  We have performed the first high-pressure magnetotransport and x-ray
diffraction measurements on ferromagnetic LaCo5, confirming the theoretically
predicted electronic topological transition driving the magneto-elastic
collapse seen in the related compound YCo5. Our x-ray diffraction results show
an anisotropic lattice collapse of the c-axis near 10 GPa that is also
commensurate with a change in the majority charge carriers evident from
high-pressure Hall effect measurements. The coupling of the electronic,
magnetic and lattice degrees of freedom is further substantiated by the
evolution of the anomalous Hall effect, which couples to the magnetization of
the ordered state of LaCo5.
