CMY 716: Lecture 4 and 5 Comprehensive Exam Questions With Elaborated Answers.

What is a space group? - correct answer a group of symmetry operations which leave the infinitely extended, regularly repeating pattern of a crystal unchanged Draw the space groups P1 bar, P2, Pm and give equivalent positions - correct answer pg. 26 Draw the space groups P2 subscript 1, P2/m, C2, C222, P222, P4 subscript 1 and give equivalent positions - correct answer London Draw the space groups P2 subscript 1/m, P2 subscript 1/c and P2/c and give equivalent positions - correct answer London Draw the space groups Cm and Cc and give equivalent positions - correct answer London What are equivalent positions? - correct answer The objects (the motifs) related by symmetry within the unit cell take up what are called general equivalent positions What are special positions? - correct answer 1. Positions that coincide with a symmetry element. 2. e.g. (0,0,0) coincides with an inversion centre. 3. ONLY special positions for inversion, rotation, lattice conditions, mirrors and NOT screw axes - because screw axes have a translational component. NB! Exercise 1 and 2! - correct answer Handouts What are Miller indices used for? - correct answer Used to distinguish between different directions in a plane Miller planes practice pg. 33 - correct answer pg. 33 Give the Miller indices for a BCC (body-centered cell) - correct answer summaries True or false? X-rays are diffracted by electrons and not Miller planes however these planes do go through electron clouds of the crystal - correct answer True General points on Miller indices and planes NB! - correct answer 1. When a miller index=0 then the plane is parallel to that axis 2. the smaller the miller index the closer the plane is to being parallel to that axis 3. the larger the miller index the closer the plane is to being perpendicular to that axis. Draw the following planes in a unit cell: (100), (200), (12(bar)1), (121), (11(bar)1) - correct answer summaries What does the diffraction pattern depend on? - correct answer The diffraction pattern depends on the unit cell, arrangement of molecules within the unit cell (x,y,z), atom types,.... Give and prove Bragg's law - correct answer When the X-ray is incident onto a crystal surface, its angle of incidence, θ, will reflect with the same angle of scattering, θ. And, when the path difference, d is equal to a whole number, n, of wavelength, λ, constructive inter