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Cite This: J. Chem. Educ. 2019, 96, 1449−1452 pubs.acs.org/jchemeduc
Inquiry-Based Experiment with Powder XRD and FeS2 Crystal:
“Discovering” the (400) Peak
N. Stojilovic*,†,‡ and D. E. Isaacs†
† ‡
Department of Physics and Astronomy and Department of Chemistry, University of WisconsinOshkosh, Oshkosh, Wisconsin
54901, United States
S
*Supporting Information
ABSTRACT: We discuss how a powder X-ray diffraction (XRD) system can
be used to probe large pyrite (FeS ) crystals to reveal a peak generally not
2
documented in the literature. The ability to detect this peak is attributed to the
use of a large crystal, which gives large signal intensities. This type of
experiment provides a research-like experience and gives students the
opportunity to deepen their understanding of diffraction orders. In this
experiment students are first challenged to be creative and determine how to
mount a mineral crystal in a powder XRD system and then practice critical
thinking in order to determine the origin of the unknown XRD peak. This
experiment may also be generalized to crystals other than pyrite.
KEYWORDS: Upper-Division Undergraduate, Physical Chemistry, Inquiry-Based/Discovery Learning, Solid State Chemistry,
Student-Centered Learning, X-ray Crystallography
■ INTRODUCTION peak intensities, missing peaks, and preferred orientation. They
In traditional laboratories students are often instructed on what are comparing an XRD pattern of a single crystal to that of the
to measure and how, and thus they do not fully develop powder and try to figure out why an XRD peak visible in the
problem-solving and critical thinking skills. Many instructors single crystal specimen is absent from the XRD pattern of the
use laboratories to strengthen the concept covered in lectures, pyrite powder. Students generally cannot find the (400) peak
but it seems that laboratories are not effective in this role. For in the literature and have to trust their own calculations and
example, Wieman and Holmes compared final test results from arguments. This activity mimics scientific research since it
two large introductory physics courses, one with and one engages students in critical thinking.
1 Pyrite (FeS ), also known as “fool’s gold”, is a mineral with
without an associated “traditional” lab component. They 2
found no observable effect on the final exam performance on pale brass-yellow appearance and metallic luster. Although it
questions involving topics covered in the laboratory. Their commonly forms cubes, octahedral and pentagonal dodecahe-
Downloaded via UNIV OF WISCONSIN-OSHKOSH on September 4, 2019 at 19:54:02 (UTC).findings question the effectiveness of laboratories as a means todra forms can also be found.15 XRD experiments on pyrite
See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.increase mastery of the lecture content. The laboratories aresamples are typically done using the powdered mineral;
more effective when their goal is to teach experimental however, in this paper, we will demonstrate how large pyrite
2 cubes found in nature can be probed using a powder XRD
practices.
Providing an undergraduate research experience through system. We will also show how probing bulk pyrite cubes can
3 and improves their
courses stimulates students’ curiosity reveal a higher-order diffraction peak not easily found in the
4 5
conceptual understanding and critical thinking skills. literature. Other minerals or single crystals could be studied
Crystallography laboratory activities have been successfully using powder XRD systems. This type of activity can easily be
incorporated into various experimental chemistry courses for expanded and is best suited as an upper-level inquiry-based lab.
undergraduate students.6−13 In a recent inquiry-based activity,
a powder XRD instrument was used to probe an Al O (0001)
2 3 ■ EXPERIMENTAL DETAILS
single crystal, and through the discovery of the origin of the
peak doublet, students could learn about X-ray generation, Pyrite minerals found in nature, with relatively large cubic faces
14 In the (Figure 1), were studied. In XRD experiments using systems
electronic transitions, and spin−orbit coupling.
present activity, the puzzle is not the origin of the doublet
but the origin of the higher-order diffraction peak not given in Received: February 3, 2019
some reference XRD patterns (JCPDS file 42-1340). In this Revised: May 29, 2019
activity students are learning about diffraction orders, relative Published: June 19, 2019
©2019 American Chemical Society and
Division of Chemical Education, Inc. 1449 DOI:10.1021/acs.jchemed.9b00099
J. Chem. Educ. 2019, 96, 1449−1452
Journal of Chemical Education Activity
primarily designed for powders, the positioning of the crystal
(height in particular) is critical.
Figure 2. Powder XRD of a typical cubic FeS2 crystal found in nature.
