MERIEDRIC TWINNING IN NEW COSi3P3 SEMICONDUCTING COMPOUND. CRYSTAL STRUCTURE AND RAMAN SPECTROSCOPY. H. Vincent, J. Kreisel, O. Chaix-Pluchery and R. Madar LMGP (URA CNRS 1109), ENS de Physique de Grenoble, 38402 St Martin d'Hères, France.
New cobalt phospho-silicide CoSi3P3 has been synthesized and its crystal structure determined by single crystal X-ray diffraction. CoSi3P3 crystallizes with the apparent orthorhombic symmetry : a = 5.900(1) Å, b = 5.703(1) Å, c = 12.736(1) Å. Only one extinction rule is observed, it concerns reflections 0k0 with k = 2n+1. Wilson statistic and non-linear optical tests excluding some symmetry center, attempt to solve the structure in P2212 group was done but unsuccessfull. Assuming the presence of pseudo 21 screw-axis along the other principal directions, crystal structure was then solved in the space group P212121 by direct method and Fourier difference analysis.
Results of this first study being not very satisfactory, the crystal structure was refined in P21 space group, that is a subgroup of P212121.This refinement did not give better results ; crystallographic and weighted factors were so much mediocre R = 0.065, Rw = 0.084, and P and Si atom distribution on non-metallic sites was not clearly established. Polarized Raman spectra of single crystals confirming the monoclinic symmetry, meriedric twinning has been considered. A last refinement including four twinned domains gave much better results : agreement factors are R = 0.037, Rw = 0.044 , and P and Si atoms are completely ordered on non-metallic sites.
The crystal structure can be described by zigzag chains of octahedra occupied by Co atoms and zigzag chains of tetrahedra occupied by P or Si atoms. All these infinite chains are parallel to the b-axis. Two chains of a same kind are separated each other by a chain of other kind. Magnetic susceptibility and electronic conductivity have been measured between 300 K and 10 K. CoSi3P3 is diamagnetic at room temperature and paramagnetic below 60 K. CoSi3P3 is semiconducting at room temperature with an energy gap of about 0.12 eV.