Valence instability of Eu in EuPd$_3$B$_x$ ($ 0 < x < 0.53$)

In a joint theoretical and experimental study large series of intermetallic compounds EuPd$_3$B$_x$ and GdPd$_3$B$_x$ are characterized by X-ray diffraction, metallography, EPMA and chemical anlysis assessing the range of formation up to $x < 0.53$ and $x < 0.42$, respectively. Density functional based electronic structure calculations predict a valence change in EuPd$_3$B$_x$ above $x = 0.19(0.02)$ from a non-magnetic Eu$^{3+}$ state into a magnetic Eu$^{2+}$ state which is reflected in a discontinuity of the lattice parameter. Consistent with the calculations X-ray diffraction data show a kink in the lattice parameter at $x = 0.22(0.02)$. X-ray absorption spectroscopy measurements assign this kink to a transition into a heterogeneous mixed valence state of Eu. The influence of external pressure on the valence instability will be discussed.