ON THE CRYSTAL-STRUCTURES OF THE MONOFLUO ROSELENITES MSEO2F (M=K, RB, CS)

The alkali metal monofluoroselenites MSeO2F (M = K, Rb, Cs) have been prepared as single crystals. KSeO2F crystallizes in the space group P2 (1)/m and contains distorted trigonalpyramidal SeO2F- anions, the geom etry of which has been determined for the first time. These complex an ions are bridged by long-range Se-F interactions to linear chains. RbS eO2F and CsSeO2F crystallize in the space group Pm3BARm. In contrast t o KSeO2F, the trigonal-pyramidal SeO2F- anions in these latter two flu oroselenites are disordered. A phase transition between the ordered an d disordered modification has been established in the case of KSeO2F. Vibrational spectra have been recorded and are correlated to the resul ts of the crystal structure determination.