Crystal field and magnetic properties of ErH2

The Mössbauser effect of the 80.6-keV resonance in ${}_{}{}^{166}{}_{}{}^{}\mathrm{Er}$ has been used to study the magnetic and electronic properties of Er${\mathrm{H}}_2$. The onset of magnetic hyperfine splitting shows a magnetic transition occurring at 2.4 ± 0.1 K. The extrapolated saturation hyperfine field is 2480 ± 20 k${\mathrm{O}}_{\mathrm{e}}$, corresponding to a magnetic moment of $(2.75\mathrm{}\pm 0.08){\mu }_B$ on the Er ion in the ordered state. Measurements of the hyperfine splitting in an external magnetic field show that the crystal-field ground state is a ${\Gamma }_6$ Kramers doublet. These data, as well as an analysis of previously published magnetic-susceptibility data, indicate that the first excited crystal-field level is ≳ 150 K above the ground state. Measurements of the hyperfine spectra of Er as a solute in Y${\mathrm{H}}_2$ give a ${\Gamma }_7$ ground state, which can arise from a small change in the ratio of fourth-order to sixth-order crystal-field parameters on going from this case to that of Er${\mathrm{H}}_2$. All of the above results can be understood on the basis of a "hydridic" (i.e., negatively charged) model for hydrogen in this compound.