Ag2ZnSnSe4 and Cu2ZnSnSe4 are quaternary semiconductors belonging to the adamantine compound family, contain only abundant elements, which makes these materials promising candidates for engineering on their base of different high-efficient and low-cost photovoltaic devices. (Ag1-xCux)2ZnSnSe4 solar cells with Ag alloying recently reached efficiency of 10.2%. By neutron powder diffraction our group could show that Cu2ZnSnSe4, Cu2ZnSnS4 and Cu2ZnGeSe4 as well as solid solutions of these compounds occur in the kesterite structure. Moreover intrinsic point defects, like copper vacancies and Cu-Zn anti-sites, have been verified. Because these defects have a critical influence on the electronic properties of the material, detailed knowledge on point defect type and concentration is crucial. The aim of the experiment is to get correlated information about changes in lattice parameters and cation site occupancies of (Ag1-xCux)2ZnSnSe4 over the whole range of x from 0 to 1.