I Could XY crystal be a special case of X2YZ crystal?

1. Jan 3, 2017

qnach

There are some types of crystals we call them XY type (i.e. NaCl) or X2YZ type? Could XY type be a special case of X2YZ type by assigning Y=Z?

2. Jan 4, 2017

M Quack

Yes.
you could see Si (X-type) as a special type of the GaAs (XY) type.
There are even order-disorder phase transitions where at high temperature the X-position is filled with 50% atom A and 50% atom B, and at low temperature X=100% A and Y=100% B. Beta-brass is an example. More complicated versions exist, and I am sure you can find XY <--> X2YZ