Single-Crystal Growth and Classification of EuAlF5 and Solid Solutions M(II)1−xEuxAlF5 (M=Ca, Sr, Ba) within the Structural Family of Tetragonal M(II)M(III)F5 Compounds

Abstract

The compound EuAlF5, as well as the solid solutions Ca0.19(1)Eu0.81(1)AlF5, Sr0.15(1)Eu0.85(1)AlF5, Sr0.55(1)Eu0.45(1)AlF5, Sr0.77(1)Eu0.23(1)AlF5, and Ba0.62(1)Eu0.38(1)AlF5, crystallize in colorless tetragonal columns. These have been prepared by solid state reactions at 900°C, starting from mixtures of the binary fluorides. According to Vegard's rule the solid solution Sr1−xEuxAlF5 shows a linear dependence of the crystal volume on the molar ratio Eu/Sr. All crystal structures have been refined from single-crystal diffractometer data. EuAlF5 and the M1−xEuxAlF5 (M=Ca, Sr) compounds obtained are isotypic with β-SrAlF5. They crystallize in a superstructure in space group I41/a (no. 88) with 64 formula units and lattice parameters a≈19.9 Å, c≈14.3 Å. The structure is characterized by chains of trans-corner-sharing [AlF4/2F2/1] and branched [AlF5/1F1/2] octahedra forming a channel structure. Inside the channels isolated ordered dimeric units [AlF4/1F2/2]2 are located. The divalent metal atoms show coordination numbers 8 and 9; they connect the [AlF6] octahedra into a three-dimensional structure. Ba0.62(1)Eu0.38(1)AlF5 is isotypic with the corresponding Sr compound Ba0.43(1)Sr0.57(1)AlF5, and it crystallizes with 16 formula units and lattice parameters a=14.3860(7) Å, c=7.2778(3) Å in space group I4/m (no. 87). The network structure is identical with that of EuAlF5. Instead of the dimeric units, infinite chains [AlF4/1F2/2]∞ of trans-corner-sharing [AlF6] octahedra extending along the c- axis are located inside the channels. The bridging fluorine atoms of this chain show large anisotropic displacement parameters, but no superstructure reflections have been observed for this compound.