Dear Vahideh,

Please read the following text:

Non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by integers; most often, in such materials, some small percentage of atoms are missing or too many atoms are packed into an otherwise perfect lattice work.[not verified in body] Contrary to earlier definitions, modern understanding of non-stoichiometric compounds view them as homogenous, and not mixtures of stoichiometric chemical compounds. Since the solids are overall electrically neutral, the defect is compensated by a change in the charge of other atoms in the solid, either by changing their oxidation state, or by replacing them with atoms of different elements with a different charge. Many metal oxides and sulfides have non-stoichiometry examples; for example, stoichiometric iron(II) oxide, which is rare, has the formula FeO, whereas the more common material is nonstoichiometric, with the formula Fe0.95O. Non-stoichiometric compounds exhibit special electrical or chemical properties because of the defects; for example, when atoms are missing, the other atoms can move through the solid more rapidly. 

Tantalum carbides form a family of binary chemical compounds of tantalum and carbon with the empirical formula TaCx, where x usually varies between 0.4 and 1. They are extremely hard, brittle, refractory ceramic materials with metallic electrical conductivity. They appear as brown-gray powders which are usually processed by sintering. Being important cermet materials, tantalum carbides are commercially used in tool bits for cutting applications and are sometimes added to tungsten carbide alloys.[4] 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.[5][6] Only tantalum hafnium carbide may have a slightly higher melting point of about 3942 °C,[7] whereas the melting point of hafnium carbide is comparable to that of TaC.

Therefore when x is a fraction of 1 in TaCx (not integers), the compound is a non-stoichiometric compound.

https://en.wikipedia.org/wiki/Tantalum_carbide

Hoping this will be helpful,

Rafik

Thanks for your explanation Dear Rafik Karaman,

I want to know about experimental methods, such as FTIR. Is it a useful technique?

Dear Vahideh,

FTIR will not give you a clear picture on the composition. I suggest you do HRTEM, high-resolution transmission electron microscopy or/and MMC, metal matrix composite.

Rafik

"...Tantalum carbides form a family of binary chemical compounds of tantalum and carbon with the empirical formula TaCx, where x usually varies between 0.4 and 1..."

This is not correct! It is better to describe tantalum monocarbide by formula TaC1-x, where 0.01 < x < 0.25 (the inequality depends strongly on temperature). However, there are other tantalum carbides: semicarbides - alpha-Ta2+xC and beta-Ta2+-xC, which have their individual limits for x; but also epsilon-Ta3C2+-x, dzeta-Ta4C3-x and Ta6C5+-x with various individual values of x. All the tantalum carbides are non-stoichiometric and have different crystal structure, so you need to find for every your sample (!) the C/Ta ratio (composition) and lattice parameter (for cubic tantalum monocarbide it varies from 0.440 to 0.446 nm).

It is easier to imagine that for stoichiometric solid compounds at the fixed temperature and pressure the relationship between lattice parameter and composition is a point, but for non-stoichiometric compounds this is a curve... Then, only prepare single-phase samples of your compound and, using simple chemical analysis and X-ray lattice parameter measurements, find the point or maybe curve composition vs. lattice parameter correlations.

FTIR and HRTEM are not useful completely for this purpose.

Data on non-stoichiometric and stoichiometric carbides you can find in Chapter 2 - Carbon (Graphene/Graphite) in the 1st Volume of my book on UHTM (available on my ResearchGate account). The 2nd Volume of the book, which is directly devoted to refractory carbides of Ti, Zr, Hf, V, Nb and Ta, is in preparation now on my desk.

Unfortunately, there are many experienced chemists, who have no practice with these compounds, and only few books on this topic are available.

Thank you very much Dr. Shabalin for your detailed answer