Question

W (9)Explain the following terms: (9%) (a) Fermi level (b) p-type semiconductors (c) type- II superconducting (10) TiO, has a Rutile structure (a) What is the coordination number (C.N.) does each Ti atom have? (b) How many Ti atoms are at the body center of the unit cell (c) How many Oxygen atoms are at the quadrants of the bottom face in a unit cell? (9%) (11) Use the r./r.ratio to determine the C.N. for (a) NaCl (Na=113 pm, CI = 167 pm) (b) ZnS (Zn2+ = 74 pm, S = 170 pm) (6%) (12) Write down the unit cell parameters and allowed Bravais lattice for the following crystal system? (a) hexagonal (b) tetragonal (6%) 22 (13) Explain the meaning for the following symbols used in Bravais lattice? (a) P (b)I (0) C (9%)

question 9 10 11 12 13

Answer 1

9 (a) Fermi Level: it is the highest occupied orbital in a metal at absolute zero.

(b) p-Type semiconductors: This is an extrinsic type semiconductor in which the current carriers are positive holes and this semiconductor can be prepared by doping Si and Ge is with group 13.

(c) Type II superconductivity materials are the alloys made up of ceramics which behave as super conductors even at high temperature, yetterium barium copper oxide is the best example of it.

Q10. (a) In Rutile, the coordination number of Titanium is 6 (octahedral) while that for oxygen is 3(trigonal planer)

(b) only one Ti atom is at the body center while other eight are at the corners of the unit cell,

(c) Two Oxygen atoms are at the bottom of the quadrants and as these are shared with other unit cells, total oxygen will be ½ X2 =1

Q11. The radius ratio gives information about coordination numbers:

Ratio                                      Coordination

0.225-0.414                                        4

0.414-0.732                                        6

Therefore R⁺/R^- (NaCl) = 113/167 = 0.677, suggest, the coordination number of 6

And for R⁺/R^-(ZnS) = 74/170 = 0.435, suggest, the coordination number of 6 for both the ions, but in this case the coordination number is 6 fot Zn⁺² as well as S^-2, this is the serious drawback of radius ratio rules, keep in mind that radius ratio only predicts (does not confirm) the coordination number.

Q12. (a) Hexagonal: a = b ≠ c; a = b = 90°; g =120°

(b) Tetragonal: a = b ≠ c; a = b = g = 90°

Q13. Bravaise lattice can be symbolized and the symbols have specific meaning;

(a)Symbol P: this symbol suggests that all the atoms (species) are present at the corners

(b) Symbol I: this symbols suggests that atoms (species) are present at the body centers

(c) Symbol C: this symbols suggests that atoms (species) are present at the face centers