The body-centered tetragonal structure (tI2) and the alpha -U-type structur
e (oC4) appear in the elements Ce and Pr as new special structures under pr
essure. The structural behaviour of binary alloys between the elements Ce,
Pr and La is the subject of the present study using energy-dispersive X-ray
diffraction under pressures up to 40 GPa. Changes of axial ratios with pre
ssure and alloy composition are analysed. The tI2 structure is stable in Ce
-alloys with a characteristic axial ratio close to 1.70, slightly depending
on alloy composition. The axial ratio increases with Pr content reaching c
/a = root3 but decreases with La content. The oC4 structure in Pr-alloys wi
th La and Ce exhibits special axial ratios b/a approximate to 2 and c/a app
roximate to root3 depending only slightly on pressure and composition. The
stability of these structures at the characteristic axial ratios is discuss
ed in terms of special symmetries in the atomic arrangements. (C) 2001 Else
vier Science B.V. All rights reserved.