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I followed this and it gives an example of how to identify a crystal structure using XRD data.

I have an unknown alkali halide where two peaks were found at $2\theta =25.66$ and $2\theta = 28.3$. I used a copper source for the x-rays. I need to figure out the crystal structure from this. Using the example in the link above, I got

$$\sin ^2 (\theta _1)=0.04931022$$

$$\sin ^2 (\theta _2)=0.05976132$$

Therefore,

$$\frac{\sin ^2 (\theta _1)}{\sin ^2 (\theta _2)} = 0.82511934$$

The PDF only mentions what the result will be for BCC (0.5) and FCC (0.75). Can I reliably conclude that this must be a FCC crystal?

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  • $\begingroup$ No, you can't. Two peaks are way below enough to make any sense. $\endgroup$ – Ivan Neretin Oct 17 '18 at 10:17

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