Ammonium fluorosilicate Structure

  Ammonium fluorosilicate has three above polymorphs: α-(NH4)2SiF6 anatomy is cubic (space accumulation Fm3m, No. 225) and corresponds to the mineral cryptohalite. The β anatomy is trigonal (scalenohedral) and occurs in attributes as mineral bararite.A third (γ) anatomy was apparent in 2001 and articular with the hexagonal 6mm symmetry. In all three configurations, the (SiF6)2− octahedra are abiding in layers. In the α form, these layers are erect to directions. In the β- and γ- forms, the layers are erect to the c-axis.The silicon atoms of α-(NH4)2SiF6 (alpha), accept cubic close(st) packing (CCP). The γ anatomy has hexagonal close(st) packing and the β-(NH4)2SiF6 has archaic hexagonal packing.In all three phases, 12 fluorine atoms acquaintance the (NH4)+.
  Although bararite was claimed to be metastable at allowance temperature,it does not arise one polymorph has anytime angry into another.Still, bararite is brittle abundant that cutting it for spectroscopy will aftermath a little cryptohalite.Even so, ammonium fluorosilicate assumes a trigonal anatomy at pressures of 0.2 to 0.3 GPa. The acknowledgment is irreversible. If it is not bararite, the appearance is at atomic actual carefully related.
  The hydrogen bonding in (NH4)2SiF6 allows this alkali to change phases in means that accustomed salts cannot. Interactions amid cationsand anions are abnormally important in how ammonium salts change phase.(To apprentice added about the β-structure, see Bararite.)