  The sudden decrease in molar volume exhibited by most lanthanides under high
pressure is often attributed to changes in the degree of localization of their
4f-electrons. We give evidence, based on electrical resistivity measurements of
dilute Y(Gd) and Y(Tb) alloys to 120 GPa, that the volume collapse transitions
in Gd and Tb metals have different origins, despite their being neighbors in
the periodic table. Remarkably, the change under pressure in the magnetic state
of isolated Pr or Tb impurity ions in the nonmagnetic Y host appears to closely
mirror corresponding changes in pure Pr or Tb metals. The collapse in Tb
appears to be driven by an enhanced negative exchange interaction between 4f
and conduction electrons under pressure (Kondo resonance) which, in the case of
Y(Tb), dramatically alters the superconducting properties of the Y host, much
like previously found for Y(Pr). In Gd our resistivity measurements suggest
that a Kondo resonance is not the main driver for its volume collapse. X-ray
absorption and emission spectroscopies clearly show that 4f local moments
remain largely intact across both volume collapse transitions ruling out 4f
band formation (delocalization) and valence transition models as possible
drivers. The results highlight the richness of behavior behind the volume
collapse transition in lanthanides and demonstrate the stability of the 4f
level against band formation to extreme pressure.
