NONSTOICHIOMETRY IN RARE-EARTH FLUORIDES

We report the results of a systematic survey of the rare-earth fluoride systems at 1500°C. Stable fluorite phases with ideal composition LnF2 were observed in the Sm, Eu, and Yb systems. A compound with this structure was also observed in the Tm system, but equilibrium was not established. The stable composition range of this phase in the Sm and Eu systems extends from LnF2.00 to LnF2.00 A second phase of unresolved structure extends from LnF2.25 to LnF2.25 in the Sm and Eu systems. Additional unresolved phases are also present in the Yb system. SmF3 (orthorhombic) is stoichiometric within experimental error. The lattice parameters of the fluorite phases decrease as fluorine is added. Bulk density measurements and X-ray densities indicate that the defect structure is due to F- interstitials. With the possible exception of the Pr and Nd systems, where slight color changes were observed in the LnF3, no reactions occur between Ln and LnF3 at temperatures to 1600°C. Mössbauer spectra of the Eu system indicate that the europium is in either a Eu2+ or Eu3+ state for all phases in the system. The relaxation time for any charge transfer process is greater than 10-8 sec (the lifetime of the 151Eu nuclear excited state). Mössbauer spectra are consistent with the existence of the intermediate phases mentioned above. A few observations were made on mixed systems containing Sm + Pr and Sm + Nd fluorides. © 1969.