. 6, MAY 1969
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