Figure 1. Pyrite mineral held by transparent adhesive tape. Theinset shows the (400) peaks due to Kα and Kα X-ray lines. The
1 2
numbers in parentheses are Miller indices, which indicate crystal
planes.
In order to probe different cubic faces of various crystals, the
samples were directly mounted on transparent adhesive tape
with the X-ray beam passing through it. The signal intensity
from single crystals is so large that the attenuation effect of the
transparent adhesive tape on the XRD pattern is insignificant.
Wide and fast scans can be used to reveal the locations of the
peak(s), and then narrow high-resolution scans can be used to
resolve the doublets. Samples are inexpensive, harmless, and
readily available and can often be analyzed without any special
sample preparation. Students performing XRD experiments
should have prior radiation safety training. Pyrite powders were
also probed, and their diffraction patterns were compared to
those of pyrite crystals. The powder was prepared after
crushing and grinding stacked pyrite crystals with the agate
mortar and pestle and was mounted on the standard glass
sample holder with 0.2 mm depth.
■ EXPERIMENTS AND DISCUSSION
Powder XRD instruments are generally used for powdered
specimens. Thus, mounting and positioning large crystals for
diffraction experiments in a typical powder XRD system Figure 3. XRD pattern of FeS2 powder. The peak at about 69°
requires the students to think creatively, and assigning Miller present in the single crystal is typically not detectable in the powder.
indices to peaks not reported in the reference XRD patterns
requires some critical thinking. The XRD pattern of a cubic
face of a natural FeS crystal is displayed in Figure 2. When a
2
pentagonal dodecahedron face is probed, the most intense the case of X-ray diffraction from a crystal is given by Bragg’s
peak corresponds to another set of planes (see Supporting law
Information). Figure 3 shows an XRD pattern from FeS
2 2dnsin θλ= (1)
powder. Although the large (200) peak observed from the
cubic face of the single crystal is also seen in our FeS powder,
2 where d is the spacing between reflecting crystal planes, n is the
the second peak (doublet of much lower intensity), displayed diffraction order (n = 1, 2, 3, ...), θ is the angle of incidence
in the inset of Figure 2, is barely visible in the powdered measured from the face of the crystal, and λ is the wavelength
sample. Thus, students try to “discover” what it represents by of incident X-rays.
calculating the peak positions corresponding to different The separation of the (hkl) planes (h, k, and l are the Miller
diffraction orders and making a direct comparison with the indices) is denoted by d for a cubic crystal lattice and is given
observed peaks. This peak doublet, which reveals the Kα and hkl
1 by
Kα splitting, is better resolved at higher values of Bragg’s
2
angles. 222
The positions of possible peaks can be calculated using 1 = hk++l
d2 a2 (2)
theory. The expression for finding the interference maxima in hkl
1450 DOI:10.1021/acs.jchemed.9b00099
J. Chem. Educ. 2019, 96, 1449−1452
Journal of Chemical Education Activity
If all three Miller indices are multiplied by the same integer n
(diffraction order), the separation is reduced by that factor and
can be calculated using
222
()nh ++(nk) (nl) 222
1 2hk++l 2 1
d2 = a2 =n a2 =n d2
nh,,nk nl hkl
(3)
or
d
d = hkl
nh.,nk nl n (4)
In a cubic lattice with lattice constant a, the spacing between
reflecting planes is given by eq 2. Therefore, the angles at
which the (hkl) planes diffract X-rays are given by
222
hkl
++
sin θλ= (5)
2a
Since the crystal structure of pyrite is primitive cubic,
reflections are allowed for any integer values of h, k, and l. Figure 4. Two hour long narrow XRD scan of FeS2 powder. The
We can predict the reflections that can be observed by doublet peak at about 69° not given in the JCPDS file 42-1340
determining the possible integer values of the sum of the reference card can be detected in the powder prepared from grinding
squares of the Miller indices. Note that the (400) peak is cubic pyrite crystals.
theoretically possible but generally not observable in pyrite
samples. The reason is that usually the samples are in the form intensities are to be determined by diffraction from powders, it
of powder and the (400) signal intensity is below or at the is essential that there is no preferred orientation in the powder
detection limit. Using a large single crystal, the resulting signal since the preferred orientation can produce systematic errors in
intensities are orders of magnitude greater than those from peak intensities.
