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By adding a hot aqueous solution containing KF and K 2 [CO 3 ] to another hot aquatic brine of Pr[NO 3 ] 3  ⋅ 5 H 2 O, Sm[NO 3 ] 3  ⋅ 5 H 2 O or Eu[NO 3 ] 3  ⋅ 5 H 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  RE FO 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.

Synthesis, crystal-structure refinement and properties of bastnaesite-type PrF[CO 3 ], SmF[CO 3 ] and EuF[CO 3 ]