Iron monosilicide (FeSi) is an

intermetallic compound An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic bonding, metallic alloy that forms an ordered solid-state Chemical compound, compoun ...

, 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. Silicides are structurally closer to borides than to carbides. Similar to borides and carb ...

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

that occurs in nature as the rare mineral

naquite Naquite is an iron silicide mineral with the formula 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 ...

. It is a narrow-bandgap

semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

with a room-temperature

electrical resistivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

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 similar to that 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 parallele to body diagonals (along 3-fold axes) from the positions of sodium and chloride. The crystal therefore loses symmetry, but retains the 2-fold screw axes and the 3-fold axes of the sodium chloride crystal structure. As a result of this displacement, an iron atom is about as close to a silicon atom on the same 3-fold axis as to any of the other six silicon atoms around it (instead of being

\sqrt 3

time further away as in sodium chloride). This means that each iron atoms has seven nearby silicon atoms, and conversely each silicon atom sits in a similar (

enantiomorphic In chemistry, a molecule or ion is called chiral () if it cannot be superposed on its mirror image by any combination of rotations, translations, and some conformational changes. This geometric property is called chirality (). The terms are d ...

) 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. In 1948 Linus Pauling investigated the nature of the bonds in iron monosilicide.Acta Crystallogr. (Sep 1948). 1, 212-216
The nature of the bonds in the iron silicide, FeSi, and related crystals
' Linus Pauling and A. M. Soldate

References

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

See also

References