Questions

1. From the TGA trace provided, determine the composition just before cooling, the maximum oxygen content on cooling and the composition of the room temperature product. Use these data to account for the preparative conditions used in the experiment. Why is the sample not superconducting after stage 1?
   
   
2. Identify any impurities in your X-ray diffraction profile and determine whether the orthorhombic or tetragonal phase predominates.
   
   
3. Show that the average charge q on the copper in your compound is given by
   
   

   q =	v1/w1	+ 1
   	v2/w2

   
   where v₁ is the titre in the determination of the Cu(III) content, v₂ the titre obtained when analysing the total copper content, and w₁, w₂ the corresponding weights of sample. Calculate q and hence determine the oxygen stoichiometry of the product. Suggest another method of measuring the oxygen content.
   
   
4. Examine the unit cell diagram and answer the following questions:
   
1. How many distinct copper coordination sites are there, and what are their geometies?
   
2. How are these units linked together into an infinite array?
   
3. What are the coordination numbers of yttrium and barium?
   
4. Which structure is formed if the vacant oxygen sites (broken circles) are also filled?
   
   
5. Comment on the fact that the superconducting properties of the materials can deteriorate on standing for long periods.
   
   
6. La₂CuO₄ adopts the K₂NiF₄ structure in which the Cu(II) cation has a Jahn-Teller distorted, quasi-octahedral coordination with four short bonds in the equatorial plane and two long axial bonds. The distorted octahedra share vertices in two dimensions. Explain whether simple band theory can correctly describe the observation that La₂CuO₄ is an insulator. Why does the substitution of strontium for lanthanum in La_2-xSr_xCuO₄ produce metallic conductivity?