Allotropy in Some Rare-Earth Metals at High Pressures.

Allotropes of lanthanum, cerium, praseodymium, and neodymiumc have been observed at elevated pressures with an x-ray diffraction camera which incorporates a diamondanvil, high-pressure cell. In each case a high-pressure modification was observed which has a face-centered cubic structure. At room temperature the unit cell dimensions (alphao) for the high-pressure face-centered cubic structures and approximate pressures at which they were determined are as follows: La, 5.17 A(23 kb); Ce, 4.82 A(15 kb); Pr, 4.88 A(40 kb); and Nd, 4.80 A(50 kb). The unit cell dimensions for the high-pressure forms of La, Pr, and Nd apparently have never been reported.