First-Principles Molecular Dynamics Study of the Structure of MgSiO3 Melt at High Temperatures and High Pressures
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摘要: 基于第一性原理分子动力学方法，计算了MgSiO3熔体在0~144 GPa、2 000~6 000 K的微观结构及其随压力、温度的变化特征。计算的近零压2 000 K下O—Si、O—Mg和O—O对分布函数的第一峰值位置分别为0.163 5、0.197 0和0.269 5 nm，与实验结果吻合很好。随着压力和温度的变化，MgSiO3熔体结构发生了显著变化，尤其是随着压力增加，结构变得更致密；当密度为4.59 g/cm3时，原子间的平均键长随温度(小于5 000 K)增加而减小，在常压和更高的压力下，原子间的平均键长随温度变化不明显。在133 GPa、4 000 K条件下，MgSiO3熔体的O—Si、O—Mg和O—O平均键长分别为0.161 0、0.183 5和0.230 0 nm；从地表常压到核幔边界压力，平均Si—O配位数从4变到6，桥氧数目比例由31.3%增高到72.9%。MgSiO3熔体微观结构的认识对了解地幔内硅酸盐流体性质及其对地幔动力学的影响有重要意义。
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关键词:
- MgSiO3熔体 /
- 第一性原理分子动力学 /
- 对分布函数 /
- 平均配位数
Abstract: The microstructures of MgSiO3 melt and their variation with temperature and pressure were investigated based on first-principles molecular dynamic simulations at high pressures (0-144 GPa) and high temperatures (2 000-6 000 K).The calculated first peak positions of the pair correlation function of O—Si, O—Mg and O—O under the condition of 0 GPa and 2 000 K are 0.163 5, 0.1 970 and 0.269 5 nm, respectively, which are consistent with the previous experimental values.As the pressure and temperature change, the structure of MgSiO3 melt undergoes a significant change.Especially when the pressure increases, the structure becomes denser.When the temperature is below 5 000 K, the average bond lengths between two atoms decrease with the increasing temperature with density 4.59 g/cm3.While under nomal or higher pressure, the average bond length change with the increasing temperature is not obvious.At 133 GPa and 4 000 K, the average bond lengths of O—Si, O—Mg and O—O are 0.161 0, 0.183 5 and 0.230 0 nm, respectively; the average Si—O coordination number increases from 4 to 6, and the number of bridging oxygen ratio increases from 31.3% to 72.9%, from atmospheric pressure to the core-mantle boundary.The knowledge of MgSiO3 melt microstructure is important to understand the mantle silicate fluid nature of mantle dynamics. -
图 1 MgSiO3熔体在2 000 K和近零压(约-0.6 GPa)下的PCF图
Figure 1. The PCF graph of MgSiO3 melt at 2 000 K and close to zero pressure (about -0.6 GPa)
图 3 MgSiO3熔体在密度4.59 g/cm3时不同温度的PCF图
Figure 3. The PCF graph of MgSiO3 melt with density 4.59 g/cm3 under different temperatures
图 4 MgSiO3熔体原子间平均键长随温度和压力的变化
Figure 4. The average interatomic bond lengths of MgSiO3 melt changes with temperature and pressure
图 5 MgSiO3熔体原子平均配位数随温度的变化
Figure 5. The coordination number of MgSiO3 melt changes with temperature and pressure
表 1 MgSiO3熔体在不同温压下的PCF峰值位置
Table 1. The calculation positions of PCF peaks of MgSiO3 melt under different pressures and temperatures
(nm) ρ/(g/cm3) T/(K) O—O O—Mg O—Si rp1 rm rp2 rp1 rm rp2 rp1 rm rp2 2.71
This work2 000 0.269 5 0.415 0 0.513 0 0.197 0 0.266 5 0.432 5 0.163 5 0.219 5 0.413 5 3 000 0.272 0 0.392 5 0.505 0 0.197 0 0.275 0 0.436 5 0.162 5 0.236 0 0.411 5 4 000 0.267 0 0.397 5 0.513 5 0.196 0 0.302 0 0.413 5 0.165 0 0.241 0 0.403 5 5 000 0.275 0 0.388 0 0.508 5 0.191 5 0.292 0 0.420 0 0.164 0 0.255 0 0.402 5 6 000 0.272 0 0.388 0 0.528 0 0.203 5 0.294 5 0.419 5 0.164 0 0.226 5 0.404 5 5.16
This work3 000 0.229 0 0.303 5 0.436 5 0.184 0 0.279 0 0.392 5 0.162 0 0.228 0 0.383 0 4 000 0.230 0 0.302 0 0.436 0 0.183 5 0.269 0 0.388 5 0.161 0 0.231 5 0.371 5 6 000 0.224 5 0.308 0 0.438 5 0.185 0 0.272 5 0.384 5 0.159 0 0.233 0 0.367 5 Waseda[10] 1 970 0.212 0 0.162 0 2.57
Karki[5]2 500 0.267 5 0.397 5 0.505 5 0.196 5 0.290 5 0.429 5 0.162 5 0.236 5 0.389 0 3 000 0.267 5 0.397 5 0.507 0 0.196 5 0.292 0 0.429 5 0.162 5 0.237 5 0.383 5 4 000 0.267 5 0.388 5 0.507 5 0.196 5 0.295 0 0.430 5 0.162 5 0.243 5 0.386 0 5 000 0.267 5 0.392 5 0.510 5 0.196 5 0.307 5 0.432 5 0.162 5 0.246 5 0.384 0 6 000 0.266 5 0.389 5 0.515 0 0.195 5 0.307 5 0.434 5 0.162 5 0.247 5 0.371 0 5.14
Karki[5]3 000 0.231 5 0.301 5 0.427 5 0.184 5 0.262 5 0.400 5 0.162 5 0.234 5 0.375 5 4 000 0.229 5 0.301 0 0.430 5 0.184 5 0.272 5 0.395 5 0.162 0 0.229 5 0.375 0 6 000 0.225 5 0.306 0 0.432 5 0.183 5 0.275 0 0.389 5 0.159 5 0.237 5 0.370 5 
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