First, a single crystal allows us to study the anisotropy of properties. I work with two-dimensional and one-dimensional objects, which is very important. For example, single crystals of titanium trisulfide at 4 K shows the relationship between resistivity along the principal crystallographic axes of 1: 100: 100000. It is clear that the nature of the conduction along these axes will be completely different.

Second, to study of the electronic structure of these materials our group uses techniques of photoemission spectroscopy in different variants. These techniques are very sensitive to the surface state due to of the escape depth of photoelectrons small (no more than 10 A). Single crystals allow to obtain completely fresh surface by cleavage in a high vacuum chamber directly into the spectrometer. For such a surface of the ceramic samples will not be able to get, because there will always be cleaved crystal faces previously been in contact with air, and adsorb various impurities. Therefore, even if we do not study the distribution of electrons in momentum space, which definitely requires a single-crystal sample, it is still high quality spectra can only be obtained on single crystals.

Third. Transmission electron microscopy (TEM) require preparation foil sufficiently thin to be transparent to electrons. Typically, this is 1000 - 500 A. Get a foil made of ceramic non-metallic material is very difficult, and in many cases impossible. Monocrystalline same samples allow to prepare such foil by technology such as graphene - with adhesive tape.

The last of my knowledge, but probably not the last of the existing reasons to use single crystals consists in close to the theoretical density. For example, the use of films can not completely guarantee the absence of pores. In many cases, the comparison of these objects with the single crystal reveals the role of discontinuities.