The Unusual Coordination Sphere in the Octaammine Calcium(II) Ions of [Ca(NH3)8]Br2 and [Ca(NH3)8]I2 and the Thermal Decomposition of the Iodide to [Ca(NH3)6]I2

We report on the synthesis and crystal structure of the octaammine calcium(II) halides [Ca(NH3)8]Br2, and [Ca(NH3)8]I2, which were synthesized by the reaction of the respective calcium( II) halides with dry liquid ammonia. The compounds form colorless crystals which crystallize isotypically at 123 K with Z = 4 in the orthorhombic space group Pnma with a = 12:0478(3), b = 7:4406(2), c = 15:7216(4) Å, V = 1409:33(6) Å3 for the bromide, and a = 12:1113(4), b = 7:7706(3), c = 16:7145(6) Å, V = 1573:0(1) Å3 for the iodide. Instead of the expected tetragonal antiprism for the eightfold-coordinated Ca2+ ions, we observed a coordination polyhedron best described as a twofold capped trigonal prism. After evaporation of the liquid ammonia and warming of [Ca(NH3)8]I2 to room temperature, [Cu(NH3)6]I2 was obtained as a colorless powder. The hexaammine calcium(II) iodide crystallizes isotypically to [Mn(NH3)6]I2 (CaF2 type) in the cubic space group Fm3̅m with a = 11:18580(6) Å, V = 1399:59(1) Å3, Z = 4 at 293 K

Structural Studies on Ba2SrMg4F14 and Ba2MM´Mg4F14 (M= Ca, Sr, Ba)

The new barium strontium magnesium fluoride Ba2SrMg4F14 has been prepared as an almost single-phase colorless powder by precipitating amorphous precursors and heating them at 650 °C. The compound crystallizes in the space group P42/mnm (no. 136) with a = 12.45514(4), c = 7.46092(3) Å, V = 1157.42(1) Å3 and Z = 4. It is isostructural with the previously known Ca analog, Ba2.2Ca0.8Mg4F14. The structure is built up from a channel-forming network of tetrahedral (MgF6)4 units linked by bridging fluorine atoms. The channels contain the Ba2+ ions (CN = 11) and Sr2+ ions (CN = 8, CaF2 type-related environment). Solid solutions with composition Ba2(Sr1−xCax)Mg4F14 with x = 0.13(1), 0.36(1) and 0.51(1) as well as Ba2(Sr0.83(1)Ba0.17(1))Mg4F14 were synthesized and characterized by powder X-ray diffraction.