JAHN-TELLER-ORDERING IN COMPOUNDS A2[MnF3(SO4)](A = K, Rb, NH4, Cs)
Werner Massa, Ute Jacobs, Jürgen Pebler
Fachbereich Chemie der
Philipps-Universität, D-35032 Marburg, Germany
E-mail: massa@ps1515.chemie.uni-marburg.de
Keywords: manganese(III),
fluorosulfatomanganates(III), crystal structures, Jahn-Teller
ordering, phase transition, magnetism
Compounds A2[MnF3(SO4)], first described by Edwards [1] and Battacharjie [2] attracted our attention because the published orthorhombic structure of the K compound [1] showed the compressed octahedral geometry for the [MnF4O2] units, unusual for the Jahn-Teller ion Mn3+. We found for all four title compounds (for K below 310 K) isotypic monoclinic structures (space group P21/c, see Table) with chain anions built from [MnF4O2] units alternatingly elongated
Cs2[MnF3(SO4)] 298 K |
(NH4)2[MnF3(SO4)] 298 K |
Rb2[MnF3(SO4)] 213 K |
K2[MnF3(SO4)] 190 K |
| a = 736,54(4) pm | a = 729,91(3) pm | a = 727,1(1) pm | a = 722,3(1) pm |
| b = 1161,10(6) pm | b = 1115,66(8) pm | b = 1109,1(1) pm | b = 1074,4(1) pm |
| c = 921,05(4) pm | c = 881,25(5) pm | c = 877,6(1) pm | c = 844,3(1) pm |
| = 92,30(1)° | = 91,512(4)° | = 92,26(1)° | = 91,68(1)° |
| R = 0,0292 | R = 0.0248 | R = 0.0334 | R = 0.0473 |
along the bridging F-Mn-F axis or along the O-Mn-O axis to the bridging sulfato ligand (antiferrodistortive Jahn-Teller-ordering). According to this ordering scheme, 1D antiferromagnetic properties were detected with reduced exchange energies and inverse dependence of the bridge angle. The K compound undergoes at 310 K a reversible phase transition, probably of 2nd order to an orthorhombic supergroup Pnca. In this phase by a 2D dynamical Jahn-Teller effect the seemingly compressed octahedral geometry appears corresponding to Edwards structure determination.