Describing the unit cells Flashcards
how is the first layer of a simple cubic unit cell arranged?
so that each sphere touches 4 others in the layer
how is the second layer of a simple cubic unit cell arranged?
arranged the same way as the first layer; so that each sphere sits directly on top of the the sphere on the layer below it
is the simple cubic unit cell efficient?
no it is very inefficient
- only 52% of the total unit cell volume is occupied by spheres
What is the only element that packs like the simple cubic unit cell?
polonium
how is the first layer of the bcc unit cell arranged?
just like the first layer of the simple cubic unit cell (touching four other spheres in this layer)
how is the 2nd layer of the bcc unit cell arranged?
arranged in the same way as the first layer, but it is instead placed in the HOLLOWS between the spheres of the first layer
how is the 3rd layer of the bcc unit cell arranged?
placed in the HOLLOWS OF THE 2ND LAYER; so that the first and 3d layers line up
true or false; the bcc unit cell has a greater packing efficiency than the simple cubic unit cell
true
how efficient is the packing of the bcc unit cell?
68%
what elements pack in the bcc unit cell
Cr, Fe, all G1 elements
how many spheres touch in the fcc unit cell per layer?
6
how is the 2nd layer of fcc unit cell arranged?
(arranged in the same way) but placed in the hollows of the 1st layer
how is the 3rd layer of fcc unit cell arranged?
placed in the hollows of the second layer in 2 different ways:
- cubic closest packing
- hexagonal closest packing
What does it mean by hexagonal closest packing?
(ABA packing)
3rd layer is placed directly over the spheres of the 1st layer (top view there will be holes seen)
what does it mean by cubic closest packing?
(ABC packing); 3rd layer is placed directly over the spaces in the first layer (no gaps can be seen from top view)
