Group 8 element
Group 8 is a group of chemical elements in the periodic table. It consists of iron, ruthenium and hassium, they are all transition metals. Like other groups, the members of this family show patterns in electron configuration in the outermost shells, resulting in trends in chemical behavior. "Group 8" is the modern standard designation for this group, adopted by the IUPAC in 1990. In the older group naming systems, this group was combined with group 9 and group 10 and called group "VIIIB" in the Chemical Abstracts Service "U. S. system", or "VIII" in the old IUPAC "European system". Group 8 should not be confused with "group VIIIA" in the CAS system, group 18, the noble gases. While groups of the periodic table are sometimes named after their lighter member, the term iron group does not mean "group 8". Most it means a set of adjacent elements on period 4 of the table that includes iron, such as chromium, iron and nickel; the first three elements are hard silvery-white metals. Hassium has not been isolated in macroscopic pure form, its properties have not been conclusively observed
A light-emitting diode is a semiconductor light source that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons; this effect is called electroluminescence. The color of the light is determined by the energy required for electrons to cross the band gap of the semiconductor. White light is obtained by using multiple semiconductors or a layer of light-emitting phosphor on the semiconductor device. Appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared light. Infrared LEDs are used in remote-control circuits, such as those used with a wide variety of consumer electronics; the first visible-light LEDs were of low intensity and limited to red. Modern LEDs are available across the visible and infrared wavelengths, with high light output. Early LEDs were used as indicator lamps, replacing small incandescent bulbs, in seven-segment displays. Recent developments have produced white-light LEDs suitable for room lighting.
LEDs have led to new displays and sensors, while their high switching rates are useful in advanced communications technology. LEDs have many advantages over incandescent light sources, including lower energy consumption, longer lifetime, improved physical robustness, smaller size, faster switching. Light-emitting diodes are used in applications as diverse as aviation lighting, automotive headlamps, general lighting, traffic signals, camera flashes, lighted wallpaper and medical devices. Unlike a laser, the color of light emitted from an LED is neither coherent nor monochromatic, but the spectrum is narrow with respect to human vision, functionally monochromatic. Electroluminescence as a phenomenon was discovered in 1907 by the British experimenter H. J. Round of Marconi Labs, using a crystal of silicon carbide and a cat's-whisker detector. Russian inventor Oleg Losev reported creation of the first LED in 1927, his research was distributed in Soviet and British scientific journals, but no practical use was made of the discovery for several decades.
In 1936, Georges Destriau observed that electroluminescence could be produced when zinc sulphide powder is suspended in an insulator and an alternating electrical field is applied to it. In his publications, Destriau referred to luminescence as Losev-Light. Destriau worked in the laboratories of Madame Marie Curie an early pioneer in the field of luminescence with research on radium. Hungarian Zoltán Bay together with György Szigeti pre-empted led lighting in Hungary in 1939 by patented a lighting device based on SiC, with an option on boron carbide, that emmitted white, yellowish white, or greenish white depending on impurities present. Kurt Lehovec, Carl Accardo, Edward Jamgochian explained these first light-emitting diodes in 1951 using an apparatus employing SiC crystals with a current source of battery or pulse generator and with a comparison to a variant, crystal in 1953. Rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide and other semiconductor alloys in 1955.
Braunstein observed infrared emission generated by simple diode structures using gallium antimonide, GaAs, indium phosphide, silicon-germanium alloys at room temperature and at 77 kelvins. In 1957, Braunstein further demonstrated that the rudimentary devices could be used for non-radio communication across a short distance; as noted by Kroemer Braunstein "…had set up a simple optical communications link: Music emerging from a record player was used via suitable electronics to modulate the forward current of a GaAs diode. The emitted light was detected by a PbS diode some distance away; this signal was played back by a loudspeaker. Intercepting the beam stopped the music. We had a great deal of fun playing with this setup." This setup presaged the use of LEDs for optical communication applications. In September 1961, while working at Texas Instruments in Dallas, James R. Biard and Gary Pittman discovered near-infrared light emission from a tunnel diode they had constructed on a GaAs substrate. By October 1961, they had demonstrated efficient light emission and signal coupling between a GaAs p-n junction light emitter and an electrically isolated semiconductor photodetector.
