Shear transformation in the layered compound KAlF4 : low temperature phase structure and transformation mechanism

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Abstract
The layered compound KA1F4 undergoes a structural phase transition in the vicinity of 250 K. Both phases have been studied by X-ray and electron diffraction and by profile refinement of the neutron powder diffraction patterns. The room temperature structure (tetragonal space group D5 4h — P 4/mbm — Z = 2 ; a = b = 5.045 Å, c = 6.159 A) derived from the T1A1F4 type is confirmed. We have determined the low temperature structure (monoclinic, P 2 1/m, Z = 4; am = 7.340 A, bm = 7.237 A, cm = 6.407 Å, β= 106.8° at 4 K) closely related to the KFeF4 structure. From structural argument it is shown that this first order transition is mainly characterized by a gliding of the successive sheets in the [100] direction. This can explain the 16° misorientation of twinned microcrystals resulting from the transition as shown by X-ray diffraction and by electron microscopy

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