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 [$10\overline{1}0$], the easy direction in terbium.