High quality nonstochiometric Tb- and Gdbased single crystals Tb 0.81Ca0.19F2.81 (TCF), Tb0.76Sr 0.24F2.76 (TSF), Gd0.85Ca0.15F 2.85 (GCF), Gd0.83Sr0.17F2.83 (GSF) are analyzed in crystallographic detail by single-crystal X-ray diffraction measurements. TSF shows only main reflections, and its structure is refined to a disordered P63/mmc structural model. Additional satellite reflections observed in GSF lead to an ordered twinned P3̄c1 structural model. The other two crystals TCF and GCF cannot be assigned uniquely to any of these space groups. Instead, their structures are interpreted as the coexistence of both space groups in separate domains.
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