Volume dependence of magnetic hyperfine fields in Eu-intermetallic compounds

High-pressure Mössbauer studies on ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ intermetallics (Eu${\mathrm{Al}}_2$, Eu${\mathrm{Al}}_4$, Eu${\mathrm{Cu}}_2$, Eu${\mathrm{Cu}}_5$, Eu${\mathrm{Sn}}_3$, ${\mathrm{Eu}}_{0.5}$ ${\mathrm{Yb}}_{0.5}$ ${\mathrm{Sn}}_3$, Eu${\mathrm{Zn}}_2$, and Eu${\mathrm{Pt}}_2$) provide information about the volume dependence of the effective hyperfine field $B_{\mathrm{eff}}$ at the ${\mathrm{Eu}}^{2+}$ nucleus. These measurements have been performed at 4.2 K and pressures up to 65 kbar. The results show that (i) the volume dependence of the transferred hyperfine field $B_{\mathrm{thf}}$ is the dominating contribution to $\frac{\partial B_{\mathrm{eff}}}{\partial \ln V}$; (ii) the magnitude of $\frac{\partial B_{\mathrm{thf}}}{\partial \ln V}$ can be explained only if one assumes the participation of $s$, $p$, and $d$ conduction electrons in the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction responsible for $B_{\mathrm{thf}}$; and (iii) the exchange integrals for non-$s$ conduction electrons ($J_{p,f}$ and/or $J_{d,f}$) are more sensitive to volume changes than that for $s$ conduction electrons ($J_{s,f}$).