Crystal structures of binary alloys between La, Ce and Pr under pressure

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.