This communication from the Journal of the American Chemical Society (J. Am. Chem. Soc. 2009, 131, 17084-17086. doi:10.1021/ja9080666) describes the use of classic solid state chemistry to dope Mn3+ in two different host oxide structures to create new blue pigments. The key to the blue color is the unusual trigonal bipyramidal coordination of the Mn3+ ion in these structures. Discussion of this paper in class provides an opportunity to discuss solid solution chemistry in extended structures, including both their synthesis and characterization as well as illustrate the application of crystal field theory to understand the color of a transition metal doped oxide. An extensive list of discussion questions is provided so that the learning activity can be tailored to a variety of different curricular uses and student backgrounds.
After reading and discussing this paper, a student will be able to:
- Describe the basic considerations in the design, synthesis, and characterization of solid solutions in extended structures.
- Identify and describe the distinguishing features of several different mixed metal oxide structures.
- Given structural information, apply the principles of crystal field theory to explain the color and electronic spectroscopy of a transition metal ion doped oxide.
An extensive list of potential discussion questions was developed by Barbara Reisner and Maggie Geselbracht, two faculty trained as solid state chemists so that other faculty would have a range to choose from and use, depending on their curricular goals. We believe that this is an ideal paper to introduce extended solids into the inorganic curriculum with the “hook” for both faculty and students of an easy connection to coordination chemistry.
The first time this learning object was used in the classroom was by Barb at James Madison University in Fall 2009 in a second semester Inorganic Chemistry Course. This paper was not originally used as a literature discussion but instead turned into a lecture. The lecture was used to tie up a unit on solid state chemistry, and the figures in the paper to discuss solid state structures and ionic radii; basic crystallography, powder diffraction, and Vegard’s Law; and crystal field theory.
Maggie used this literature discussion activity as the final conference for her sophomore-level inorganic chemistry course at Reed College in Spring 2011. She selected 8 of the discussion questions from the full list and provided them to her students in advance of the conference meeting. This shortened list is available as an attachment above. Students were asked to read the paper and write out the answers to the discussion questions prior to the discussion. The solutions document to these 8 questions is available to registered faculty users on VIPEr.
When the literature discussion was used at Reed in Spring 2011, the discussion questions were collected and graded on a 10 point scale, 1 point for each question with the exception of #5, worth 2 points, plus an additional point for effort.
Of the 17 students that turned in discussion questions, 2 students (12%) earned 9.5 points out of 10, 7 students (41%) earned 8-9 points, 4 students (24%) earned 7-8 points, and 4 students (24%) earned 6-7 points.
On question 1 (ionic radii), most students did not cite the source of their data or did not specify a coordination number for the ions, and fractions of a point were deducted for these omissions. On question 2 (bond distances), 7 of 17 (41%) students did not calculate predicted bond distances from the ionic radii. In general, students did very well on questions 3 and 4. Question 5 proved more of a challenge. Common errors included no drawings of the d-orbitals in the trigonal prismatic crystal field or unsatisfactory explanations for the crystal field splitting pattern. Only 5 of 17 students (29%) provided the correct answer of magnetism on question 6. Other answers were incomplete or did not explain how the experiment would verify the presence of high-spin Mn3+. On question 7, nearly all students successfully converted the energy scale to photon wavelengths, but 4 students mislabeled the electromagnetic region, as either all infrared or all ultraviolet. Six of 17 students (35%) answered question 8 correctly. The most common error was no explicit indication that transitions in the octahedral geometry are symmetry forbidden by the LaPorte selection rule, whereas this rule is relaxed in trigonal bipyramidal coordination.
Mas Subramanian's group at Oregon State University has followed up this paper with several additional papers based on the principle of using trigonal bipyramidal coordination sites in oxides to synthesize new intensely colored inorganic pigment materials. For example, a 2011 communication in Inorganic Chemistry (Inorg. Chem. 2011, 50, 10-12.) describes the synthesis and characterization of more blue solids. I developed a problem set question and an exam question for my sophomore level inorganic course on this second paper. The link to this additional learning object can be found above in Related Activities.
I adapted this problem to include on the final exam for my inorganic chemistry course this spring. I was initially inspired by the idea of printing the exam in color and including the beautiful Figure 3 from the article. I also liked that I could quiz the students on a range of topics from our course. I asked them to explain the formula and the significance of the 0.1, to determine the oxidation state of Mn and number of d electrons, to identify the trigonal bipyramidal MO5 shape's point group, to determine the symmetry labels of the five metal d orbitals (using the point group), to sketch the crystal field splitting diagram and fill it with electrons... Then I also provided energy levels for the trigonal bipyramidal complex and asked them to do an LFSE calculation to compare with octahedral. In the last two parts of the question, I asked about selection rules and asked them to explain how we could determine experimentally if the Mn was high spin or low spin. The question was worth 31 points out of 130 on the exam.
I'm sure my students didn't quite love this problem as much as I did. But they did fairly well. I don't have the breakdown by individual section, but I can see that scores ranged from 21 to 31 (out of 31 total).
I will very likely continue to use this problem in my course, either as homework or a literature discussion or for review. (Or on exams.)
And the interest in new blues continues. Read this interesting article about nanoparticles and "quantum blue"!
Your students may find this interesting... the color of this pigment is the inspiration for a Crayola crayon named called Bluetiful.
What a great example of STEAM!