Molecular dynamics simulation of aluminous perovskite

富镁钙钛矿是下地幔中含量最丰富的矿物，被认为是该地区氧化铝Al203的主相。我们用分子动力学方法对铝钙钛矿(Mg，Al)(Al，Si)O_3进行了研究，以预测氧化铝对其物理性能的影响。固溶体的制备方法有两种：1.用Al(3+)离子随机取代钙钛矿中的Mg(2+)和Si(4+)离子。用Al(3+)离子取代钙钛矿中相邻的{Mg(2+)+Si(4+)离子对，以满足局部电荷平衡.虽然后一种固溶体模型给出的热值较低，但两种模型给出的结果基本相同，包括以下特征：1.在P=0 GPA和T=298K时，晶胞参数c和晶胞体积随氧化铝含量的增加而显著增大，这与在常压和温度下获得的粉末X射线衍射数据一致(例如，Irifune等人，1996；Kubo，1999)。在较高的压力下，氧化铝的晶胞参数c和晶胞体积的增加变小。在40-50 GPa时，体积几乎不受铝含量的影响。富Al_2O_3的钙钛矿比不含铝的钙钛矿具有更强的可压缩性，这与15 Gpa以下高压原位X射线研究的结果一致(Kubo等人，1999；Zhang和Weidner，1999)。具有软锰矿成分的钙钛矿(Mg_3Al_2Si3012)的零压下体积弹性模量值大于20%，小于镁硅系钙钛矿。但随着压力的增大，压缩系数的差值变小。在MgSi03-Al203二元系中观察到正的超额热焓。

Mg-rich perovskite, the most abundant mineral of the lower mantle, is thought to be the host phase of alumina Al203 in the region. We have applied molecular dynamics (MD) method to aluminous perovskite (Mg, Al)(AL, Si)O3 in order to predict the effect of alumina on the physical properties. The solid solution was made by the following two methods :1. Random substitution of Mg(2+) and Si(4+) ions in MgSi03 perovskite with Al(3+) ions.2. Substitution of adjacent pairs of {Mg(2+) ion + Si(4+) ion} in MgSi03 perovskite with Al(3+) ions, so as to satisfy the local charge balance.Although the latter solid solution model gives a little lower values of enthalpy, both of the models give almost the same results, including the following characteristics :1. At P=0 GPa and T= 298 K, the unit-cell parameter c and the cell volume increase remarkably with the increase of alumina content, which is consistent with the powder X-ray diffraction data obtained under atmospheric pressure and temperature (e.g., Irifune et al., 1996 ; Kubo, 1999).2. Under the higher pressures, the increase in cell parameter c and cell volume with alumina gets smaller. At around 40-50 GPa, the volume is almost insensitive to the content of alumina.3. The Al203-enriched perovskite is more compressible than aluminum-free perovskite, which is consistent with the results of high-pressure in situ X-ray studies below 15 GPa (Kubo et al., 1999 ; Zhang and Weidner, 1999). The value of the bulk modulus at zero pressure for the perovskite with pyrope composition (Mg3Al2Si3012) is more than 20 %, smaller than that for MgSi03 perovskite. However, the difference of the value of the compressibility gets smaller with the increase of pressure.4. Positive excess enthalpy for MgSi03-Al203 binary system is observed.