Self-Assessment: Crystalline Materials, Exam Solutions

3.091 OCW Scholar Self-Assessment Exam Crystalline Materials Solution Key Write your answers on these pages. State your assumptions and show calculations that support your conclusions. RESOURCES PERMITTED: PERIODIC TABLE OF THE ELEMENTS, TABLE OF CONSTANTS, AN AID SHEET (ONE PAGE 8½" × 11"), AND A CALCULATOR. NO BOOKS OR OTHER NOTES ALLOWED. Exam 2, Problem #1 z z y y x (012) x [111] (a) For each unit cell above, draw the crystallographic feature indicated and label it clearly. (b) Named after Salvadore Dali, dalium (Da) is BCC. Its molar volume is 6.66 cm3/mol. Calculate the density of atoms in (001) of Da. Express your answer in atoms/cm2. (c) Here is the (011) plane in a unit cell of magnesium oxide (MgO) which is FCC. Indicate the positions of all atoms lying in the plane. Represent atoms as 2-dimensional slices of space-filling spheres. The values of ionic radii are Mg2+ = 0.65 Å and O2– = 1.34 Å. Your sketch need not be drawn to scale; however, you must convey relative values of the ionic dimensions. Exam 2, Problem #2 (a) You discover that someone has been using your x-ray generator and has changed the target/anode. To determine the chemical identity of the new target, you go ahead and operate the x-ray generator and find the wavelength, λ, of the Kα peak to be 0.250 Å. What element is the target made of? (b) Hilary Sheldon conducts an experiment with her x-ray diffractometer. A specimen of tantalum (Ta) is exposed to a beam of monochromatic x-rays of wavelength set by the Kα line of titanium (Ti). Calculate the value of the smallest Bragg angle, θhkl, at which Hilary can expect to observe reflections from the Ta specimen. DATA: λ K of Ti = 2.75 Å; α lattice constant of Ta, a = 3.31 Å (c) Sketch the emission spectrum (intensity versus wavelength) of an x-ray target that has been bombarded with photons instead of with electrons. Assume that the incident photons have more than enough energy to dislodge K-shell electrons in the target. On your spectrum label the features associated with Kα radiation, Kβ radiation, and Lα radiation. With photons, we expect to see the characteristic lines but NO Bremsstrahlung, because the interaction between photons and atoms in the target causes no deflection. Exam 3, Problem #1 Silver bromide (AgBr) has rock salt crystal structure, i.e., FCC Bravais lattice with the ion pair, Ag+ and Br− as basis. The dominant defect in AgBr is the Frenkel disorder. (a) Does the Frenkel disorder in AgBr create vacancies of Ag+, vacancies of Br−, or both? Explain. The ionic radii are 0.67 Å for Ag+ and 1.96 Å for Br−. We expect the smaller ion to form interstitials and create vacancies. Here, Ag+ is the smaller ion, so there should be Ag+ interstitials and vacancies on the Ag+ sublattice. (b) Calculate the temperature at which the fraction of Frenkel defects in a crystal of AgBr exceeds 1 part per billion = 1 ppb = 10–9. The enthalpy of Frenkel defect formation, ΔHF, has a value of 1.16 eV / defect, and the entropic prefactor, A, has a value of 3.091. Exam 3, Problem #2 (b) On each of three separate drawings of one face of an FCC unit cell, indicate one of each of the following: (1) substitutional impurity; (2) vacancy; (3) interstitial impurity. Final Exam, Problem #3 (b) Calculate the atomic packing density along [011] direction of aluminum (Al). Express your answer in units of atoms cm–1. Final Exam, Problem #4 Give the rotational symmetry of each of the following patterns. Express your answer as n-fold. 5-fold 3-fold 4-fold