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Four new mixed-valent chain alkali metal ( A ) sulfido ferrates of the general structure familyA   1  +  xFe  x  II    Fe   1  −  x   III    S  2${A}_{1+x}\left[{\text{Fe}}_{x}^{\text{II}}{\text{Fe}}_{1-x}^{\text{III}}{\text{S}}_{2}\right]$were synthesized in the form of tiny green-metallic needles from nearly stoichiometric melts reacting elemental potassium with natural pyrite ( A  = K) or previously prepared Rb 2 S/Cs 2 S 2  with elemental iron and sulfur ( A  = Rb/Cs). The crystal structures of the compounds were determined by means of single crystal X-ray diffraction: In the (3+1)D modulated structure of K 7.15 [FeS 2 ] 4  (space group  Ccce (00 σ  3 )0 s 0,  a  = 1363.87(5),  b  = 2487.23(13),  c  = 583.47(3) pm,  q  = 0,0,0.444,  R 1 = 0.055/0.148,  x  = 0.787), a position modulation of the two crystallographically different undulated[FeS4  /  2]∞  1${}_{\infty }{}^{1}\left[{\text{FeS}}_{4/2}\right]$tetrahedra chains and the surrounding K cations is associated with an occupation modulation of one of the three potassium sites. In the case of the new monoclinic rubidium ferrate Rb 4 [FeS 2 ] 3  ( x  = 1  3$\frac{1}{3}$; space group  P 2 1 / c ,  a  = 1640.49(12),  b  = 1191.94(9),  c  = 743.33(6) pm,  β  = 94.759(4)°,  Z  = 4,  R 1 = 0.1184) the undulation of the tetrahedra chain is commensurate, the repetition unit consists of six tetrahedra. In the second new Rb ferrate, Rb 7 [FeS 2 ] 5  ( x  = 0.4; monoclinic, space group  C 2/ c , K 7 [FeS 2 ] 5 -type;  a  = 2833.9(2),  b  = 1197.36(9),  c  = 744.63(6) pm,  β  = 103.233(4)°,  Z  = 4,  R 1 = 0.1474) and its isotypic mixed Rb/Cs-analog Rb 3.6 Cs 3.4 [FeS 2 ] 5  ( a  = 2843.57(5),  b  = 1226.47(2),  c  = 759.890(10) pm,  β  = 103.7170(9)°,  R 1 = 0.0376) the chain buckling leads to a further increased repetition unit of 10 tetrahedra. For all mixed-valent ferrates, the Fe–S bond lengths continuously increase with the amount ( x ) of Fe(II). The buckling of the chains is controlled through the local coordination of the S atoms by the variable number of A cations of different sizes.

New mixed-valent alkali chain sulfido ferrates A 1+ x [FeS 2 ] ( A = K, Rb, Cs; x = 0.333–0.787)