On August 8, 1962, Biard and Pittman filed a patent titled "Semiconductor Radiant Diode" based on their findings, which described a zinc-diffused p–n junction LED with a spaced cathode contact to allow for efficient emission of infrared light under forward bias. After establishing the priority of their work based on engineering notebooks predating submissions from G. E. Labs, RCA Research Labs, IBM Research Labs, Bell Labs, Lincoln Lab at MIT, the U. S. patent office issued the two inventors the patent for the GaAs infrared light-emitting diode, the first practical LED. After filing the patent, Texas Instruments began a project to manufacture infrared diodes. In October 1962, TI announced the first commercial LED product, which employed a pure GaAs crystal to emit an 890 nm light output. In October 1963, TI announced the first commercial hemispherical LED, the SNX-110; the first visible-spectrum LED was developed in 1962 by Nick Holonyak, Jr. while working at General Electric. Holonyak first reported his LED in the journal Applied Physics Letters on December 1, 1962.
M. George Craford, a former graduate student of Holonyak, invented the first yellow LED and improved the brightness of red and red-orange LEDs by a factor of ten in 1972. In 1976, T. P. Pearsall created the first high-brightness, high-efficiency LEDs for optical fiber telecommunicat
Group 3 element
Group 3 is a group of elements in the periodic table. This group, like other d-block groups, should contain four elements, but it is not agreed what elements belong in the group. Scandium and yttrium are always included, but the other two spaces are occupied by lanthanum and actinium, or by lutetium and lawrencium; when the group is understood to contain all of the lanthanides, its trivial name is the rare-earth metals. Three group 3 elements occur naturally: scandium and either lanthanum or lutetium. Lanthanum continues the trend started by two lighter members in general chemical behavior, while lutetium behaves more to yttrium. While the choice of lutetium would be in accordance with the trend for period 6 transition metals to behave more to their upper periodic table neighbors, the choice of lanthanum is in accordance with the trends in the s-block, which the group 3 elements are chemically more similar to, they all are silvery-white metals under standard conditions. The fourth element, either actinium or lawrencium, has only radioactive isotopes.
Actinium, which occurs only in trace amounts, continues the trend in chemical behavior for metals that form tripositive ions with a noble gas configuration. So far, no experiments have been conducted to synthesize any element that could be the next group 3 element. Unbiunium, which could be considered a group 3 element if preceded by lanthanum and actinium, might be synthesized in the near future, it being only three spaces away from the current heaviest element known, oganesson. In 1787, Swedish part-time chemist Carl Axel Arrhenius found a heavy black rock near the Swedish village of Ytterby, Sweden. Thinking that it was an unknown mineral containing the newly discovered element tungsten, he named it ytterbite. Finnish scientist Johan Gadolin identified a new oxide or "earth" in Arrhenius' sample in 1789, published his completed analysis in 1794. In the decades after French scientist Antoine Lavoisier developed the first modern definition of chemical elements, it was believed that earths could be reduced to their elements, meaning that the discovery of a new earth was equivalent to the discovery of the element within, which in this case would have been yttrium.
Until the early 1920s, the chemical symbol "Yt" was used for the element, after which "Y" came into common use. Yttrium metal was first isolated in 1828 when Friedrich Wöhler heated anhydrous yttrium chloride with potassium to form metallic yttrium and potassium chloride. In 1869, Russian chemist Dmitri Mendeleev published his periodic table, which had empty spaces for elements directly above and under yttrium. Mendeleev made several predictions on the upper neighbor of yttrium. Swedish chemist Lars Fredrik Nilson and his team discovered the missing element in the minerals euxenite and gadolinite and prepared 2 grams of scandium oxide of high purity, he named it scandium, from the Latin Scandia meaning "Scandinavia". Chemical experiments on the element proved. Nilson was unaware of Mendeleev's prediction, but Per Teodor Cleve recognized the correspondence and notified Mendeleev. Metallic scandium was produced for the first time in 1937 by electrolysis of a eutectic mixture, at 700–800 °C, of potassium and scandium chlorides.
In 1751, the Swedish mineralogist Axel Fredrik Cronstedt discovered a heavy mineral from the mine at Bastnäs named cerite. Thirty years the fifteen-year-old Vilhelm Hisinger, from the family owning the mine, sent a sample of it to Carl Scheele, who did not find any new elements within. In 1803, after Hisinger had become an ironmaster, he returned to the mineral with Jöns Jacob Berzelius and isolated a new oxide which they named ceria after the dwarf planet Ceres, discovered two years earlier. Ceria was independently isolated in Germany by Martin Heinrich Klaproth. Between 1839 and 1843, ceria was shown to be a mixture of oxides by the Swedish surgeon and chemist Carl Gustaf Mosander, who lived in the same house as Berzelius: he separated out two other oxides which he named lanthana and didymia, he decomposed a sample of cerium nitrate by roasting it in air and treating the resulting oxide with dilute nitric acid. Since lanthanum's properties differed only from those of cerium, occurred along with it in its salts, he named it from the Ancient Greek λανθάνειν.
