An Electron Diffraction Study on the Crystal Structure of a New Modification of Chromium

Fine particles of chromium were produced by evaporation and condensation of chromium in argon at low pressures (Kimoto et al.: Japan. J. appl. Phys. 2 (1963) 702). Electron diffraction studies have shown that the particles have the ordinary b. c. c structure of chromium when the argon contains a small amount of air or oxygen but they have a new structure when the argon is pure. The new structure has a cubic primitive lattice with lattice constant a 0 =4. 588±0. 001 Å. The unit cell contains eight chromium atoms; the probable space group is Pm3; one atom occupies position (a) 000, one atom occupies position (b) \ (121212\) and the remaining six atoms distribute themselves so that on the average one-quarter of an atom occupies each of the 24-fold position (1) x y z ; where \ (x=14-u\), y = v, \ (z=12+w\) with parameters u =0. 2/100, v =4/100 and w =1/100. This is a disordered atomic arrangement derivable from the β-tungsten structure. The shortest Cr-Cr distance is 2. 34 Å. The new modification transforms into the ordinary b. c. c. form above 400°C.