Distinguishing the scattering contributions of isoelectronic atomic species by
means of conventional x-ray- and/or electron diffraction techniques is a
difficult task. Such a problem occurs when determining the crystal structure
of compounds containing different types of atoms with equal number of
electrons. We propose a new structural model of Cu(InxGa1−x)3Se5 which is
valid for the entire compositional range of the CuIn3Se5–CuGa3Se5 solid
solution. Our model is based on neutron and anomalous x-ray diffraction
experiments. These complementary techniques allow the separation of scattering
contributions of the isoelectronic species Cu+ and Ga3+, contributing nearly
identically in monoenergetic x-ray diffraction experiments. We have found that
CuIII3Se5 (III=In,Ga) in its room temperature near-equilibrium modification
exhibits a modified stannite structure (space group I4¯2m). Different
occupation factors of the species involved, Cu+, In3+, Ga3+, and vacancies
have been found at three different cationic positions of the structure
(Wyckoff sites 2a, 2b, and 4d) depending on the composition of the compound.
Significantly, Cu+ does not occupy the 2b site for the In-free compound, but
does for the In-containing case. Structural parameters, including lattice
constants, tetragonal distortions, and occupation factors are given for
samples covering the entire range of the CuIn3Se5–CuGa3Se5 solid solution. At
the light of the result, the denotation of Cu-poor 1:3:5 compounds as
chalcopyrite-related materials is only valid in reference to their
composition
