# Solid state and Bragg's Equation

A frst order reflection from (111) plane is observed for LiX with 2theta =24.6 degree (X-ray of wavelength 1.54 angstrom). Assuming LiX to be a cubic crystal system, calculate the length of the side of the unit cell in angstrom. This is a question of JAM 2014, here, the length is considered as a=2a for ionic system in a solution provided by Arihant's book. Why is it so? Is it correct?

• That's what happens when you are reading a book written by a troll. Indeed, bluntly saying $a=2a$ is a great way of leaving your readers confused and dumbstruck. – Ivan Neretin Jan 22 at 13:52
• $a = 2a$ is a mistake of some kind. – MaxW Jan 22 at 15:25

If LiX crystallizes like NaCl, then the unit cell length is twice the distance of the shortest Li - X distance. The magnitude of the answer given in the book looks like it could be an interatomic distance, but the calculation does not make any sense to me (it does say $$a = 2a$$, as @Ivan_Neretin already pointed out in the comments. Also, when a calculation drops the units midway and sticks them back in the result, it demonstrates a certain nonchalance that does not instill confidence in the source.