Unusual Coexistence of Magnetic and Nonmagnetic Mo6 Octahedral Clusters in a Chalcohalide Solid Solution:Synthesis,X-ray Diffraction,EPR,and DFT Investigations of Cs3Mo6I_6~iI_(2-x)~iSe_x~iI6~a

摘要

The Cs3Mo6I_6~iI_(2-x)~iSe_x~iI6~a series has been obtained by a solid-state route.There is evidence for a solid solution between the composi tions Cs3Mo6I6~iI_(0.8)~iSe_(1.2)~iI6~a and Cs3Mo6I_6~iI_(0.4)~iSe_(1.6)~iI6~a (space group:R3c,Z=6;a = 16.7065(4),c = 20.5523(4) A,V=4967.8(2) A3 and a = 16.6354(3),c =20.5444(4) A,V= 4923.7(2) A3,respec tively).The structure of this new series is based on magnetic [Mo6I6~iSe_2~iI6~a]~(3-) and diamagnetic [Mo6I7~iSe~iI6~a]~(3-) units with 23 and 24 valence electrons per Mo6 cluster,respectively.For a particu lar x,the structure of Cs3Mo6I6~iI_(2-x)~iSe_xI6~a is based on a mix ture of (x-1) [Mo6I6~iSe2~iI6~a]~(3-) with (2-x)[Mo6I7~iSe~iI6~a]~(3-).This leads to an average [Mo6I6~iI_(2-X)~iSe_xI6~a]~(3-) ionic unit deduced from single-crystal X-ray diffraction investigations.The two inner positions of the average face-capped [Mo6I_(8-x)~iSe_x~iI_6~a]~(3-) ionic units (located on the threefold axis of the unit) are randomly occupied by iodine and selenium,whereas the other ligand positions are fully occupied by iodine.Low-temperature electron paramagnetic resonance (EPR) studies reveal a signal split into two components with g > g.The reciprocal double integration intensity of the EPR signal versus T graph reveals a typical Curie law behavior.A density functional theory (DFT) study indicates that occupation of the inner position on the threefold axis by selenium atoms is preferred energetically among the three possible distributions of selenium atoms.The comparison of experimental and theoretical g values confirms the crystallo-graphic analysis and agrees with the axial elongation of the Mo6 cluster within the crystal structure.