Crystal structure study of new lanthanide silicates with silico-carnotite structure

The crystal structures of new rare earth-based silicate compounds (Ca3Eu2Si3O12, Ca3Gd2Si3O12, Ca3Dy2Si3O12, Ca3Er2Si3O12 and Ca3Lu2Si3O12) have been determined using powder X-ray diffraction. From Rietveld refinement calculations on the collected powder patterns we observe a different distribution of the rare earth ions on the three available crystal sites characterized by different coordination numbers, depending on the ionic radius of the rare earth ion. The reasons of the instability of the silico-carnotite structure for lanthanide ions larger than Eu3+ have been deduced. In addition, in order to detect crystal phase transitions, the powder patterns of Ca3Eu2Si3O12 and Ca3Sm2Si3O12 samples have been collected as a function of the temperature (RT-1000°C range), but no phase transitions have been observed. Synchrotron X-ray diffraction allows us the accurate determination of the RE3+ ions distribution on the three available crystal sites of the silico-carnotite structure. [Display omitted] ► The structure of the Ca3M2Si3O12 (M=Eu, Gd, Dy, Er and Lu) was determined. ► Different distribution of RE3+ ions on the three available crystal sites was observed. ► The instability of the silico-carnotite structure for RE=La→Sm was discussed.