The optical spectra of ${\mathrm{U}}^{4+}$ and ${\mathrm{U}}^{5+}$ doped into hafnon (HfSi${\mathrm{O}}_4$) and thorite (ThSi${\mathrm{O}}_4$) are reported. However, no significant information was obtained on the validity of a previous crystal-field fit of the absorption spectrum of ${\mathrm{U}}^{4+}$ doped into zircon (ZrSi${\mathrm{O}}_4$). New experiments confirm the interpretation of Judd and Runciman of the Zeeman behavior of zircon: ${\mathrm{U}}^{5+}$.

• Received 22 April 1977

DOI:https://doi.org/10.1103/PhysRevB.18.185