## 3 Results and Discussion
### Elastic Properties
When the pressure is greater than 70 GPa, the G/B is inversely proportional to the pressure, which manifests in the brittleness of Co\({}_{2}\)Ti increasing with the increase in pressure when the pressure is less than uniaxial 60 GPa while the brittleness decreases with the pressure when the pressure is higher than 70 GPa. Comparing Tables 3 and 4, the G/B of Co\({}_{2}\)Ti is always larger than that of Co\({}_{2}\)Zr except when the pressure is 10 and 30 GPa, which illustrates that the toughness of Co\({}_{2}\)Ti is weaker than that of Co\({}_{2}\)Zr. Additionally, with its G/B value fluctuating around 0.55, the ductility of Co\({}_{2}\)Ti varies little with pressure, being situated between toughness and brittleness. Young's modulus presents the resistance of materials against tension. However, Poisson's ratio is defined as the ratio of transverse contraction strain versus longitudinal extension strain when materials are stretched. Poisson's ratio makes known the bonding properties within materials. [PERSON] et al. [42] put forward that Poisson's ratio for pure covalent compounds is small (usually 0.1); for typical metals, the value is 0.33; for the ionic covalent crystal, it is between 0.2 and 0.3. The values of Co\({}_{2}\)Ti are all over 0.3, which also confirms the metallic characteristic of Co\({}_{2}\)Zr in the pressure range from approximately 0 to 100 GPa, whereas, the values of Co\({}_{2}\)Zr are larger than 0.33, expect for at 10 and 30 GPa, indicating the Co\({}_{2}\)Zr is metallic but is an ionic covalent crystal at 10 and 30 GPa. Anisotropy refers to a property that all or part of the physical or chemical properties of materials varies in direction, exhibiting the divergence in different directions. As an important property of crystal, anisotropy has significant value as it performs specifically in the elastic modulus, hardness, fracture resistance, yield strength, thermal expansion coefficient, thermal conductivity and so on. In the study of the anisotropy of crystals, we often use a dimensionless parameter A to represent it, that is, A = [(C\({}_{12}\) + 2C\({}_{44}\))/C\({}_{11}\)] - 1 [38]. The anisotropic parameter A of Co\({}_{2}\)Zr and Co\({}_{2}\)Ti at 0 K and various pressures (0-100 GPa) has been given in the Tables 3 and 4. Table 3 shows that when the pressure is 20 GPa, there is a abnormal anisotropic parameter A = 0.219 in Co\({}_{2}\)Zr. Except for 20 GPa, we can find that the anisotropy reaches its highest degree at the pressure of 30 GPa. When less than 30 GPa, the degree of anisotropy increases with the increasing of pressure while the trend is contrary when the pressure is more than 30 GPa. In addition, the degrees of anisotropy are equal at 20 and 70 GPa. The anisotropy of Co\({}_{2}\)Ti varies regularly without any abnormal conditions. From 0 to 70 GPa, the degree of anisotropy increases with the increasing of pressure, whereas that is quite contrary above 70 GPa. We know that the anisotropy of Co\({}_{2}\)Zr is larger than that of Co\({}_{2}\)Ti only at 10 and 30 GPa by comparing Tables 3 and 4. Finally, Figure 1a,b exhibit variation curves of elastic constants C\({}_{\bar{\mathrm{j}}}\), elastic modulus E, shear modulus G and adiabatic bulk modulus B\({}_{\mathrm{S}}\) with pressure of these two crystals at 0 K. The elastic constant C\({}_{11}\) stands for the elasticity in length and the longitudinal strain causes changes of it while C\({}_{12}\) and C\({}_{44}\) dominated by transverse strain, respectively, represent the elasticity in shape [31]. Figure 1a shows that in Co\({}_{2}\)Zr, the changes in C\({}_{11}\) and C\({}_{12}\) with pressure are not monotonic, with the figures under 10 and 30 GPa lower than adjacent pressure. So, it is possible that the physical properties of Co\({}_{2}\)Zr become abnormal, the reason for which is unknown under the present study. In addition, we can find that C\({}_{44}\) increases monotonously with pressure. In all, however, as the pressure increases, C\({}_{11}\) rises faster than C\({}_{12}\) and C\({}_{44}\).