Comparative Studies of the Structure and Microstructure of Zn 2x (CuB III ) 1-x X 2 Semiconductors (B III =Ga,In; X=S,Se,Te) Susan Schorr 1 , Gerald Wagner 2 , Michael Tovar 3 , and Denis Sheptyakov 4 1 Solar Energy Research, Hahn-Meitner-Institut Berlin, Glienicker Str. 100, Berlin, 14109, Germany 2 Institute of Mineralogy, Crystallography and Materials Science, University Leipzig, Scharnhorststr. 20, Leipzig, 04275, Germany 3 Structural Research, Hahn-Meitner-Institut Berlin, Glienicker Str. 100, Berlin, 14109, Germany 4 Laboratory for Neutron Scattering, ETH Zurich & PSI Villigen, Villigen, 5232, Switzerland ABSTRACT Powder samples of (ZnX) 2x (CuBX 2 ) 1-x mixed crystals (B=Ga,In; X=S,Se,Te) were prepared by solid state reaction of the elements (annealing) for the entire composition range. Then they were cooled down with rates between 2 ñ 42 K/h. Structural parameters (e. g. lattice parameter, tetragonal deformation η) were determined by X-ray and neutron diffraction experiments. The microstructure was studied by transmission electron microscopy (TEM) and HRTEM. The chemical composition was measured by EDX analysis on the transmission electron microscope. The chemical disorder process in the 2(ZnX) x (CuBX 2 ) 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 type of the three-valent cation only and is independent of the type of the anion. The habit of the tetragonal domains is like a flat discus, its short extension lies always parallel to the 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 t 1/2 , i.e. it is controlled by diffusion. In the sulfide and selenide systems with In as three-valent cation the tetragonal domains are ëinfectedí by stannite type (or CuAuI type) cation ordering independent of their orientation, whereas in the telluride system CuPt type cation ordering was observed. The tetragonal (ZnX) 2x (CuGaX 2 ) 1-x mixed crystals exhibit only chalcopyrite type structure and orientation domains without any type of cation ordering. In the single phase regions outside the two-phase field the lattice parameters follow Vegardís rule, whereas in the 2-phase field the both phases try to match in the a-b-plane (a tetr ~a cub ), causing an increase of the tetragonal lattice parameter c and herewith a strong increase of the tetragonal deformation η. INTRODUCTION Solid solutions of D II X VI and A I B III X VI 2 compounds (D=Zn; A=Cu; B=Ga, In and X=S, Se Te) are of interest due to possible photovoltaic applications. The advantage would be a variation of the band gap from the large value of the binary wide band gap semiconductor ZnX towards the band gap of the ternary chalcopyrite type end member CuBX 2 . This enables to create tailor- made photovoltaic materials [1,2]. Both end members of the (ZnX) 2x (CuBX 2 ) 1-x solid solution series belong to the adamantine compound family, their structures are closely related but non-isotype. The chalcopyrite-type Mater. Res. Soc. Symp. Proc. Vol. 1012 © 2007 Materials Research Society 1012-Y03-05