Synthesis, crystal-structure refinement and properties of bastnaesite-type PrF[CO${}_3$], SmF[CO${}_3$] and EuF[CO${}_3$]

By adding a hot aqueous solution containing KF and K${}_2$[CO${}_3$] to another hot aquatic brine of Pr[NO${}_3$]${}_3$ ⋅ 5H${}_2$O, Sm[NO${}_3$]${}_3$ ⋅ 5H${}_2$O or Eu[NO${}_3$]${}_3$ ⋅ 5H${}_2$O with a 1.3 times excess of the anion-providing solution, amorphous water-insoluble powders of PrF[CO${}_3$], SmF[CO${}_3$] and EuF[CO${}_3$] can be obtained. Through hydrothermal treatment at 210 °C for five days crystalline powders could be synthesized and their crystal structure was refined with Rietveld methods based on PXRD data. The named compounds crystallize in the bastnaesite-type structure with a = 710.912(12) pm, c = 976.811(6) pm for the praseodymium, a = 704.77(2) pm, c = 971.83(4) pm for the samarium and a = 700.734(6) pm, c = 969.066(8) pm for the europium compound, all hexagonal with Z = 6. Upon heating them, the compounds lose CO${}_2$ and fluoride oxides REFO emerge. Thermogravimetric experiments with crystalline samples show thermal stability up to 420 °C for PrF[CO${}_3$], 400 °C for SmF[CO${}_3$] and 340 °C for EuF[CO${}_3$], but decomposition below 200 °C for the amorphous ones. Infrared spectroscopy confirms only marginal portions of [OH]${}^{-}$ instead of F${}^{-}$ anions in all cases. The RE${}^{3+}$ cations are coordinated by 9 + 2 anions at distances between 236 and 254 pm plus 326 pm to F${}^{-}$ anions and oxygen atoms bonded to carbon as oxocarbonate anions [CO${}_3$]${}^{2-}$. Triggered by ultraviolet radiation, the bulk sample of EuF[CO${}_3$] shows a poor red luminescence.