New orthoferrates of rubidium and cesium: alpha-, beta-Cs-5[(FeO4)-O-III] and A(7)(I)[(FeO4)-O-IV][(FeO4)-O-V] (A(I) = Rb, Cs)

The four title compounds, all crystallising with new crystal structure types (monoclinic, space group P2(1)/c), are orthoferrates with isolated [FeO4] tetrahedra. In the two ferrates(III), alpha-Cs-5[FeO4] (a = 1133.92(10), b = 1269.49(13), c = 725.05(6)pm, beta = 99.073(7)degrees, R1 = 0.0289) and beta-Cs-5[FeO4] (a = 880.78(13), b = 1067.4(2), c = 1115.7(2) pm, beta = 97.354(3), R1 = 0.0259), these tetrahedra are of nearly ideal tetrahedral geometry (d(Fe-O) = 189-192 pm; angle(O-Fe-O) = 108-111degrees) consistent with the d(5) electron configuration of Fe. In contrast, in the mixed valence orthoferrates A(7)[FeO4](2) (A=Rb/Cs, a = 637.19(5)/666.0(1), b = 1047.39(8)/1097.4(2), c = 2070.66(14)/2156.6(4) pm, beta = 92.47(1)/92.83(1)degrees, R1 = 0.0677/0.0466), that can be synthesized starting from A, AO(2) and Fe2O3 under oxidizing conditions, iron assumes the oxidation states +IV and +V. The Fe-O distances in the two crystallographically independent tetrahedra are in the range of 174 to 179 pm and thus between those in ferrates(IV) und (V). As a result of the d(4) and d(3) electron configuration of Fe respectively, the [FeO4] tetrahedra show significant Jahn-Teller distortion (angleO-Fe-O = 102-121degrees).