Er 3 O 2 F 5 : Ein Erbiumoxidfluorid mit Vernier ‐Struktur

Er 3 O 2 F 5 : An Erbium Oxide Fluoride with Vernier ‐Type Structure Attempts to synthesize multinary erbium‐trifluoride derivatives ( e. g. Er 3 F[Si 3 O 10 ], Er 4 F 2 [Si 2 O 7 ][SiO 4 ], CsEr 2 F 7 , and RbEr 3 F 10 ) from mixtures of ErOF‐contaminated erbium trifluoride (ErF 3 ) itself and appropriate other components (such as Er 2 O 3 and SiO 2 or CsF and RbF, respectively) frequently resulted in the formation of pale pink, transparent, lath‐shaped single crystals of Er 3 O 2 F 5 (orthorhombic, Pnma ; a = 562.48(5), b = 1710.16(14), c = 537.43(4) pm; Z = 4) as by‐product, typically after seven days at 800 °C and regardless of the applied reaction‐container material (evacuated torch‐sealed silica or silica‐jacketed arc‐welded tantalum capsules). Its crystal structure, often described as a vernier‐ type arrangement consisting of two interpenetrating and almost misfitting lattices (ErOF and ErF 3 ), contains two crystallographically different Er 3+ cations in the eight‐ and seven‐ plus ‐one‐fold anionic coordination of bicapped trigonal prisms. Whereas (Er1) 3+ carries four O 2− and F − anions each, (Er2) 3+ resides in the neighbourhood of only two O 2− , but five plus one F − anions. As the main structural feature, however, one can consider O 2− ‐centred (Er 3+ ) 4 tetrahedra which share common edges to form linear double strands of the composition $^{1}_{\infty}\rm \{[O_{2}Er_{3}]^{5+}\}(d(O^{2-}-Er^{3+}) = 220-234 pm, CN = 4)$ . Running parallel to the [100] direction and assembling like a hexagonal closest rod‐packing, their electroneutralization and three‐dimensional interconnection is achieved by three crystallographically independent F − anions (d(F − −Er 3+ ) = 221 − 251 plus 281 pm) in three‐ and two‐ plus ‐two‐fold coordination of the Er 3+ cations, respectively.