Iron monosilicide (FeSi) is an intermetallic compound, a

silicide A silicide is a type of chemical compound that combines silicon and a usually more electropositive element. Silicon is more electropositive than carbon. In terms of their physical properties, silicides are structurally closer to borides than t ...

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

that occurs in nature as the rare mineral

naquite Naquite is a mineral of iron monosilicide, FeSi. It was discovered in the 1960s in Donetsk Oblast in Soviet Union, and named fersilicite, but was not approved by the International Mineralogical Association. It was later rediscovered in the Nagq ...

. It is a narrow-bandgap

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

with a room-temperature

electrical resistivity Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...

of around 10 kΩ·cm and unusual magnetic properties at low temperatures. FeSi has a cubic crystal lattice with no

inversion center In geometry, a point reflection (also called a point inversion or central inversion) is a geometric transformation of affine space in which every point is reflected across a designated inversion center, which remains fixed. In Euclidean or ...

; 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 In mathematics, physics and chemistry, a space group is the symmetry group of a repeating pattern in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of the pattern that ...

''P''23 (no. 198). It is similar to the structure of

sodium chloride Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...

, 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 counterions, in iron monosilicide the atoms are all displaced parallel to body diagonals (along threefold axes) from the positions of sodium and chloride. The crystal loses all indirect (chirality reversing) symmetry elements, all fourfold and twofold rotation axes, and many of the threefold axes and translations, but retains some of the twofold screw axes and threefold 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 . The atoms of the other kind are displaced as well, with one of the four at (''y'',''y'',''y''). If ''x'' = −''y'' = ± ≈ ±0.1545 then each ion will be equidistant from seven atoms of the other kind, at a distance of ≈ 0.53523, rather than six at a distance of 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 threefold 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 threefold screw axes without there being a symmetry element taking one to the other. (Threefold 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 Linus Carl Pauling ( ; February 28, 1901August 19, 1994) was an American chemist and peace activist. He published more than 1,200 papers and books, of which about 850 dealt with scientific topics. ''New Scientist'' called him one of the 20 gre ...

investigated the nature of the bonds in iron monosilicide.

References

{{Silicides Iron compounds Transition metal silicides Iron monosilicide structure type

See also

References