The Crystal Structure of KCuF₃

The crystal structure of potassium trifluorocuprate (II) KCuF 3 has been determined by an X-ray analysis. The structure was refined by Fourier method. The crystals are tetragonal, \(a{=}√2a₀{=}5.855\) and c =2 c 0 =7.852Å; space group D 4 h 18 – I 4/ mcm , with four formula units (KCuF 3 ) in the unit cell, where a 0 and c 0 designate the lattice constants of the fundamental pseudo-perovskite structure. This superstructure is due to a displacement of fluorine ions along the copper-fluorine-copper bonds only in the c plane. The atoms are in the following positions: 4 K + in ( a ): (0, 0, 0; \(1/2\), \(1/2\), \(1/2\))+(0, 0, \(1/4\); 0, 0, \(3/4\)): 4 Cu 2+ in ( d ): (0, 0, 0; \(1/2\), \(1/2\), \(1/2\))+(0, \(1/2\), 0; \(1/2\), 0, 0): 4 F - in ( b ): (0, 0, 0; \(1/2\), \(1/2\), \(1/2\))+(0, \(1/2\), \(1/4\), \(1/2\), 0, \(1/4\)): 8 F - in ( h ): (0, 0, 0; \(1/2\), \(1/2\), \(1/2\))±( x , \(1/2+x\), 0; \(1/2-x\), x , 0) with x =0.228. In this structure a Cu 2+ ion is surrounded by a distorted octahedron of F - ions with copper-fluorine distances of 2.25, 1.96 and 1.89Å.