Strontium fluorite

Goldschmidt predicted from his empirical rule that calcium chloride would not have the fluorite structure, and he states that on investigation he has actually found it not to crystallize in the cubic system. Our theoretical deduction of the transition radius ratio allows us to predict that of the halides of magnesium, calcium, strontium and barium only calcium fluoride, strontium fluoride and chloride, and barium fluoride, chloride,... 

In this discussion, two mutually canceling simplifications have been made. For the transition value of the radius ratio the phenomenon of double repulsion causes the inter-atomic distances in fluorite type crystals to be increased somewhat, so that R is equal to /3Rx-5, where i has a value of about 1.05 (found experimentally in strontium chloride). Double repulsion is not operative in rutile type crystals, for which R = i M + Rx- From these equations the transition ratio is found to be (4.80/5.04)- /3i — 1 = 0.73, for t = 1.05 that is, it is increased 12%. But Ru and Rx in these equations are not the crystal radii, which we have used above, but are the univalent crystal radii multiplied by the constant of Equation 13 with z placed equal to /2, for M++X2. Hence the univalent crystal radius ratio should be used instead of the crystal radius ratio, which is about 17% smaller (for strontium chloride). Because of its simpler nature the treatment in the text has been presented it is to be emphasized that the complete agreement with the theoretical transition ratio found in Table XVII is possibly to some extent accidental, for perturbing influences might cause the transition to occur for values a few per cent, higher or lower. 

The favored defect type in strontium fluoride, which adopts the fluorite structure, are Frenkel defects on the anion sublattice. The enthalpy of formation of an anion Frenkel defect is estimated to be 167.88 kJ mol-1. Calculate the number of F- interstitials and vacancies due to anion Frenkel defects per cubic meter in SrF2 at 1000°C. The unit cell is cubic, with a cell edge of 0.57996 nm and contains four formula units of SrF2. It is reasonable to assume that the number of suitable interstitial sites is half that of the number of anion sites. 

Now there must be twice as many fluoride ions as strontium iocs, so the coordination number of the strontium ion must be twice as large as that of fluoride. Coordination numbers of 8 (Sr2 +) and 4 (F ) are compatible with the maximum allowable coordination numbers and with the stoichiometry of the crystal. Strontium fluoride crystallizes in the fluorite lattice (Fig. 4.3). 

In this chapter, the commonly held belief that the terms ceramic and brittle are synonymous has been shown not to be exactly tenable. Oxide single crystals with the rock-salt structure can be deformed plastically at liquid helium temperatures, while strontium titanate shows an inverse BDT and can be deformed plastically at ambient temperatures and below. Oxides with the fluorite structure can be deformed... 

Strontium group Baryte Cerussite Dolomite Fluorite Gypsum Halite Strontianite Strontium sulfate Lithopone Strontium -white Sulfopone Deer et al. (1992) 610-611 Heaton (1928) 105 Mayer (1991) 57 Rouveret Walter (1998) Walter et al. (1998) Zerr Riibencamp (1906/1908) 85... 

The difluorides of strontium, barium, cadmium and mercury all adopt the fluorite crystal stmcture. The difluorides of magnesium and zinc adopt a mtile -type stmcture where each metal cation is surrounded by six F anions at the comers of an octahedron, and each anion is surrounded by three metal caritnis in a planar trigonal arrangement [19]. 

Six and eight, respectively in Figure 13.5, calcium is in approximately octahedral y coordination, and in SrCl2, strontium has the fluorite structure of Figure 13.6. 

---