Pure lanthanum metal was first isolated in 1923. Lutetium was independently discovered in 1907 by French scientist Georges Urbain, Austrian mineralogist Baron Carl Auer von Welsbach, American chemist Charles James as an impurity in the mineral ytterbia, thought by most chemists to consist of ytterbium. Welsbach proposed the names cassiopeium for element 71 and aldebaranium for the new name of ytterbium but these naming proposals were rejected, although many German scientists in the 1950s called the element 71 cassiopeium. Urbain chose the names neoytterbium for lutecium for the new element; the dispute on the priority of the discovery is documented in two articles in which Urb
Group 4 element
Group 4 is a group of elements in the periodic table. It contains the elements titanium, zirconium and rutherfordium; this group lies in the d-block of the periodic table. The group itself has not acquired a trivial name; the three Group 4 elements that occur are titanium and hafnium. The first three members of the group share similar properties. However, the fourth element rutherfordium, has been synthesized in the laboratory. All isotopes of rutherfordium are radioactive. So far, no experiments in a supercollider have been conducted to synthesize the next member of the group, it is unlikely that they will be synthesized in the near future. Like other groups, the members of this family show patterns in its electron configuration the outermost shells resulting in trends in chemical behavior: Most of the chemistry has been observed only for the first three members of the group; the chemistry of rutherfordium is not established and therefore the rest of the section deals only with titanium and hafnium.
All the elements of the group are reactive metals with a high melting point. The reactivity is not always obvious due to the rapid formation of a stable oxide layer, which prevents further reactions; the oxides TiO2, ZrO2 and HfO2 are white solids with high melting points and unreactive against most acids. As tetravalent transition metals, all three elements form various inorganic compounds in the oxidation state of +4. For the first three metals, it has been shown that they are resistant to concentrated alkalis, but halogens react with them to form tetrahalides. At higher temperatures, all three metals react with oxygen, carbon, boron and silicon; because of the lanthanide contraction of the elements in the fifth period and hafnium have nearly identical ionic radii. The ionic radius of Zr4+ is 79 picometers and that of Hf4+ is 78 pm; this similarity results in nearly identical chemical behavior and in the formation of similar chemical compounds. The chemistry of hafnium is so similar to that of zirconium that a separation on chemical reactions was not possible.
The melting points and boiling points of the compounds and the solubility in solvents are the major differences in the chemistry of these twin elements. Titanium is different from the other two owing to the effects of the lanthanide contraction; the table below is a summary of the key physical properties of the group 4 elements. The four question-marked values are extrapolated. British minerologist William Gregor first identified titanium in ilmenite sand beside a stream in Cornwall, Great Britain in the year 1791. After analyzing the sand, he determined the weakly magnetic sand to contain iron oxide and a metal oxide that he could not identify. During that same year, minerologist Franz Joseph Muller produced the same metal oxide and could not identify it. In 1795, chemist Martin Heinrich Klaproth independently rediscovered the metal oxide in rutile from the Hungarian village Boinik, he named it for the Titans of Greek mythology. Martin Heinrich Klaproth discovered zirconium when analyzing the zircon containing mineral jargoon in 1789.
He deduced that the mineral contained a new element and named it after the known Zirkonerde. However, he failed to isolate the newly discovered zirconium. Cornish chemist Humphry Davy attempted to isolate this new element in 1808 through electrolysis, but failed. In 1824, Swedish chemist Jöns Jakob Berzelius isolated an impure form of zirconium, obtained by heating a mixture of potassium and potassium zirconium fluoride in an iron tube. Hafnium had been predicted by Dmitri Mendeleev in 1869 and Henry Moseley measured in 1914 the effective nuclear charge by X-ray spectroscopy to be 72, placing it between the known elements lutetium and tantalum. Dirk Coster and Georg von Hevesy were the first to search for the new element in zirconium ores. Hafnium was discovered by the two in 1923 in Copenhagen, validating the original 1869 prediction of Mendeleev. There has been some controversy surrounding the discovery of hafnium and the extent to which Coster and Hevesy were guided by Bohr's prediction that hafnium would be a transition metal rather than a rare earth element.
