Z.T.Y. Liu

Structural, mechanical and electronic properties of 3d transition metal nitrides in cubic zincblende, rocksalt and cesium chloride structures: a first-principles investigation

We report systematic results from ab initio calculations with density functional theory on three cubic structures, zincblende (zb), rocksalt (rs) and cesium chloride (cc), of the ten 3d transition metal nitrides. We computed lattice constants, elastic constants, their derived moduli and ratios that characterize mechanical properties. Experimental measurements exist in the literature of lattice constants for rs-ScN, rs-TiN and rs-VN and of elastic constants for rs-TiN and rs-VN, all of which are in good agreement with our computational results. Similarly, computed Vickers hardness (HV) values for rs-TiN and rs-VN are consistent with earlier experimental results. Several trends were observed in our rich data set of 30 compounds. All nitrides, except for zb-CrN, rs-MnN, rs-FeN, cc-ScN, cc-CrN, cc-NiN and cc-ZnN, were found to be mechanically stable. A clear correlation in the atomic density with the bulk modulus (B) was observed with maximum values of B around FeN, MnN and CrN. The shear modulus, Youngs modulus, HV and indicators of brittleness showed similar trends and all showed maxima for cc-VN. The calculated value of HV for cc-VN was about 30 GPa, while the next highest values were for rs-ScN and rs-TiN, about 24 GPa. A relation (H(V) is proportional to θ(D)(2)) between HV and Debye temperature (θD) was investigated and verified for each structure type. A tendency for anti-correlation of the elastic constant C44, which strongly influences stability and hardness, with the number of electronic states around the Fermi energy was observed.

First-principles phase diagram calculations for the rocksalt-structure quasibinary systems TiN–ZrN, TiN–HfN and ZrN–HfN

We have studied the phase equilibria of three ceramic quasibinary systems Ti1-x Zr x N, Ti1-x Hf x N and Zr1-x Hf x N (0  ⩽  x  ⩽  1) with density functional theory, cluster expansion and Monte Carlo simulations. We predict consolute temperatures (T C), at which miscibility gaps close, for Ti1-x Zr x N to be 1400 K, for Ti1-x Hf x N to be 700 K, and below 200 K for Zr1-x Hf x N. The asymmetry of the formation energy ΔE f(x) is greater for Ti1-x Hf x N than Ti1-x Zr x N, with less solubility on the smaller cation TiN-side, and similar asymmetries were predicted for the corresponding phase diagrams. We also analyzed different energetic contributions: ΔE f of the random solid solutions were decomposed into a volume change term, [Formula: see text], and a chemical exchange and relaxation term, [Formula: see text]. These two energies partially cancel one another. We conclude that [Formula: see text] influences the magnitude of T C and [Formula: see text] influences the asymmetry of ΔE f(x) and phase boundaries. We also conclude that the absence of experimentally observed phase separation in Ti1-x Zr x N and Ti1-x Hf x N is due to slow kinetics at low temperatures. In addition, elastic constants and mechanical properties of the random solid solutions were studied with the special quasirandom solution approach. Monotonic trends, in the composition dependence, of shear-related mechanical properties, such as Vickers hardness between 18 to 23 GPa, were predicted. Trends for Ti1-x Zr x N and Ti1-x Hf x N exhibit down-bowing (convexity). It shows that mixing nitrides of same group transition metals does not lead to hardness increase from an electronic origin, but through solution hardening mechanism. The mixed thin films show consistency and stability with little phase separation, making them desirable coating choices.

Thermal equation of state of silicon carbide

A large volume press coupled with in-situ energy-dispersive synchrotron X-ray was used to probe the change of silicon carbide (SiC) under high pressure and temperature (P-T) up to 8.1 GPa and 1100 K. The obtained pressure–volume–temperature data were fitted to a modified high-T Birch-Murnaghan equation of state, yielding values of a series of thermo-elastic parameters, such as the ambient bulk modulus KTo = 237(2) GPa, temperature derivative of the bulk modulus at a constant pressure (∂K/∂T)P = −0.037(4) GPa K−1, volumetric thermal expansivity α(0, T) = a + bT with a = 5.77(1) × 10−6 K−1 and b = 1.36(2) × 10−8 K−2, and pressure derivative of the thermal expansion at a constant temperature (∂α/∂P)T = 6.53 ± 0.64 × 10−7 K−1 GPa−1. Furthermore, we found the temperature derivative of the bulk modulus at a constant volume, (∂KT/∂T)V, equal to −0.028(4) GPa K−1 by using a thermal pressure approach. In addition, the elastic properties of SiC were determined by density functional theory through the calculation of...

Diffusion in CdS of Cd and S vacancies and Cu, Cd, Cl, S and Te interstitials studied with first-principles computations

We present an ab initio study of the diffusion profiles in CdS of native, Cd and S vacancies, and interstitial adatoms Cd, S, Te, Cu, and Cl. The global minimum and saddle point positions in the bulk unit cell vary for different diffusing species. This results in a significant variation, in the bonding configurations and associated strain energies of different extrema positions along the diffusion paths for various defects. The rate-limiting diffusion barriers range from a low of 0.42 eV for an S interstitial to a high of 2.18 eV for a S vacancy. The rate-limiting barrier is 0.66 eV for Cu and Te interstitials, 0.76 eV for Cl interstitial, 0.87 eV for Cd interstitial and 1.09 eV for the Cd vacancy. The 0.66 eV barrier for a Cu interstitial is in good agreement with experimental values in the range of 0.58–0.96 eV reported in the literature. We report an electronic signature in the projected density of states for the s-and d-states of the Cu interstitial at the saddle point and global minimum energy position. In addition, we have examined the relative charge transfer experienced by the interstitials at the extrema positions through Bader analysis.