X-ray diffraction analysis of stan nite, wurtz-stan nite and pseudo-cubic quaternary compounds by Rietveld method

Room temperature X-ray powder diffraction measurements were carried out on nine polycrystalline samples of the Cu₂B^IIC^IVX₄ (B=Mn, or Fe, or Co; C=Si, or Ge, or Sn; X=S, or Se or Te) magnetic semiconductor compounds. The diffraction patterns were used to show the equilibrium conditions and to derive crystalline parameters values. The results showed that four of these compounds have a tetragonal stan nite structure with space group 142 m(N^o 121), two and orthorhombic wurtz-stan nite structure with space group Pmn2₁(N^o 31) and three of them and orthorhombic pseudo-cubic structure with space group F222 (N^o 22). In each case, the structure was refined using the Rietveld method. When the obtained atomic parameter values for the tetragonal compounds were plotted as a function of molecular weight W, it was found that the values of the atomic positions, the cation-anion bond distances, tetragonal distortion and internal distortion of the compounds containing S and /or Se lay on different lines. Also, it was found that when the experimental points of the cation-anion bond distances d_Cu-VI, d_II-VI and d_IV-VI were plotted against the effective lattice parameter a_c = (V/N)^1/3, a linear variation of these distances with a_c was obtained. Values of the ionic energy gap C_i and homopolar energy gap E_h using the Phillips-Van Vechten scheme, with the present experimental crystallographic results as well as using the atomic data, were determined. It was found that the observed and predicted values of C_i and E_h lie on the same straight line. (Author)