powdered samples allowing detection of the (400) peak. To successfully complete this inquiry-based experiment,
The doublet profile of the (400) peak can be explained by students should know Bragg’s law, be familiar with Miller
Kα1 (1.54050 Å) and Kα2 (1.54434 Å) lines whose indices, and be able to calculate the angles of Bragg’s peaks
wavelengths are so close in value that in typical powder using the theory presented above. Students generally do not
XRDpatternsthey may appear as single peaks. The wavelength demonstrate a deep understanding of the concept of diffraction
of the Cu Kα radiation reported in the literature actually order, and this type of activity helps them learn about higher
corresponds to (2Kα +Kα )/3 since the Kα line is about 2 diffraction orders through “discovery”. Searching and reading
1 2 16 1 scientific literature (primarily books and articles) should be an
times larger than that of Kα . Separation between Bragg
2 integral part of the activity. Since the answer is not trivial, this
peaks due to Kα and Kα radiation increases with diffraction
1 2 activity allows students to engage in scientific ways of thinking.
angle θ. The inset in Figure 2 shows how these two X-ray lines
can be separated when single crystals are probed using powder They will have to create a hypothesis and determine a way to
XRD systems. test it. This experiment is an inquiry-based type of lab activity
Using eq 5, one can solve for the angles corresponding to suited for students taking physical chemistry or advance
Kα and Kα (400) reflections. The calculated separation physics laboratories but can also be given as an independent
1 2 study. The activity was given to 29 students taking either
between these two peaks is 0.19° and matches the measured
one shown in Figure 4. The intensity of the (400) peak is general physics or independent study. Students either worked
more than 100 times smaller than that of the largest peak, and on the activity individually or in groups of two. In a four week
thus the (400) peak is not reported in some reference X-ray period, none of the students “discovered” the origin of the
diffraction patterns like JCPDS file 42-1340 (see Supporting “unknown” peak but gained a lot from working on it and
Information). These reference X-ray diffraction patterns exercising critical thinking. A set of questions was then given to
typically have the intensity of the greatest peak assigned a students to help them focus on relevant concepts and terms
value of 100, and the intensity of the smallest peak is not less (see the Supporting Information). The lecture on XRD
than 1. The fact that one cannot find the (400) peak in this covered relevant sections from Atkins’ Physical Chemistry
17
reference card index gives students an opportunity to textbook. Theuniversity has access to the Journal of Chemical
“discover” it via critical thinking. Education and SciFinder. Learning objectives were learning
Oneofthereasons we see the (400) peak is the fact that our from mistakes (wrong hypothesis), developing critical thinking
XRDpattern of pyrite powder has a somewhat greater relative skills, and deepening understanding of the term diffraction
intensity of the (200) peak compared to other peaks from the order.
reference pattern. This suggests a slightly preferred (non- SUMMARY
random) orientation of the crystallites in our powder prepared ■
from grinding pyrite cube crystals. Therefore, how powder is We discussed an activity which mimics scientific research and
prepared can play a role in detecting the (400) reflection. The in which students can exercise their creativity and critical
crystallites should have all possible orientations in finely thinking which are considered essential scientific skills. The
ground powders. The crystallite shapes within a powder give fact that the given reference X-ray diffraction pattern of pyrite
rise to what is known as a preferred orientation. If peak does not show the (400) peak provides students the
1451 DOI:10.1021/acs.jchemed.9b00099
J. Chem. Educ. 2019, 96, 1449−1452
Journal of Chemical Education Activity
opportunity to “discover” it in the diffractogram of a single (7) Lyle, S. J.; Flaig, R. W.; Cordova, K. E.; Yaghi, O. M. Facilitating
crystal using a powder XRD system. Once students mount the Laboratory Research Experience Using Reticular Chemistry. J. Chem.
sample and observe unexpected results, they may come up with Educ. 2018, 95 (9), 1512−1519.
their own experiments to test their hypothesis. The question (8) Enemark, J. H. Introducing Chemists to X-ray Structure
can be posted in class (physics, chemistry, materials science, Determination. J. Chem. Educ. 1988, 65 (6), 491−493.
etc.) without performing the actual experiment if students are (9) Glusker, J. P. Teaching Crystallography to Non Crystallogra-
given XRD data and this reference pattern. Students can collect phers. J. Chem. Educ. 1988, 65 (6), 474−477.
or can be given diffractograms of different crystal faces of the (10) Goldstein, B. M. Introduction to the Crystallographic
Literature: A Course for the Nonspecialist. J. Chem. Educ. 1988, 65
same sample and can repeat experiments on various mineral or (6), 508−512.
