Magneto-elastic lattice collapse in YCo5

The isomorphic collapse of crystalline lattices under pressure is a rare and intriguing phenomenon - the most famous examples being samarium sulphide and cerium metal. Both lattices are cubic under ambient conditions and collapse isomorphically under pressure, remaining cubic with similar to 15% volume reduction(1-3). In SmS the transition results from a change of the 4 f chemical valence. The collapse in Ce is connected with the altering contributions of the 4 f electrons to the chemical bonding, the details of which are currently much debated(4,5). In contrast, YCo5 is a hexagonal metallic compound with a stable valence and no 4 f electrons. Here, we present a combination of high-pressure X-ray diffraction measurements and density functional electronic-structure calculations to demonstrate an entirely different type of isomorphic transition under hydrostatic pressure of 19 GPa. Our results suggest that the lattice collapse is driven by magnetic interactions and can be characterized as a first-order Lifshitz transition, where the Fermi surface changes topologically. These studies support the existence of a bistable bonding state due to the magneto-elastic interaction.