While titanium and zirconium, as abundant elements, were discovered in the late 18th century, it took until 1923 for hafnium to be identified. This was only due to hafnium's relative scarcity; the chemical similarity between zirconium and hafnium made a separation difficult and, without knowing what to look for, hafnium was left undiscovered, although all samples of zirconium, all of its compounds, used by chemists for over two centuries contained significant amounts of hafnium. Rutherfordium was first detected in 1966 at the Joint Institute of Nuclear Research at Dubna. Researchers there bombarded 242Pu with accelerated 22Ne ions and separated the reaction products by gradient thermochromatography after conversion to chlorides by interaction with ZrCl4. 24294Pu + 2210Ne → 264−x104Rf → 264−x104RfCl4 The production of the metals itself is difficult due to their reactivity. The formation of oxides and carbides must be avoided to yield workable metals; the oxides are reacted with chlorine to form the chlorides.
The chlorides of the metals are reacted with magnesium, yielding
Aluminium nitride is a nitride of aluminium. Its wurtzite phase is a wide band gap semiconductor material, giving it potential application for deep ultraviolet optoelectronics. AlN was first synthesized in 1877, but it was not until the middle of the 1980s that its potential for application in microelectronics was realized due to its high thermal conductivity for an electrically insulating ceramic. Aluminium nitride is stable at high temperatures in inert atmospheres and melts about 2200 °C. In a vacuum, AlN decomposes at ~1800 °C. In the air, surface oxidation occurs above 700 °C, at room temperature, surface oxide layers of 5-10 nm have been detected; this oxide layer protects the material up to 1370 °C. Above this temperature bulk oxidation occurs. Aluminium nitride is stable in hydrogen and carbon dioxide atmospheres up to 980 °C; the material dissolves in mineral acids through grain boundary attack, in strong alkalies through attack on the aluminium nitride grains. The material hydrolyzes in water.
Aluminium nitride is resistant to attack from most molten salts, including cryolite. Aluminum nitride can be patterned with a Cl2-based reactive ion etch. AlN is synthesized by the carbothermal reduction of aluminium oxide in the presence of gaseous nitrogen or ammonia or by direct nitridation of aluminium; the use of sintering aids, such as Y2O3 or CaO, hot pressing is required to produce a dense technical grade material. Epitaxially grown thin film crystalline aluminium nitride is used for surface acoustic wave sensors deposited on silicon wafers because of AlN's piezoelectric properties. One application is an RF filter, used in mobile phones, called a thin film bulk acoustic resonator; this is a MEMS device. Aluminium nitride is used to build piezoelectric micromachined ultrasound transducers, which emit and receive ultrasound and which can be used for in-air rangefinding over distances of up to a meter. Metallization methods are available to allow AlN to be used in electronics applications similar to those of alumina and beryllium oxide.
AlN nanotubes as inorganic quasi-one-dimensional nanotubes, which are isoelectronic with carbon nanotubes, have been suggested as chemical sensors for toxic gases. There is much research into developing light-emitting diodes to operate in the ultraviolet using gallium nitride based semiconductors and, using the alloy aluminium gallium nitride, wavelengths as short as 250 nm have been achieved. In May 2006, an inefficient AlN LED emission at 210 nm was reported. There are multiple research efforts in industry and academia to use aluminum nitride in piezoelectric MEMS applications; these include resonators and microphones. Among the applications of AlN are opto-electronics, dielectric layers in optical storage media, electronic substrates, chip carriers where high thermal conductivity is essential, military applications, as a crucible to grow crystals of gallium arsenide and semiconductor manufacturing. Boron nitride Aluminium phosphide Indium nitride Aluminium oxynitride Jaime Andrés Pérez Taborda.
C. Caicedo. "Deposition pressure effect on chemical and optical properties of binary Al-nitrides". Optics & Laser Technology. 69: 92–103. Bibcode:2015OptLT..69...92P. Doi:10.1016/j.optlastec.2014.12.009. Hdl:10261/129916
Molybdenum disulfide is an inorganic compound composed of molybdenum and sulfur. Its chemical formula is MoS2; the compound is classified as a transition metal dichalcogenide. It is a silvery black solid that occurs as the mineral molybdenite, the principal ore for molybdenum. MoS2 is unreactive, it is unaffected by dilute acids and oxygen. In appearance and feel, molybdenum disulfide is similar to graphite, it is used as a dry lubricant because of its low friction and robustness. Bulk MoS2 is a diamagnetic, indirect bandgap semiconductor similar to silicon, with a bandgap of 1.23 eV. MoS2 is found as either molybdenite, a crystalline mineral, or jordisite, a rare low temperature form of molybdenite. Molybdenite ore is processed by flotation to give pure MoS2; the main contaminant is carbon. MoS2 arises by thermal treatment of all molybdenum compounds with hydrogen sulfide or elemental sulfur and can be produced by metathesis reactions from molybdenum pentachloride. All forms of MoS2 have a layered structure, in which a plane of molybdenum atoms is sandwiched by planes of sulfide ions.