crystal samples. They can probe a cubic face and compare it to (11) Bazley, I. J.; Erie, E. A.; Feiereisel, G. M.; LeWarne, C. J.;
a pentagonal one or grind the mineral to make a direct Peterson, J. M.; Sandquist, K. L.; Oshin, K. D.; Zeller, M. X-ray
comparison with the powder. One advantage of using single Crystallography Analysis of Complexes Synthesized with Tris(2-
crystals in powder XRD systems is the ability to resolve Kα1 pyridylmethyl)amine: A Laboratory Experiment for Undergraduate
and Kα lines that typically overlap and form a single peak in Students Integrating Interdisciplinary Concepts and Techniques. J.
2 Chem. Educ. 2018, 95 (5), 876−881.
XRDanalyses of powders. As they investigate the doublet peak (12) Hoang, G. T.; Kubo, T.; Young, V. G., Jr.; Kautzky, J. A.;
profile students can learn about electronic transitions Wissinger, J. E. Illustrating the Utility of X-ray Crystallography for
responsible for different X-ray lines. This type of activity Structure Elucidation through a Tandem Aldol Condensation/Diels−
could be easily incorporated in physical chemistry or advanced Alder Reaction Sequence. J. Chem. Educ. 2015, 92 (8), 1381−1384.
physics laboratories. (13) Campbell, M. G.; Powers, T. M.; Zheng, S.-L. Teaching with
the Case Study Method To Promote Active Learning in a Small
■ ASSOCIATED CONTENT Molecule Crystallography Course for Chemistry Students. J. Chem.
S Educ. 2016, 93 (2), 270−274.
*Supporting Information (14) Stojilovic, N. Using Cu Kα1/Kα2 Splitting and Powder XRD
The Supporting Information is available on the ACS System To Discuss X-ray Generation. J. Chem. Educ. 2018, 95 (4),
Publications website at DOI: 10.1021/acs.jchemed.9b00099. 598.
Details of the experiment and how this activity can be (15) Bonewitz, R. L. Rock and Gem; Dorling Kindersley Limited:
implemented (PDF, DOCX) New York, NY, 2005.
(16) Klug, H. P.; Alexander, L. E. X-Ray Diffraction Procedures For
AUTHOR INFORMATION Polycrystalline and Amorphous Materials; John Wiley & Sons, Inc.:
■ New York, 1967.
Corresponding Author (17) Atkins, P. Physical Chemistry, 6th ed.; Freeman: New York,
*E-mail: stojilovicn@uwosh.edu. 1998.
ORCID
N. Stojilovic: 0000-0002-1751-6033
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
We thank Dr. Eric Hiatt and Dr. Sheri Lense for critical
reading of the manuscript and suggestions. We also thank
anonymous reviewers for excellent comments. N.S. was
supported by UW Oshkosh Grant FDT592.
■ REFERENCES
(1) Wieman, C.; Holmes, N. G. Measuring the impact of an
instructional laboratory on the learning of introductory physics. Am. J.
Phys. 2015, 83, 972.
(2) Holmes, N. G.; Wieman, C. E. Introductory physics labs: we can
do better. Phys. Today 2018, 71, 38.
(3) Kean, K. M.; Van Zee, K.; Mehl, R. A. Unnatural Chemical
Biology: Research-Based Laboratory Course Utilizing Genetic Code
Expansion. J. Chem. Educ. 2019, 96 (1), 66−74.
(4) Seymour, E.; Hunter, A.-B.; Laursen, S. L.; DeAntoni, T.
Establishing the benefits of research experiences for undergraduates in
the sciences: First findings from a three-year study. Sci. Educ. 2004, 88
(4), 493−534.
(5) Thiry, H.; Laursen, S. L.; Hunter, A. B. What Experiences Help
Students Become Scientists? A Comparative Study of Research and
Other Sources of Personal and Professional Gains for STEM
Undergraduates. J. Higher Educ. 2011, 82 (4), 357−388.
(6) Zheng, S.-L.; Campbell, M. G. Connecting Key Concepts with
Student Experience: Introducing Small-Molecule Crystallography to
Chemistry Undergraduates Using a Flexible Laboratory Module. J.
Chem. Educ. 2018, 95 (12), 2279−2283.
1452 DOI:10.1021/acs.jchemed.9b00099
J. Chem. Educ. 2019, 96, 1449−1452
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