why X rays used for crystallography, under what condition diffraction peaks produced?

Answer 1

The wavelength of an X-ray is roughly 1 Å (0.1 nm = 10-10 m), which is on the scale of a single atom. Longer wavelength photons (such as ultraviolet radiation) would not have sufficient resolution to determine the atomic positions. At the other extreme, shorter wavelength photons such as gamma rays are difficult to produce in large numbers, difficult to focus, and interact too strongly with matter.

The crystal is placed in an intense beam of X-rays of a single wavelength, producing a series of spots called reflections. As the crystal is gradually rotated, previous reflections disappear and new ones appear; the intensity of every spot is recorded meticulously at every orientations of the crystal. Multiple data sets may have to be collected, with each covering a full rotation of the crystal and containing tens of thousands of reflection intensities.

ref: http://en.wikipedia.org/wiki/X-ray_crystallography

Answer 2

The Xrays goes into the crystal and reflection is depending of the wavelength.
If the ray first is reflectet by some material, then there will go into the crystal rays with wavelength depending of each basic chemical in the materia.
Each material in the perodic system have special quants of energy they can recieve and send out. Therfore they give rays with different wavelength and frequense.
You Will have a peak for each basic chemical.
I am not quite sure about the reflection of the waves in the crystal. May be you will observe the same peaks that is given by gitters when you send light through it.

Answer 3

the word the previous two people as searching for is diffraction (not reflection), hence X-ray diffractometry and it's the distance between the atoms that the wavelength must be comparable to-about 1angstrom. the second answer is when the Bragg law is satisfied-you look for the equation or work it out by considering constructive interference and angles at which it will occur for a given frequency of EM radiation