Iron monosilicide (FeSi) is an intermetallic compound, a silicide of iron that occurs in nature as the rare mineral naquite. It is a narrow-bandgap semiconductor with a room-temperature electrical resistivity of ca. 10,000 Ohm·cm and unusual magnetic properties at low temperatures. FeSi has a cubic crystal lattice with no inversion center; therefore its magnetic structure is helical, with right-hand and left-handed chiralities.
The structure is the prototype of the "iron monosilicide structure type", of space group P23 (no. 198). It is similar to the structure of sodium chloride, with four iron atoms and four silicon atoms in each unit cell. Whereas in sodium chloride the eight atoms are at the corners of a cube and each ion is surrounded by six counter-ions, in iron monosilicide the atoms are all displaced parallel to body diagonals (along 3-fold axes) from the positions of sodium and chloride. The crystal loses all indirect (chirality reversing) symmetry elements, all 4-fold and 2-fold rotation axes, and many of the 3-fold axes and translations, but retains some of the 2-fold screw axes and 3-fold axes of the sodium chloride crystal structure. As an atom is moved from (0,0,0) to (x,x,x), the other three atoms of the same kind move as well, creating a chiral arrangement except when x is a multiple of 1/4. The atoms of the other kind are displaced as well, with one of the four at (y,y,y). If then each ion will be equidistant from seven atoms of the other kind, at a distance of rather than six at a distance of 1/2 and one at a distance of in the sodium chloride structure.
In the case of iron monosilicide, x=0.13652 and y=0.8424 (or −0.1576), or x=−0.13652 and y=0.1576, so the seven silicon atoms around an iron atom are not all the same distance from the iron atom. Each silicon atom sits in a similar cage of iron atoms. The cages only have 3-fold rotational symmetry, with three slightly different interatomic distances between the central atom and the seven surrounding atoms (to 3 atoms, 3 atoms, and 1 atom respectively). There are right-handed and left-handed 3-fold screw axes without there being a symmetry element taking one to the other. (Three-fold screw axes also exist in sodium chloride but are related by a mirror.) This means that iron monosilicide crystals exist in two different enantiomorphs, depending on the signs of x and y.
In 1948 Linus Pauling investigated the nature of the bonds in iron monosilicide.