Crystal and molecular structure of CCl₄ III: A high pressure polymorph at 10 kbar

The crystal and molecular structure of a high pressure form of carbon tetrachloride (CCl₄ III) was determined at approximately 10 kbar using a diamond‐anvil beryllium pressure cell and a modified Buerger‐type precession camera. CCl₄ III crystallizes in the monoclinic system with a unit cell of the following dimensions: $a=9.079± 0.012 Å, b=5.764± 0.003 Å, c=9.201± 0.004 Å$⁠, and β=104.29±0.05°. The space group is P2₁/c with four molecules per unit cell. Observed infrared spectra indicate that the CCl₄ molecule exhibits regular tetrahedral symmetry at 10 kbar. By using an approximation to the repulsion energy in a least‐squares refinement procedure considering only nonbonded $Cl–Cl$ interactions, an approximate structure was obtained. Subsequent structure factor calculations using a grid‐point sampling procedure yielded a final structure with a reliability factor of 9.56%. CCl₄ III is isostructural with SnBr₄, and contains closest nonbonded $Cl–Cl$ distances of 3.49 Å, significantly less than the normal Van der Waals separation of 3.6 Å. Taking four molecules per unit cell, the calculated density is 2.190 g cm⁻³. A more compact phase than CCl₄ III was predicted in the CCl₄ system on the basis of packing efficiency and the predicted phase (CCl₄ IV) was subsequently verified by visual observation in microscopic studies at pressures in the 35–40 kbar range and temperatures up to 500°C.