The Influence Of External Conditions And Impurities On Elastic Constants Of Diamond

In this paper, we use molecular dynamics method to study the effect of temperature(l-lOOOK), pressure(0-60Gpa) and defects on the elastic properties of diamond.We build a supercell including 216 atoms (vacancy system contains 511 atoms)with diamond unit cell and derive the bulk material through the three-dimensional period boundary conditions. Through zero temperature geometry optimization, we find the stable structure of substitutional impurities. The impurities include a single atom of nitrogen, phosphorus, sulfur, and vacancy with neutral charge. We calculate the elastic constants of diamond by fluctuational method. The main work of this paper includes three parts,and the details are given below:1. Through the formation energy calculation, we find that substitutional-doped structure is more stable than the vacancy structure, and the interstitial-doped diamond is the most unstable structure. As the formation energy of interstitial impuritites is more than 90eV, it is difficult to exist in diamond stably. The formation energy of substitutional nitrogen and vacancy increases with increasing pressure, so substitutional nitrogen and vacancy are harder and harder to form. But it is contrary to substitutional phosphorus and sulfur structure,so they are easy to form at high pressure.2. Different impurities can bring different effect on the elastic constants of diamond. However, the trend of doped systems, with increasing pressure and temperatue, is in agreement with pure diamod. At the same pressure and temperature, vacancy makes elastic constants C₁₁,C₄₄, and bulk modulus decrease by about lOGPa, 4GPa, and 3GPa, respectively,but C₁₂ increase about 1.5GPa. Substitutional phosphorus makes elastic constants C₁₁ , C₄₄ ,and bulk modulus decrease by about 3GPa,1.5Gpa, and 2Gpa, respectively, but C₁₂is changed little. Substitutional nitrogen makes elastic constants C₁₂ decrease by about 2GPa, but C₁₁, C₄₄, and bulk modulus is changed little. However, substitutional sulfur has little effect on the elastic properties.3. The bulk modulus and the elastic constants of the diamond systems increase linearly with increasing pressure, so the systems are compressed harder and harder with increasing pressure. The rate of C₁₁ and C₁₂ increased with pressure is thrice as C₄₄.And C₁₂ is more sensitive to the pressure.4. The bulk modulus and the elastic constants of the diamond systems decrease linearly with increasing temperature, so the systems are more easily compressed with the temperature increased. But the elastic constants decreased with the temperature are slower. The rate of C 11 and C₁₂ decreased with the temperature is twice as C₄₄. And C₄₄ is less affected by the temperature.We have studied the effect of temperature, pressure and point defects on the elastic properties of diamond by using molecular dynamics. Our results are significant to the study of the thermodynamic properties of diamond.