Amorphous MoS3:: clusters or chains?: The structural evidence

In spite of extensive studies, using a variety of physical and chemical techniques, no consensus has yet been reached even as to the basic structural unit present in a-MoS3, a material with interesting catalytic and electrochemical properties. Here we present neutron diffraction data for a-MoS3 which are compared in both reciprocal- and real-space with the predictions of different structural models. We conclude that a-MoS3 is built from MoS6 groups ([r(MoS)] = 2.44 +/- 0.01 Angstrom) linked together by shared sulfurs to form Mo2S9 units, with the two molybdenum atoms forming Mo-Mo bonds at 2.75 +/- 0.02 Angstrom. The presence of triangular MO3 units in a-MoS3 is ruled out by the poor fit to the data of models based on these clusters. The results are consistent with chain models built from Mo2S9 units, with two of the bridging sulfur atoms bonded as a disulfide group. These units link together to form extended chains by sharing of the terminal monosulfides, creating a second (non-bonded) Mo-Mo distance at around 3.3 Angstrom. (C) 1998 Published by Elsevier Science B.V. All rights reserved.