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I already have the answer to my question, but what I would like to know is why. My question is:

**why in the question below is the plane 111 used to find ao and not another plane?**

An ordered compound of NiAl has a cubic structure with one formula unit/cell. One Al atom at the origin and one Ni at 1/2 1/2 1/2. The nine lines of lowest bragg angle on a diffraction pattern have d-spacing of: 4.05, 2.86, 2.34, 2.03, 1.81, 1.65, 1.43, 1.35, 1.28.

**QUESTION**: Evaluate the unit cell dimensions of NiAl.

the formula to use is: dhkl=ao/sqrt(h^2+k^2+l^2)

**The answer is**: ao= sqrt(3)*2.34=

__4.05A__Thank you for any clarification on this topic.