Final Exam

December 21, 2000

7. O. Gourdon, J. Hanko, F. Boucher, V. Petricek, M.-H. Whangbo, M. G. Kanatzidis, and M. Evain (p. 1398-1409) calculated the electronic band structure for K_1/3Ba_2/3AgTe₂. Would you expect this material to be a p-type or n-type semiconductor? Hint: assume that the metal ions exist in their most common oxidation states.

Answer:

K_1/3Ba_2/3AgTe₂: the hint tells us that the metal oxidation states are K⁺, Ba²⁺, and Ag⁺, requiring the Te to be –[(1/3)(+1) + (2/3)(+2) + (+1)]/2 = –4/3. Since the natural oxidation state of Te is Te^2–, there is an excess of positive charge on the Te atoms, implying the the compound is a p-type semiconductor.