What's the significance of the nickel crystal in the Davisson and Germer experiment? Davisson and Germer, in 1927, conducted an successful experiment to prove the existence of matter waves. What's the meaning of using the nickel crystal? I mean, why didn't they use any other materials? (Or, did they?)
 A: They were looking at the scattering of electrons from samples of nickel out of general interest, and because their nickel target was polycrystalline the electrons came off pretty much in all directions with a smooth angular distribution, which they carefully measured. 
Under the electron bombardment the sample of nickel got hot. At this point in their experiment the glass vessel broke, air rushed in to the vacuum, and their nice nickel sample oxidised. 
To repurify it they heated it up (in vacuum) to drive the oxygen off, and let it cool. This annealed the sample, and gave them one big crystal instead of lots of little ones.  
Then when they started their experiment again, to their surprise the electrons came out in particular directions not just a smooth spread, because they now had a single crystal target.  
A: Serendipity.
Davisson and Germer were conducting experiments to investigate the "reflection" of electrons from the surface of nickel to find out how flat the surface was.  
Whilst conducting their experiments in a vacuum chamber some air got in and oxidised the surface of the nickel. To remove the oxide layer they heating the nickel to a high temperature. During the heating process the poly-crystalline nickel turned into relatively large nickel crystals.  
When they conducted their experiments subsequently they found that there were a number of unexpected peaks. They published their results and were surprised to find out later at a conference that they attended that Max Born had used their data and interpreted it as showing the wave nature of electrons!
