Incommensuration and other structural anomalies in rhenium disilicide

The structure of solidification-processed rhenium disilicide is investigate d by X-ray diffraction and transmission electron microscopy. It is usually reported to have the tetragonal C11(b) MoSi2 structure and the stoichiometr y of ReSi2. In the present study it is found that rhenium disilicide exhibi ts an off-stoichiometric Si-deficient composition of Re4Si7. In addition, s everal structural instabilities are observed by electron microscopy includi ng firstly an orthorhombic distortion of about 0.6% from the tetragonal C11 (b) MoSi2 structure with spot splitting across the {110} twin plane, second ly an incommensuration along the a axis with a periodicity of 1.3 nm, third ly another longer period modulation along the c axis, fourthly a weak monoc linic distortion (beta approximate to 90.4 degrees) in some specimens, as r evealed by spot splitting in the high-order Laue zone reflections of [100] diffraction patterns, accompanied by twinning on the (001) planes, and fift hly much larger distortions in some [010] patterns plus a chevron structure in the image which may result from a displacive transformation. These stru ctural anomalies are probably related to the stoichiometry of Re4Si7 which may result in a composite modulated structure where the Si sublattice has a larger average periodicity along the a axis than the Re sublattice, as is the case with manganese silicides.