CHE 450G: Practical Inorganic Chemistry

U of KY Dept of Chemistry

Preparation of YBa₂Cu₃O_7-x

Addenda to Girolami et. al. Experiment #1

Introduction

While the introduction in your textbook is good, it fails to discuss a key relationship between the oxygen content of your superconductor and its structure. Therefore, you will need to read at least one of the first 3 references listed below before you attempt to draw conclusions from your data (and well before the night your report is due):
• C & E News, December 21, 1992, pages 24-41 (very readable).
• Cava, Scientific American, Aug 1990, pages 42-49. Very readable with nice color graphics. Available in Young library, oversize section, call number T1.S5.
• Cheetham and Dey Solid State Chemistry: Compounds, 1992 (on reserve). Read relevant parts of the chapter on superconductivity.
• Sleight, Science, 1988, 242, (Dec 16), p 1517-1527. (more complex)

In addition, we will be performing powder X-ray diffraction on your product, so you will need to read the following items:

• West, Anthony Solid State Chemistry and its Applications. Chapter 2.1 (Solid State Reactions)
• ibid, Chapter 5 (powder X-ray diffraction...this is a very long chapter; just try to get the basic idea for now)

Synthesis (Day 1)

Do not follow the procedure in your text book. Your starting materials for this experiment will be Y₂O₃, BaCO₃ and CuO. It is up to you to determine the proper amounts of reagents to use for this reaction, but you should try to make 3 to 5 grams of product (your yield will be nearly quantitative). Make sure you come up with a balanced equation for the synthesis of your product and record your calculations in your notebook!

Follow this generic procedure:

1. Place your raw materials in a mortar and pestle. Question: Why are we doing this?

2. Add enough acetone to form a thick slurry (makes it easier to grind).

3. Grind your slurry until you have a homogeneous gray powder. Continue grinding until all of the acetone has evaporated (10 to 15 minutes total).

4. Place your powder into an alumina boat and place it into a tube furnace. The one we're using is shown below. Fire your material overnight at 940 degrees. It is not necessary to flow oxygen in this first step, although you can try it if you wish. Arrange with your instructor to come in the following day for approximately 30 minutes to perform the following two steps.

   

5. Your instructor will turn the furnace off the following morning. Remove your material from the furnace using a push rod. Let your product cool to room temperature. Place it in a mortar and pestle and regrind the material (don't bother using acetone). Question: Why are we doing this?

6. Press your product into pellets using the pneumatic pellet press. Place your pellets and any leftovers in the tube furnace and heat it under a slow flow of pure oxygen at 940 degrees Celsius overnight. Question: why are we doing this? Your instructors will turn off the furnace the following morning.

Characterization (Day 2)

Characterize your alleged superconductor by the following three methods. For the first two methods, you will need to regrind a portion of your material (without acetone). Be sure to save one of your pellets for the third method!
1. Iodometric Titration. Follow the procedure given in your textbook.
   We have not tried this procedure before, but iodometric titrations are very sensitive to oxygen (but not water). You can attempt to do the titration on the Schlenk line if you wish, or you may even perform this in a glove box if you prefer. The decision is up to you.

2. Powder X-ray Diffraction. Powder XRD is often used by chemists, physicists, geologists and material scientists to characterize unknown microcrystalline materials. Please read the appropriate background readings before attempting this experiment.
   To use X-ray equipment you have to be a certified operator and have a radiation badge through the University. Therefore, your TA will run the XRD of your sample for you, but you will accompany him to the XRD lab. He will explain the workings of the unit.

   Your instructor will demonstrate how to prepare a sample for the XRD during the first laboratory period.

   Interpret your XRD spectrum, taking particular note to account for any likely impurity phases, such as the "green phase". You will have to go to the library and look up XRD spectra of authentic samples of these materials (the reference in Angelici is not acceptable for comparison). You must consult the primary literature to write your lab report!

3. Magnetic Levitation The T_C of your material is above that of liquid nitrogen. Therefore we will test your sample to see if it exhibits the Meissner Effect. Question: What the heck is the Meissner effect?
   1. Place your oxygen-treated pellet into a shallow dish (a cut-off styrofoam coffee cup works well). Carefully fill the dish with liquid nitrogen. Caution: Liquid nitrogen is -196 degrees C (77 K) and can cause instantaneous frostbite! The nitrogen will boil off very rapidly at first -- add more liquid nitrogen as necessary.

   2. Your TA will give you a very powerful (but small) neodynium magnet. Use a pair of non-magnetic tweezers to place the magnet above your cooled sample. If your sample is superconducting, the magnet will levitate above the sample. If so, measure the distance that the magnet is elevated. Be sure the magnet is dry before returning it to the instructor.

      If you find that your sample levitates your magnet at room temperature, you may give your sample to me in return for an A for the course (I, of course, will retain the patent rights and the claim to a Nobel prize in chemistry)

If we had time, we would perform resistivity or SQUID measurements to determine the T_C of your material. In addition, we could perform a Rietveld analysis of your material to try and "solve" the crystal structure. Finally, we could determine the oxygen content using thermogravimetric analysis (TGA) as a comparison to our titration. These are powerful tools that you may want to consider in your independent or development experiments.

For your report...

Use CrystalDesigner to generate drawings of the structure of your product in your lab report. You will need the atomic coordinates and crystallographic space group of the material before you will be able to generate these drawings -- this means looking up some original literature references and trying out the software earlier than the night before your lab report is due!

BIG HINT: If you're including a drawing of the structure...maybe you should think about discussing the structure in your report!

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