Tantalum carbide
Tantalum carbides form a family of binary
Binary compound
A binary compound is a chemical compound that contains exactly two different elements. Examples of binary ionic compounds include calcium chloride , sodium fluoride , and magnesium oxide , whilst examples of binary covalent compounds include water , carbon monoxide , and sulfur hexafluoride...

 chemical compounds of tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...

 and carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 with the empirical formula TaCx, where x usually varies between 0.4 and 1. They are extremely hard, brittle, refractory
Refraction (metallurgy)
In metallurgy, refraction is a property of metals that indicates their ability to withstand heat. Metals with a high degree of refraction are referred to as refractory. These metals derive their high melting points from their strong intermolecular forces...

A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

 materials with metallic electrical conductivity. They appear as brown-gray powders which are usually processed by sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...

. Being important cermet
A cermet is a composite material composed of ceramic and metallic materials. A cermet is ideally designed to have the optimal properties of both a ceramic, such as high temperature resistance and hardness, and those of a metal, such as the ability to undergo plastic deformation. The metal is used...

 materials, tantalum carbides are commercially used in tool bit
Tool bit
The term tool bit generally refers to a non-rotary cutting tool used in metal lathes, shapers, and planers. Such cutters are also often referred to by the set-phrase name of single-point cutting tool. The cutting edge is ground to suit a particular machining operation and may be resharpened or...

s for cutting applications and are sometimes added to tungsten carbide
Tungsten carbide
Tungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...

 alloys. The melting points of tantalum carbides peak at about 3880 °C depending on the purity and measurement conditions; this value is among the highest for binary compounds. Only tantalum hafnium carbide
Tantalum hafnium carbide
Tantalum hafnium carbide is a refractory chemical compound with a general formula TaxHf1-xCy, which can be considered as a solid solution of tantalum carbide and hafnium carbide...

 has a distinctly higher melting point of about 4215 °C, whereas the melting point of hafnium carbide is comparable to that of TaC.


TaCx powders of desired composition are prepared by heating a mixture of tantalum and graphite powders in vacuum or inert gas atmosphere (argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

). The heating is performed at temperature of about 2000 °C using a furnace or an arc-melting setup. An alternative technique is reduction
Smelting is a form of extractive metallurgy; its main use is to produce a metal from its ore. This includes iron extraction from iron ore, and copper extraction and other base metals from their ores...

 of tantalum pentoxide by carbon in vacuum or hydrogen gas atmosphere at a temperature of 1500-1700 °C. This method was used to obtain tantalum carbide back in 1876, but it lacks control over the stoichiometry of the product.

Crystal structure

TaCx compounds have a cubic
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....

 (rock-salt) crystal structure for x = 0.7–1.0; the lattice parameter increases with x. TaC0.5 has two major crystalline forms symmetry. The more stable one has an anti-cadmium iodide
Cadmium iodide
Cadmium iodide, CdI2, is a chemical compound of cadmium and iodine. It is notable for its crystal structure, which is typical for compounds of the form MX2 with strong polarization effects.-Uses:...

-type trigonal structure which transforms upon heating to about 2000 °C into a hexagonal lattice with no long-range order for the carbon atoms.
FormulaSymmetryTypePearson symbol
Pearson symbol
The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a crystal structure, and was originated by W.B. Pearson. The symbol is made up of two letters followed by a number. For example:* Diamond structure, cF8...

Space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...

NoZρ (g/cm3)a (nm)c (nm)
TaC Cubic NaCl cF8 Fmm 225 4 14.6 0.4427
TaC0.75 Trigonal hR24 Rm 166 12 15.01 0.3116 3
TaC0.5 Trigonal anti-CdI2 hP3 Pm1 164 1 15.08 0.3103 0.4938
TaC0.5 Hexagonal hP4 P63/mmc 194 2 15.03 0.3105 0.4935

Here Z is the number of formula units per unit cell, ρ is the density calculated from lattice parameters.


The bonding between tantalum and carbon atoms in tantalum carbides is a complex mixture of ionic, metallic and covalent contributions, and because of the strong covalent component these carbides are very hard and brittle materials. For example, TaC has a microhardness of 1800 kg/mm2 and an elastic modulus
Elastic modulus
An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically when a force is applied to it...

 of 285 GPa, whereas the corresponding values for tantalum are 110 kg/mm2 and 186 GPa. The hardness, yield stress and shear stress
Shear stress
A shear stress, denoted \tau\, , is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section...

 increase with the carbon content in TaCx. Tantalum carbides have metallic electrical conductivity, both in terms of its magnitude and temperature dependence. TaC is a superconductor
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...

with a relatively high transition temperature of TC = 10.35 K.

The magnetic properties of TaCx change from diamagnetic for x ≤ 0.9 to paramagnetic at larger x. An inverse behavior (para-diamagnetic transition with increasing x) is observed for HfCx, despite it has the same crystal structure as TaCx.
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