Pressure-induced structural transition and antiferromagnetism in elemental terbium

Abstract

Structural and magnetic properties of rare-earth Tb metal have been studied by means of neutron powder diffraction at pressures up to 9 GPa in the temperature range 7--290 K. A structural phase transition from the initial hexagonal close-packed (hcp) to the Sm-type rhombohedral phase develops gradually at high pressures above 4 GPa. The initial ferromagnetic state in the hcp phase is suppressed and an antiferromagnetic state is developed in the pressure-induced phase. In the Sm-type structure and the temperature range below ${T}_{\mathrm{MO}}=110\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (at 9 GPa) down to 50 K, long-range order of Tb magnetic moments located in the layers resembling hexagonal close-packing type is formed with a propagation vector ${k}_{\mathrm{AF}1}=(0\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}\frac{1}{2})$, while the layers resembling cubic close-packing type remain disordered. This partial disorder disappears at temperatures below 50 K when magnetic order, including the moments in the latter layers, develops with a propagation vector ${k}_{\mathrm{AF}2}=(\frac{1}{2}\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}\frac{1}{2})$. The relative stability of the hcp and Sm-type structures under pressure was examined by density functional theory calculations, providing significant support to the experimental findings. The calculated bulk moduli of the hcp and Sm-type phases are close to the experimentally determined ones and the estimate ${P}_{0}\ensuremath{\approx}4\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ obtained for the equilibrium transition pressure is close to the onset pressure found in real material. The volume collapse at the hcp to Sm-type transition was evaluated to amount to $0.4\phantom{\rule{0.16em}{0ex}}{\AA{}}^{3}$ per atom.