Chemical Education Today

1372 Journal of Chemical Education • Vol. 75 No. 11 November 1998 • JChemEd.chem.wisc.edu

The intersection among the disciplines of science—bi-ology, chemistry, physics, and geology—is the atomic or mo-lecular description of matter. Whether it is the structure of DNA(1), benzene (2), a cuprate superconductor (3 ), or magnesiumsilicates (4), there are many examples of critical structures uponwhich fundamental theories are based (5 ). The most defini-tive method of structure determination is generally consideredto be X-ray diffraction, a technique used routinely in researchacross the sciences.

By contrast with NMR, another powerful structuralmethod, X-ray crystallography, is poorly represented at the un-dergraduate level. Crystallography is often perceived as inac-cessible owing to the mathematics, group theory, and abstractconcepts involved. However, the theory behind NMR is prob-ably equally challenging. There is a sense that a utilitarian levelof understanding of NMR is easily obtained; similarly, small-molecule crystal structures can be solved with little more knowl-edge than that required to run a computer program. Thus twoprimary factors, availability of instrumentation and ease of datacollection, seem to account for the difference in representationof these two methods at the undergraduate level.

Single-crystal X-ray diffractometers are expensive and canrequire time-intensive maintenance. As a result they are not com-monly found in primarily undergraduate institutions. Even withavailable instrumentation, the data collection for single crystalsis more involved than a simple NMR scan. One alternative is togive the student previously collected data for analysis. However,the usefulness of computer-generated data or simulated diffrac-tion experiments (6, 7 ) is limited because it does not providethe important hands-on learning that occurs in the laboratory.

We have addressed the need for stronger grounding in crys-tallography with an X-ray powder diffractometer. A powderdiffractometer is significantly cheaper and easier to maintain thana single-crystal instrument. Modern diffractometers are safe, andthe data collection requires approximately the same amount oftime as a normal proton NMR scan. In addition, powder dif-fraction has several pedagogical advantages over single-crystal X-ray diffraction because powder diffraction patterns can be analyzedwithout resorting to calculation-intensive programs. Students canlearn fundamental concepts of crystallography without the black-box aspects (at this level) of “direct methods” programs (8).

The use of X-ray powder diffractometers in the physics orgeology departments of small colleges has a long history. In thephysics department at Oberlin, diffraction is initially introducedin a solid state physics course. Basic concepts such as diffractionof light, Miller indices, Bragg’s law, and the unit cell are easilyillustrated using powder data. The diffraction patterns of hex-agonal close-packed and cubic close-packed structure types areused to discuss the structure of solids. In the Advanced Labora-tory course, superconducting samples of YBa2Cu3O7 are madeand characterized using X-ray diffraction (assuring other phases

such as Y2BaCuO5 or YBa2Cu3O6 are not present) and resistiv-ity measurements. Advances in modern instrumentation haveenabled nontraditional uses of the instrument. For example thecourse also has a laboratory to study the strain (9) in thin filmsof metals deposited on glass substrates.

The chemistry department has used the instrument in itsfirst inorganic chemistry course (primarily sophomores), and weare currently designing experiments for a synthesis laboratory.In the inorganic chemistry course students run scans on simplealkali metal halide salts to determine atomic radii and bondlengths and to compare the density based on a calculation ofthe measured unit cell with tabulated data. Thus, in additionto an introduction to crystallography, students learn how dataare used to make conclusions about trends across the periodictable. Again, only simple concepts are required for analysis, andthe laboratory supports concepts traditionally taught in class. Infact, it is not until the advanced inorganic course that studentslearn about space groups, systematic absences, and other typesof diffraction (neutron or electron diffraction).

Most significantly, the X-ray powder diffractometer hasopened new opportunities for collaboration with the physicsdepartment. There is strong interest in both departments in thestudy of materials, and the instrument is used in the researchprogram of both departments. The focus on solar cell materialsand magnetic materials has applications that draw students into re-search. There has even been a joint project with an archeologiststudying the diffraction patterns of ancient Tuscan pottery. Thereis clearly potential for many new laboratory experiments using X-ray powder diffraction to explore the structure–property relation-ships in solids, which will promote interdisciplinary activities.

Acknowledgment

This work is partially supported by a grant from theNational Science Foundation, Division of UndergraduateEducation, Instrumentation and Laboratory ImprovementProgram (DUE 9650840).

Literature Cited1. Watson, J. D.; Crick, F. H. C. Nature 1953, 171, 737. Klug, A. Na-

ture 1968, 219, 808.2. Lonsdale, K. Nature 1928, 122, 810.3. Bednorz, J. G.; Muller, K. A. Z. Phys. 1986, B64, 186.4. McElhinny, M. W. The Earth: Its Origin, Structure and Evolution; Aca-

demic: New York, 1979.5. Rossi, M.; Berman, H. J. Chem. Educ. 1988, 65, 473.6. Lisensky, G.; Kelly, T.; Neu, D. R.; Ellis, A. B. J. Chem. Educ. 1991,

68, 91.7. Spencer, B. H.; Zare, R. N. J. Chem. Educ. 1991, 68, 97.8. Stout, G. H.; Jensen, L. H. X-ray Structure Determination, a Practical

Guide, 2nd ed.; Wiley: New York, 1989; Chapter 11 (covers directmethods programs).

9. Noyan, I. C.; Huang, T. C.; York, B. R. Crit. Rev. Solid State Mater.Sci. 20(2), 1995, 125.

NSF HighlightsProjects Supported by the NSF Division of Undergraduate Education

edited bySusan H. Hixson

National Science FoundationArlington, VA 2230

Curtis T. Sears, Jr.Georgia State University

Atlanta, GA 30303

X-ray Diffraction Facility for UndergraduateTeaching and Research in Chemistry and PhysicsSarah StollDepartment of Chemistry, Oberlin College, Oberlin, OH 44074