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When a powder sample of a crystal is used to known its crystalline structure, on the diffractogram there are spikes corresponding to the constructive interference, at specific angles, by different crystallography planes.

Why each spike is associated with a single family of planes? If the particles of the powder are in random positions, there could be more than one family which contributes to constructive interference.

Thank you for your time.

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  • $\begingroup$ For a given incident angle of the X-rays on the sample, a family of planes will be diffracted at a specific angle only, which is determined by the inter-atomic spacing of the planes. This is well defined in the Bragg's law. Also, in reality, even though we say powder method, we are not grinding the crystal into fine powder, but into tiny crystallites so as to maintain its structure. There are no random orientations of the planes. A crystal is characterized by regular arrangement of planes. $\endgroup$ – UKH Dec 19 '16 at 0:45

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