Structure of lanthanum prepared by quenching from a liquid state

The structure of lanthanum quenched from a liquid state at cooling rates of 2.5 × 10⁵–10⁷ K/s is investigated. It is shown that a mixture of the face-centered cubic β-La phase stable above 583 K and the metastable lanthanum modification with a seven-layer hexagonal lattice consisting of alternating atomic layers ABCABAC… is formed in rapidly cooled foils. A mechanism is proposed for the formation of the metastable polytype, according to which the polytype is formed as a result of saturation of the main β-phase with well-ordered stacking faults.