How to determine the atomic plane without knowing it beforehand?

[toex]

What I know is that we can determine the grazing angle at which has maximum diffraction intensity, and then apply Bragg's law. But at first we have to know where is the atomic plane, otherwise the grazing angle cannot be defined. So if I have an unknown crystal, how do we actually locate the atomic plane experimentally?

Suppose I have a diffraction spectrum, and I got 2 peaks, are they the first and second order diffraction peak? But then I calculated the atomic plane spacing using 2d sin(theta) = n(wavelength),where d is the atomic plane spacing, theta is the grazing angle, I got 300% difference in the 2 d's, what may account for this?

 

[BigJDubs]

The difference in the two d values could be due to a number of factors. One possible cause is that the two peaks may not actually be the first and second order diffraction peak, but instead two different reflections from different planes. Another possibility is that the crystal structure is not perfectly regular, which can result in different lattice spacings for different atomic planes. Finally, the angle of incidence or grazing angle at which the reflection was measured might have been slightly different for the two peaks, resulting in a different d value.