NOVEL REENTRANT HIGH-PRESSURE PHASE-TRANSITION IN LANTHANUM

High presure x-ray diffraction experiments with synchrotron radiation in an extended pressure range up to 67 GPa reveal a novel phase transition of La: At about 60 GPa the low pressure fcc structure of La is recovered from the distorted fcc (d-fcc) phase. The reappearance of the high symmetry cubic structure at higher pressure implies that the d-fcc phase, which also appears in other lanthanides, is not a precursor of the low symmetry structures caused by the so-called f delocalization. A possible mechanism for the reentrant behavior is considered and the phase diagram of La is discussed.