JAHN-TELLER ORDERING IN KAGOME-TYPE LAYERS OF COMPOUNDS A(2)A'MNIII(3)F12 (A=RB, CS, A'=LI, NA, K)

For a series of new Mn(III) fluorides the crystal structures have been determined by X-ray diffraction on single crystals: Cs2LiMn3F12, spac e group R (3) over bar, Z = 3 (merohedral twin), a = 7.440(1), c = 17. 267(3) Angstrom, R = 1.95%; three isostructural triclinic phases, spac e group P (1) over bar, Z = 2, Cs2NaMn3F12, a = 7.305(1), b = 7.512(1) , c = 10.376(2) Angstrom, alpha = 89.89(3)degrees, beta = 87.32(3)degr ees, gamma = 89.89(3)degrees, R = 2.75%; Rb2NaMn3F12, a = 7.115(1), b = 7.482(1), c = 10.160(2) Angstrom, alpha = 89.92(3)degrees, beta = 86 .52(3)degrees, gamma = 89.51(3)degrees, R = 2.70%; Rb2LiMn3F12, a = 6. 883(1), b = 7.481(1), c = 10.194(2) Angstrom, alpha = 89.42(3)degrees, beta = 87.16(3)degrees, gamma 90.25(3)degrees. R = 4.87%; monoclinic Cs2KMn3F12, space group C2/c, Z = 4, a = 13.112(3), b = 7.571(2), c = 12.672(3) Angstrom, beta = 108.79(3)degrees, R = 5.3%. All structures derive from the Cs2NaAl3F12 parent structure and consist of [Mn3F12] l ayers of corner-sharing octahedra of the Kagome-net-type (3+6 octahedr a units like in the hexagonal tungsten bronze layers). The [MnF6] octa hedra are strongly elongated by the Jahn-Teller effect with ratios of long to short axes of about 1.15. In all structures the long axes are oriented within the layers in an alternating wind-wheel-like pattern c orresponding to an antiferrodistortive Jahn-Teller ordering. Thus, all Mn-F-Mn bridges are asymmetric; the bridge angles are between 136 deg rees and 141 degrees. Magnetic investigations of Cs2NaMn3F12 and Cs2KM n3F12 indicate similar antiferromagnetic exchange interactions (J/k = -2.5 K), but only for the Cs2K compound has 2D and 3D ordering been fo und at low temperatures. The results are discussed in context with the Jahn-Teller ordering and the 'frustrated' exchange situation in the K agome net.