Density-functional theory investigation of hardness, stability, and electron-energy-loss spectra of carbon nitrides withC11N4stoichiometry

Abstract

The characterization of carbon nitride films with stoichiometry C 3 N 4 is heavily restricted by the problem of getting pure crystalline samples with the right C/N ratio. However, thin films with lower nitrogen concentration ͑5-25 %͒ have been found relatively easier to deposit, for example, with reactive magnetron sputtering. It is also in this range of nitrogen content that the recently discovered ''graphiticlike → fullerenelike'' phase transition has been suggested to take place. Therefore, in order to add more information to the above experimental evidence, it is important to use theoretical methods to obtain further characterization of carbon nitride models with a high C/N ratio such as that of C 11 N 4 . It is relevant to propose a cross checking on the role played by the nitrogen concentration in determining the stability, hardness, and electronic properties of CN x compounds with different stoichiometries. For the sake of simplicity we have here compared the C 3 N 4 and C 11 N 4 systems, which are isoelectronic to each other. For this purpose two C 11 N 4 phases, namely, ␣ and ␤, are presented and investigated with density-functional-theory methods within the local density approximation. These phases contain less than ϳ30% of nitrogen than the well-known C 3 N 4 and are formally derived from the so-called pseudocubic C 3 N 4 . Cohesive properties, heats of formation, bulk and elastic moduli have been calculated and a full detailed analysis of the density-of-states and energy-loss-near-edge-structure spectra is presented. We propose that the lowering of the nitrogen concentration does not prevent the finding of new ultrahard materials and indeed brings a significant increase in the cohesive energy of carbon nitrides. However, the computed enthalpies of formation have shown values that are positive and generally larger than the analog carbon-deficient phases.

Origin of the low compressibility in hard nitride spinels

“…This systematic behavior has prompted the proposal of simple correlations involving coordination polyhedral volumes 2 or nearest-neighbor distances 3,4 along with some bonding index as the key parameters to estimate bulk moduli ͑B͒ of solids. In tetrahedrally bonded compounds, the success of Cohen's equation 3 has fueled the investigation of carbon nitride polymorphs [5][6][7][8] since it predicts B values significantly larger than diamond. However, different energetic arguments rise reasonable doubts on the feasibility of the synthesis of low compressible hexagonal C 3 N 4 since a preference for N-deficient or C-rich compositions has been recognized.…”

“…However, different energetic arguments rise reasonable doubts on the feasibility of the synthesis of low compressible hexagonal C 3 N 4 since a preference for N-deficient or C-rich compositions has been recognized. 1,6,8 It is also well known that nitrides show a much more versatile response to hydrostatic pressure than oxides. For example, contrarily to the uniform behavior of general AB 2 O 4 spinels with zero pressure bulk moduli (B 0 ) clustering around 200 GPa, nitride spinels are predicted to have B 0 values ranging from 200 to 400 GPa.…”

Structural and mechanical properties of TaZrN films: Experimental and ab initio studies

“…The shear modulus is correlated to the hardness of a material, which was confirmed experimentally as being the best hardness predictor. [23][24][25] Values of elastic constants provide valuable information about the bonding characteristic between adjacent atomic planes and the anisotropic character of the bonding and structural stability. Pugh 26 introduced the ratio between the bulk and shear modulus, G / B, for polycrystalline phases, as a measure of fracture/toughness in metals.…”

“…B can also be expressed as a linear combination of two elastic constants as follows: The determination of the stress strain distribution in a polycrystalline aggregate with respect to an external load can be established by equating the uniform strain in the polycrystal to the external strain in the Voigt approximation. 23,28 Hence, the shear modulus G, Young's modulus E, and Poisson's ratio can be calculated from the following relations:…”

Peculiar magnetic properties of NC6 and NC12 layered compounds from first principles

“…The know-how-to in synthesizing such compositions with modern CVD/PVD and other growth techniques leading for instance to original compositions as C 11 N 4 Fig. 6 Variation of the magnetization with cell volume in NC 6 and NC 12 carbonitride in both 2D and 3D forms [9,10] may cast confidence that further growth experiments with smaller amounts of nitrogen might lead to new nitrogen-deficient carbonitrides with compositions ranging from NC 6 to NC 12 which can be then proposed to the physics community for further characterization, especially the unusual magnetic behavior.…”