Magnetic and elastic properties of YCo 5 and LaCo 5 under pressure

High hydrostatic pressure destroys the strong ferromagnetism in $\mathrm{Y}{\mathrm{Co}}_{5}$. The transformation proceeds in a stepwise fashion, as a first-order phase transition $({P}_{\mathrm{crit}}=18\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$, and is accompanied by an isomorphic (without change of symmetry) lattice collapse and a topological change of the Fermi surface. Accurate full-potential density-functional calculations enable us to ascribe these phenomena to a quasi-one-dimensional $3d$-like band, whose top under ambient pressure is situated $\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the Fermi level. Lattice compression drives the top of the nondispersive band---and the associated peak in the majority density of states---upward in energy, until they reach the Fermi level and the system becomes unstable. A similar transition at a somewhat higher pressure $(23\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$ is predicted for the isomorphous compound $\mathrm{La}{\mathrm{Co}}_{5}$.