Ferroelastic KIO2F2: Crystal structure and ferroelastic transformation

Abstract

Potassium fluoroiodate, KIO2F2, is ferroelastic at room temperature, crystallizing in space group Pca21 with four formulas in the unit cell. The lattice constants at 298.2 °K are a=8.39430±0.00004, b= 5.97918±0.00005, c=8.44680±0.00003 Å, for λCuKα1=1.540562 Å. The integrated intensities of 4169 reflections within a hemisphere segment of reciprocal space with radius (sinϑ)/λ=1.02 Å−1 were measured by diffractometer, and resulted in 772 symmetry-independent Fmeas and σFmeas. 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 factor R=0.046, and the δR normal probability plot is essentially linear. The atomic coordinates are in agreement with the 1940 values of Helmholz and Rogers. Within the IO2F−2 ion, F–I–F is linear (average I–F=2.000 Å) with O–I–O forming a plane (average I–O=1.768 Å) normal to F–I–F. Compressive stress along [001] ferroelastically transforms a crystal with orientation abc into one with orientation cba. The compressive strength of KIO2F2 is comparable to the coercive stress. Atomic displacements for some atoms in the ferroelastic transformation, as IO2F−2 ions rotate, are as large as 2.4 Å.