The synthesis and X-ray powder diffraction data for the long-known CaSiF6 and CaSiF6·2H2O species are reported. Their crystal structures have been determined from laboratory powder diffraction data by simulated annealing and full-profile Rietveld refinement methods. CaSiF6·2H2O was found to crystallize in the monoclinic P21/n space group with unit-cell parameters: a = 10.48107(9), b = 9.18272(7), c = 5.72973(5) Å, β = 98.9560(6)°, V = 544.733(8) Å3, and Z = 4. The crystal structure of CaSiF6·2H2O, eventually found to be isomorphous with SrSiF6·2H2O (but not with the Mg analogue—a hexahydrate phase), contains centrosymmetric [Ca(μ-H2O)2Ca]4+ dimers, interconnected by hexafluorosilicate anions, in a dense 3D framework. The crystal structure is completed by a further water molecule, terminally bound to the Ca2+ ion, which, consequently, attains a F5O3 octacoordination. Thermodiffractometric measurements allowed the determination of the linear and volumetric thermal expansion coefficients of CaSiF6·2H2O, which showed a minor contraction, along a, on heating. CaSiF6 is trigonal, space group R-3, a = 5.3497(3), c = 13.5831(11) Å, V = 336.66(5) Å3, and Z = 3, and isomorphous with several other species of MIIAIVF6 or MIAVF6 formulation, among which several silicates, germanates, and stannates.

K3Al3F12⋅nH2O (n=2,1) are the successive products of the thermal decomposition of K(H3O)2AlF6. Both structures are orthorhombic, built up from disconnected hexagonal-tungsten-bronze (HTB)-related layers, with K+ cations and H2O molecules inserted between. For n=2, there are two disconnected layers along a presenting different octahedra tilting [a=13.5135(2) Å, b=7.0433(1) Å, c=12.2252(2) Å, V=1163.60(3) Å3, Z=4, space group Pnma], whereas for n=1, the stacking is reduced to only one HTB layer along c [a=7.0523(5) Å, b=12.1005(9) Å, c=6.7057(5) Å, V=572.24(7) Å3 (at 170 °C), Z=2, space group Pmmn] after the departure of one water molecule. The thermodiffractometry ends in the α-KAlF4 form.