Lattice Parameters of Terbium and Erbium at Low Temperatures

Lattice parameters have been determined for terbium between 300 and 77^∘{}K and for erbium between 300 and 10^∘{}K. The temperature dependences of the hexagonal c axis for both elements are in qualitative agreement with a strain-dependent on M·{}M, the scalar product of the moments of adjacent planes of ferromagnetically coupled spins. Below its temperature of ferromagnetic ordering, 220^∘{}K, terbium shows an orthorhombic distortion similar in magnitude to that observed previously for dysprosium. The distortion corresponds to an expansion along b or [101̄0], the easy direction in terbium.