Structural aspecs on compounds in the CsI-(Cd/Hg)I2 system

Abstract: Structural investigations of compounds in the CsI-(Cd/Hg)I2 system have been performed by means of X-ray diffraction, thermogravimetric and calorimetric techniques. Crystals of the compounds studied were synthesized from aqueous solutions of CsI and CdI2/HgI2 by the slow evaporation of water. In total, the structure of ten compounds were determined by means of X-ray diffraction on single crystals, including two hydrates. The compounds are: Cs2CdI4 (P21/m), Cs2(Cd0.29Hg0.71)I4 (P21/m, Cs2(Cd0.34Hg0.66)I4 (P21/m), Cs2HgI4 (P21/m), Cs3CdI5 (Pnma), Cs3(Cd0.52Hg0.48)I5 (Pbca), Cs3HgI5 (Pbca), CsCdI3.H2O (Pc) and Cs2Hg3I8.H2O (Cm). These structures are built from (Cd/Hg)I4 tetrahedra and Cs ions with I and/or water oxygens as ligands. The coordination number of the Cs ions range from 8 to 11. The mixed cadmium and mercury phases are isostructural or closely related to the pure end phases. Cd is completetly replaced by Hg, and Vegard's law is approximately obeyed for the cell volume per formula unit. Also, the unexpectedly larger cell volumes observed for the cadmium containing compounds are discussed. The hydrates decompose above 350 K yielding Cs2(Cd/Hg)I4 and (Cd/Hg)I2. On mild dehydration the hydrates partially loose water of crystallization. This process is reversible at room temperature, in air. Structural relationships and trends among halides in general are discussed on the basis of the Biltz' concept. A geometrical model for calculation of tetrahedral distorsions is evaluated. In addition, two geometrical models for determination of coordination numbers are developed and tested with good results on atoms with 8 to 11 ligands.