In 1982, Voronkov presented a model describing point defect behavior during the growth of single crystal Si from a melt and derived an expression to predict if the crystal was vacancy- or self-interstitial-rich. Recently, Vanhellemont claimed that one should take into account the impact of compressive stress introduced by the thermal gradient at the melt/solid interface by considering the hydrostatic pressure dependence of the formation enthalpy of the intrinsic point defects. To evaluate the impact of thermal stress more correctly, the pressure dependence of both the formation enthalpy (Hf) and the migration enthalpy (Hm) of the intrinsic point defects should be taken into account. Furthermore, growing single crystal Si is not under hydrostatic pressure but almost free of external pressure (generally in Ar gas under reduced pressure). In the present paper, the dependence of Hf and Hm on the pressure P, or in other words, the pressure dependence of the formation energy (Ef) and the relaxation volume (vf), is quantified by density functional theory calculations. Although a large number of ab initio calculations of the properties of intrinsic point defects have been published during the last years, calculations for Si crystals under pressure are rather scarce. For vacancies V, the reported pressure dependences of HfV are inconsistent. In the present study, by using 216-atom supercells with a sufficient cut-off energy and mesh of k-points, the neutral I and V are found to have nearly constant formation energies EfI and EfV for pressures up to 1 GPa. For the relaxation volume, vfI is almost constant while vfV decreases linearly with increasing pressure P. In case of the hydrostatic pressure Ph, the calculated formation enthalpy HfI and migration enthalpy HmI at the [110] dumbbell site are given by HfI = 3.425 − 0.057 × Ph (eV) and HmI = 0.981 − 0.039 × Ph (eV), respectively, with Ph given in GPa. The calculated HfV and HmV dependencies on Ph given by HfV = 3.543 − 0.021 × Ph2 − 0.019 × Ph (eV) and HmV = 0.249 + 0.018 × Ph2 − 0.037 × Ph (eV), respectively. These results indicate that, when assuming that the pre-factors in the Arrhenius equation are not influenced, hydrostatic pressure up to 1 GPa leads to a slight increase of the thermal equilibrium concentration and diffusion of vacancies but this increase is much smaller than that of self-interstitials. The thermal stress in growing Si crystal is compressive, and thus the point defects are under internal pressure. Taking into account the differences in the enthalpies of point defects between hydrostatic pressure and internal pressure, Si crystal shifts to being V-rich with an increase in thermal stress during crystal growth.

A new carbon-nitride-related C 2 N 2 (CH 2 ) nanoplatelet was synthesized by subjecting a precursor C 3 N 4 H x O y nanoparticle in a laser-heating diamond anvil cell to the pressure of 40 GPa and temperature of 1200-2000 K. The C and N composition of the quenched sample was determined to be C 3 N 2 by using an energy dispersive X-ray spectroscope attached to a transmission electron microscope. The crystal structure and atomic positions of this C 3 N 2 were obtained through Rietveld analysis of the X-ray diffraction pattern measured using synchrotron radiation. The hydrogen composition was difficult to determine experimentally because of the several-hundred-nanometer dimensions of the sample. Firstprinciples calculation was alternatively used to discover the hydrogen composition. The synthesized C 2 N 2 (CH 2 ) was accordingly found to be an orthorhombic unit cell of the space group Cmc2 1 with lattice constants a ¼ 7:625 A, b ¼ 4:490 A, and c ¼ 4:047 A. If the CH 2 atomic unit is replaced with the CN 2 atomic unit and the bonding rearranged, the C 2 N 2 (CH 2 ) becomes the expected superhard C 3 N 4 .

The dependences of the formation enthalpy (H f ) and the migration enthalpy (H m ) of the self-interstitial I and the vacancy V in single crystal Si on hydrostatic pressure and on internal pressure were studied by calculating the formation energies and relaxation volumes. Density functional theory calculations were used with 216-atom supercells and with special attention for the convergence of the calculations. The obtained H f and H m were used to describe the impact of thermal stress on the critical pulling speed over thermal gradient ratio Γ0 crit as defined in the Voronkov criterion. A nearly linear relation between Γ0 crit (in 10 -3 cm 2 min -1 K -1 ) and σ0 (in MPa) was obtained, described by Γ0 crit ≈ 1.509 -0.023σ0. The impact of thermal stress on Γ0 crit is opposite to that of hydrostatic pressure and makes the growing Si crystal more vacancy-rich. It is important to take into account the impact of stress on the generation of intrinsic point defects in developing future large diameter defect-free crystals.

The electronic structure of IV2V2VI class semiconductors, C2N2X (X = O, NH, CH2), was investigated using first principles calculations. The crystal structures of C2N2X are isostructural with the Si2N2O compound, sinoite. The valence of the X atom is virtually two, and thus the substitution of X (X = O, NH, CH2) is isoelectronic. From the calculated density of states, the carbon 2 p orbital does not participate in the upper valence band (VB) (0 to –5 eV). The upper valence band is dominated by the N 2 p and X 2 p orbitals. The calculated optical absorption edge shifts to a lower energy as the substitution progresses from the O atom to the CH2 group. The calculated absorption edge is 7.76, 7.07, and 6.66 eV for C2N2O, C2N2(NH), and C2N2(CH2), respectively.

The experimental bulk modulus, B0, of C2N2(CH2) is determined to be 258 ± 3.4 GPa from the analysis of high-pressure (up to 30 GPa) X-ray diffraction patterns obtained using synchrotron radiation. This bulk modulus is 40% lower than that of diamond. At the level of a combined analysis of lattice constants determined experimentally and atomic positions obtained theoretically for the compression behavior of C2N2(CH2), the strength of the C–N single bond is determined to be the same as the C–C single bond in diamond. In other words, the tetrahedral frame of C2N2(CH2) which consists of CN3Cb, where Cb is a bridging carbon, is as hard as diamond. To account for the differing bulk moduli, we infer that the lower bulk modulus in C2N2(CH2) is due to the rotational freedom in the crystal at high pressures.