Potassium fluoroiodate, KIO2F2, is ferroelastic at room temperature, crystallizing in space groupPca21with four formulas in the unit cell. The lattice constants at 298.2thinsp;deg;K area=8.39430plusmn;0.00004,b= 5.97918plusmn;0.00005,c=8.44680plusmn;0.00003 Aring;, for lgr;CuKagr;1=1.540562 Aring;. The integrated intensities of 4169 reflections within a hemisphere segment of reciprocal space with radius (sinVthgr;)/lgr;=1.02 Aring;minus;1were measured by diffractometer, and resulted in 772 symmetryhyphen;independentFmeasand sgr;Fmeas. The crystal structure was solved by use of a combination of Patterson and Fourier series, and refined by the method of least squares. The final agreement factorR=0.046, and the dgr;Rnormal probability plot is essentially linear. The atomic coordinates are in agreement with the 1940 values of Helmholz and Rogers. Within the IO2Fminus;2ion, Fndash;Indash;F is linear (average Indash;F=2.000 Aring;) with Ondash;Indash;O forming a plane (average Indash;O=1.768 Aring;) normal to Fndash;Indash;F. Compressive stress along lsqb;001rsqb; ferroelastically transforms a crystal with orientationabcinto one with orientationcba. The compressive strength of KIO2F2is comparable to the coercive stress. Atomic displacements for some atoms in the ferroelastic transformation, as IO2Fminus;2ions rotate, are as large as 2.4 Aring;.
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