Published online by Cambridge University Press: 01 February 2011
Powder samples of (ZnX)2x(CuBX2)1-x mixed crystals (B=Ga,In; X=S,Se,Te) were prepared by solid state reaction of the elements and annealed with cooling rates between 2 - 42 K/h in the entire composition range. Structural parameters (e. g. lattice parameter, tetragonal deformation η) were investigated by X-ray and neutron diffraction studies. The microstructure was studied by transmission electron microscopy (TEM) and HRTEM. The chemical composition was determined by EDX analysis on the transmission electron microscope.
The chemical disorder process in the 2(ZnX)x(CuBX2)1-x solid solution series leads to a phase separation, i. e. in a certain composition range (2-phase field) two phases, tetragonal domains and a cubic matrix, coexist. Its width depends on the three-valent cation only and is independent from the size of anion. The habit of the tetragonal domains is like a flat discus, its short extension lies always parallel to their tetragonal c-axis. They are simultaneously arranged on the (100),(010) and (001) planes within the cubic matrix. After nucleation of the domains, their growth obeys t1/2, i.e. it is controlled by diffusion. In the sulfides and selenides systems with In as three-valent cation the tetragonal domains are 'infected' by stannite type (or CuAuI type) cation ordering independently of their orientation, whereas in the tellurides system CuPt type cation ordering was obtained. The tetragonal (ZnX)2x(CuGaX2)1-x mixed crystals exhibit only chalcopyrite type structure and orientation domains without another type of cation ordering.
In the single phase regions the lattice parameter follows Vegrads rule, whereas in the 2-phase field the both phases try to match in the a-b-plane (atetr∼acub ), causing an increase of the tetragonal lattice parameter c and herewith a strong increase of the tetragonal deformation η.
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