These three strata form a monolayer of MoS2. Bulk MoS2 consists of stacked monolayers, which are held together by weak van der Waals interactions. Crystalline MoS2 is found in nature as one of two phases, 2H-MoS2 and 3R-MoS2, where the "H" and the "R" indicate hexagonal and rhombohedral symmetry, respectively. In both of these structures, each molybdenum atom exists at the center of a trigonal prismatic coordination sphere and is covalently bonded to six sulfide ions; each sulfur atom is bonded to three molybdenum atoms. Both the 2H- and 3R-phases are semiconducting. A third, metastable crystalline phase known as 1T-MoS2 was discovered by intercalating 2H-MoS2 with alkali metals; this phase is metallic. The 1T-phase can be stabilized through doping with electron donors like rhenium, or converted back to the 2H-phase by microwave radiation. Nanotube-like and buckyball-like molecules composed of MoS2 are known. While bulk MoS2 in the 2H-phase is known to be an indirect-band gap semiconductor, monolayer MoS2 has a direct band gap.
The layer-dependent optoelectronic properties of MoS2 have promoted much research in 2-dimensional MoS2-based devices. 2D MoS2 can be produced by exfoliating bulk crystals to produce single-layer to few-layer flakes either through a dry, micromechanical process or through solution processing. Micromechanical exfoliation pragmatically called "Scotch-tape exfoliation", involves using an adhesive material to peel apart a layered crystal by overcoming the van der Waals forces; the crystal flakes can be transferred from the adhesive film to a substrate. This facile method was first used by Geim to obtain graphene from graphite crystals. However, it can not be employed for a uniform 1-D layers because of less adhesion of MoS2 with the substrate; the aforementioned scheme is good for Graphene only. While Scotch tape is used as the adhesive tape, PDMS stamps can satisfactorily cleave MoS2 if it is important to avoid contaminating the flakes with residual adhesive. Liquid-phase exfoliation can be used to produce monolayer to multi-layer MoS2 in solution.
A few methods include lithium intercalation to delaminate the layers and sonication in a high-surface tension solvent. MoS2 excels as a lubricating material due to its layered structure and low coefficient of friction. Interlayer sliding dissipates energy. Extensive work has been performed to characterize the coefficient of friction and shear strength of MoS2 in various atmospheres; the shear strength of MoS2 increases as the coefficient of friction increases. This property is called superlubricity. At ambient conditions, the coefficient of friction for MoS2 was determined to be 0.150, with a corresponding estimated shear strength of 56.0 MPa. Direct methods of measuring the shear strength indicate; the wear resistance of MoS2 in lubricating applications can be increased by doping MoS2 with chromium. Microindentation experiments on nanopillars of Cr-doped MoS2 found that the yield strength increased from an average of 821 MPa for pure MoS2 to 1017 MPa for 50 at. % Cr. The increase in yield strength is accompanied by a change in the failure mode of the material.
While the pure MoS2 nanopillar fails through a plastic bending mechanism, brittle fracture modes become apparent as the material is loaded with increasing amounts of dopant. The used method of micromechanical exfoliation has been studied in MoS2 to understand the mechanism of delamination in few-layer to multi-layer flakes; the exact mechanism of cleavage was found to be layer dependent. Flakes thinner than 5 layers undergo homogenous bending and rippling, while flakes around 10 layers thick delaminated through interlayer sliding. Flakes with more than 20 layers exhibited a kinking mechanism during micromechanical cleavage; the cleavage of these flakes was determined to be reversible due to the nature of van der Waals bonding. In recent years, MoS2 has been utilized in flexible electronic applications, promoting more investigation into the elastic properties of this material. Nanoscopic bending tests using AFM cantilever tips were performed on micromechanically exfoliated MoS2 flakes that were deposited on a holey substrate.
