A note on the structure of TiMnSi2 and the tetrahedrally close-packed 'pentagon–sigma' structure

The hypothetical tetrahedrally close-packed (t.c.p.) structure, which is a pentagon-triangle analogue of the hexagontriangle tr phase, mentioned by Steinmetz, Venturini, Roques, Engel, Chabot & Parth+ in their paper on the structure of TiMnSi 2 [Acta Cryst. (1982), B38, 2103-21081, has actually been found to exist in the W-Fe-Si system at approximate composition WzFeSi by Kripyakevich & Yarmolyuk [Dopov. Akad. Nauk. Ukr. RSR Set. A (1974), 36, 460-463]. This 'pentagon-sigma' structure is one of four t.c.p, structures of composition R6X 7 [R represents atoms with coordination number (CN) > 12, and X atoms with CN = 12]. The other structures are the M phase, the p phase, and a hypothetical structure which has a projection almost identical to the projection of the hypothetical structure given in Fig. 3(b) of Steinmetz et al. (1982).