T(Z) DIAGRAM AND OPTICAL-ENERGY GAP VALUES OF ZN1-ZMNZGA2SE4 ALLOYS

The T(z) diagram of the system Zn1-zMnzGa2Se4 was obtained from X-ray diffraction and differential thermal analysis measurements. It was fou nd that at lower temperatures, a single phase solid solution occurs ac ross the whole composition range and values of the lattice parameters were determined as a function of z. At higher temperatures, an order-d isorder transition occurs to what is probably a partially ordered tetr agonal structure when 0 < z < 0.83, and to a disordered defect zinc-bl ende structure when 0.83 < z < 1.0. In the T(z) diagram, it is found t hat all of the phase boundaries show discontinuities at z approximate to 0.83. Although the two terminal compounds have very similar tetrago nal structures, they have different space group symmetries, and hence the discontinuities at z approximate to 0.83 are attributed to an abru pt change in space group symmetry at this composition. The values of t he optical energy gap are found to show a similar discontinuity at the same composition. Two other similar alloy systems have recently been found to show similar behavior, and the results for the three systems are compared. (C) 1995 Academic Press, Inc.