THE CRYSTAL-STRUCTURES OF THE TETRAFLUOROMANGANATES(III) AMNF4 (A=K,RB, CS)

The crystal structures of AMnF4 compounds (A = K, Rb, Cs) were determined by X-ray diffraction on twinned single crystals: The K and Rb compound are isostructural: space group P2(1)/a, Z = 4; KMnF4: lattice constants a = 769.9(2), b = 764.4(2), c = 576.9(1) pm, beta = 90.54(3)-degrees; R/WR = 0.029/0.025 for 481 reflections; pseudomerohedral (100) twin. RbMnF4: lattice constants a = 782.2(5), b = 777.7(3), c = 605.0(2) pm, beta = 90.83(4)-degrees; R/WR = 0.0665/0.0513 for 471 reflections. The structure of CsMnF, has been refined anew in the space group P4/n (Z = 4) as a merohedral (110) twin (lattice constants a = 794.40(6), c = 633.76(9) pm; R/WR = 0.051/0.044 for 820 reflections). All three structures derive from the TlAlF4 type and show puckered [MnF4]- layers with an antiferrodistortive order of strongly Jahn-Teller distorted [MnF6] octahedra. The elongated axes (Mn-F for KMnF4: 215.8, RbMnF4 215.2, CsMnF4 216.8 pm) alternate with shorter ones (188.1, 188.3, 185.4 pm) within the layer planes. The terminal Mn- F bonds are very short (180.3, 180.6, 181.7 pm). The Mn- F- Mn bridging angles are 140.6-degrees and 146.4-degrees for the K, 148.3-degrees and 152.1-degrees for the Rb, and 161.9-degrees for the Cs compound. The magnetostructural relations and the three puckering types are discussed for all the AMnF4 layered structures.