A review of the systematic prediction of new quaternary materials is given. Many new quaternary adamantine phases are proposed. The preparation of a range of polycrystalline selenides and sulphides of composition I-III-IV-?-VI4 was undertaken. In twelve selenide compounds and three of the sulphur compounds the structure was found to be tetragonal with a c/a ratio of approximately 2. A systematic variation of unit cell volume and c/a was found over the entire range of compounds grown. Single crystals of CuGaSn?Se4, CuGaGed?Se4, AgGaSn?Se4 and AgGaGe?Se4 were grown by a modification of the Pend?lofen vapour transport method. This interesting technique proved that it is possible to transport simultaneously three non-volatile metals in a closed tube. Single crystal diffraction methods have been used to solve the crystal structure of CuGaSn?Se4. The structure was found to be the partially ordered defect chalcopyrite structure, space group I42d (D2d12) copper and gallium are on one metallic sublattice and tin and vacancy on the other of the ABX2 pattern. Room temperature lattice parameters were determined for all the compounds, and the variations of the lattice parameters with temperature up to the melting point of CuGaSn?Se4 were studied using a high temperature x-ray camera. This compound shows an increase in the tetragonal compression with temperature. The results are interpreted in terms of the thermal expansions of the A-Se (A = 1/2 (Cu + Ga)) and B-Se (B = 1/2 (Sn + ?)) bonds. The fundamental absorption edges of the compounds were studied at room temperature, and the lowest direct transitions were determined at room temperature by reflectance and wavelength modulation reflectance. The reflectance measurements of CuGaSn?Se4 were studied at different temperatures between 4K to room temperature (295K) to find the temperature dependence of the band structure. It was deduced that all the compounds have direct gaps. The principles of operation and construction details of a sensitive wavelength modulation spectrometer are described. The derivative spectra of these compounds show clear improvement of resolution over the conventional technique. The melting points and transition temperatures were determined for some of these compounds by differential thermal analysis.
|Date of Award||1981|