The yield strength of monolayer flakes was 270 GPa, while the thicker flakes were stiffer, with a yield strength of 330 GPa. Molecular dynamic simulations found the in-plane yield strength of MoS2 to be 229 GPa, which matches the experimental results within error. Bertolazzi and coworkers characterized the failure modes o
In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be classified by the chemical bonds type as follows: salt-like, covalent compounds, interstitial compounds, "intermediate" transition metal carbides. Examples include calcium carbide, silicon carbide, tungsten carbide, cementite, each used in key industrial applications; the naming of ionic carbides is not systematic. Salt-like carbides are composed of electropositive elements such as the alkali metals, alkaline earth metals, group 3 metals, including scandium and lanthanum. Aluminium from group 13 forms carbides, but gallium and thallium do not; these materials feature isolated carbon centers described as "C4−", in the methanides or methides. The graphite intercalation compound KC8, prepared from vapour of potassium and graphite, the alkali metal derivatives of C60 are not classified as carbides. Carbides of this class decompose in water producing methane. Three such examples are aluminium carbide Al4C3, magnesium carbide Mg2C and beryllium carbide Be2C.
Transition metal carbides are not saline carbides but their reaction with water is slow and is neglected. For example, depending on surface porosity, 5–30 atomic layers of titanium carbide are hydrolyzed, forming methane within 5 minutes at ambient conditions, following by saturation of the reaction. Note that methanide in this context is a trivial historical name. According to the IUPAC systematic naming conventions, a compound such as NaCH3 would be termed a "methanide", although this compound is called methylsodium. Several carbides are assumed to be salts of the acetylide anion C22–, which has a triple bond between the two carbon atoms. Alkali metals, alkaline earth metals, lanthanoid metals form acetylides, e.g. sodium carbide Na2C2, calcium carbide CaC2, LaC2. Lanthanides form carbides with formula M2C3. Metals from group 11 tend to form acetylides, such as copper acetylide and silver acetylide. Carbides of the actinide elements, which have stoichiometry MC2 and M2C3, are described as salt-like derivatives of C2−2.
The C-C triple bond length ranges from 119.2 pm in CaC2, to 130.3 pm in LaC2 and 134 pm in UC2. The bonding in LaC2 has been described in terms of LaIII with the extra electron delocalised into the antibonding orbital on C2−2, explaining the metallic conduction; the polyatomic ion C4−3, sometimes called sesquicarbide or allylenide, is found in Li4C3 and Mg2C3. The ion is linear and is isoelectronic with CO2; the C-C distance in Mg2C3 is 133.2 pm. Mg2C3 yields methylacetylene, CH3CCH, propadiene, CH2CCH2, on hydrolysis, the first indication that it contains C4−3; the carbides of silicon and boron are described as "covalent carbides", although all compounds of carbon exhibit some covalent character. Silicon carbide has two similar crystalline forms. Boron carbide, B4C, on the other hand, has an unusual structure which includes icosahedral boron units linked by carbon atoms. In this respect boron carbide is similar to the boron rich borides. Both silicon carbide and boron carbide are hard materials and refractory.
Both materials are important industrially. Boron forms other covalent carbides, e.g. B25C; the carbides of the group 4, 5 and 6 transition metals are described as interstitial compounds. These carbides are refractory; some exhibit a range of stoichiometries, e.g. titanium carbide, TiC. Titanium carbide and tungsten carbide are important industrially and are used to coat metals in cutting tools; the long-held view is that the carbon atoms fit into octahedral interstices in a close-packed metal lattice when the metal atom radius is greater than 135 pm: When the metal atoms are cubic close-packed filling all of the octahedral interstices with carbon achieves 1:1 stoichiometry with the rock salt structure. When the metal atoms are hexagonal close-packed, as the octahedral interstices lie directly opposite each other on either side of the layer of metal atoms, filling only one of these with carbon achieves 2:1 stoichiometry with the CdI2 structure; the following table shows actual structures of their carbides.
The notation "h/2" refers to the M2C type structure described above, only an approximate description of the actual structures. The simple view that the lattice of the pure metal "absorbs" carbon atoms can be seen to be untrue as the packing of the metal atom lattice in the carbides is different from the packing in the pure metal, although it is technically correct that the carbon atoms fit into the octahedral interstices of a close-packed metal lattice. For a long time the non-stoichiometric phases were believed to be disordered with a random filling of the interstices, however short and longer range ordering has been detected. In these carbides, the transition metal ion is smaller than the critical 135 pm, the structures are not interstitial but are more complex. Multiple stoichiometries are common; the best known is cementite, Fe3C, present in steels. These carbides are more reactive than the interstitial carbides.