I don't study material science, so I am useless.
Okay, I did some researching on what this is for the past 30 minutes:
n * lambda = 2d sin theta
d = (2sine theta)/(n*lambda)
Do you know what n is suppose to mean?
n is suppose to be the order? And usually it is suppose to be 1?
I looked up the copper k-alpha x rays and it's wavelength is about 1.5418..
I don't know if that is helpful or even related to this problem...
Er.. I don't know... sorry... hopefully somebody who studies this can help you way more than I can.
I should stop trying to answer questions that I don't know.