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

Abstract

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) \(\frac{1}{2}\frac{1}{2}\frac{1}{2}\) 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 \(x{=}\frac{1}{4}-u\), y = v , \(z{=}\frac{1}{2}+w\) 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.