Jan 27 Flashcards
(6 cards)
What is the convention for symmetry-related atoms in the same unit cell for atom position?
As each axis is 0-1, then you could have 1-x, y+1/2 etc. You essentially add or subtract multiples of 1 from any coordinate to get the other atom that is symmetrically related but the same atom for that molecule
What does it mean that something has 3 fold symmetry? Where would the axis be for that for a cube?
It means that something when rotated once, during that 360 degrees it appears exactly the same 3 times. This is from corner to corner for a cube, so if you look at it facing one corner, you would also see 3 sides
What is are the following crystal systems: Tetragonal, orthorhombic, rhomobhedral, hexagonal, monoclinic?
Tetragonal: It is a rectangle, lengths of 2 sides are equal.
Orthorhomic: Rectangle where none of the axis lengths are equal to eachother
Rhombohedral: All the sides are equal to eachother like a cube, all of the angles are the same but not 90 degrees like a cube
Hexagonal: like hexagonal prisim: but only a segment of it
Monoclinic is 2 angles same, no lengths are the same
Triclinic: None of the angles or lengths are the same
What are primitive vs centered unit cells?
Primitive have nodes at each of the corners. Centered have a lattice points at the corners too but also one in the center.
What are the following types of centering: Primitive, body-centered, face-centered? How many nodes do they have per unit cell?
Primitive is a node at each of the 4 corners. It has 1 node as it has 8 nodes shared with 8 unit cells.
Body centered has 1 at each corner but 1 other node in the center of it. It then has 2 nodes as 1 to itself and 1 from having 1/8 of each of the 8 corners
Face centred has a node on each corner, and one on each face. From each corner, get a total of 1 node. From each face, it is shared by another unit cell so 6 nodes/2 cells means it gets 3 nodes total from that. So 3+1=4 nodes total
