辉石结构与相变的第一性原理研究

Structural Properties and Phase Transition of Pyroxene Polymorphs from First-Principles

利用第一性原理方法研究了Mg端员辉石(MgSiO3)低压相(Pbca、P2_1/c)和高压相(C2/c、P2_1ca)在不同压力下(0~30GPa)的结构。首先计算得到了不同压力下各相的晶胞体积,并由状态方程拟合得到了体积模量,结果显示在零温零压条件下Pbca和P2_1/c(低压单斜相)的体积模量相近,C2/c的体积模量最大(比Pbca的大3%),而P2_1ca作为高压相其体积模量却比Pbca小很多。其次分析了Mg端员辉石3个轴向的压缩性,发现C2/c相的c轴比a轴难压缩,与前人研究的透辉石(MgCaSi2O6)的表现相反。P2_1/c、C2/c、P2_1ca的链角随着压力的增大而减小;Pbca其中一种链角的变化趋势和其他3个相一样,而另一种链角先随着压力的增大而减小,在达到7GPa后开始增大,可能表示相的不稳定或开始相变。最后通过研究不同相之间的焓值差,讨论了Mg端员辉石在低温高压下可能存在的相变情况。

In order to evaluate the structures of magnesium end-member pyroxene polymorphs（MgSiO3）under different pressures,first-principles theoretical calculation on low-and high-pressure phases,with space group Pbca,P21/c,C2/c,P21 carespectively,was conducted under pressures up to 30 GPa.The bulk moduli of polymorphs were obtained from fitting the third-order Birch-Murnaghan equation with calculated pressure-volume data.The C2/c phase had the largest modulus（an increase of3% compared to Pbca）under zero-temperature and zero-pressure,whereas little difference was observed between moduli of Pbcaand P21/c,and the high-pressure phase P21 cashowed a smaller value than Pbca.Moreover,the results of axial compression showed that the c-axis was harder to compress than a-axis in C2/c,which was opposite to the previous first-principle results on diopside（MgCaSi2O6）.The angels of SiO4 tetrahedral chains in P21/c,C2/c,and P21 cadecreased monotonically as a function of pressure while in Pbca,which had two kinds of angles,one showed the same trend as the aforementioned three polymorphs and the other increased monotonically above 7GPa,implying an unstable structure or the onset of a new phase transition.The static enthalpy differences among the four polymorphs indicated the possible phase transitions of the pyroxene under low-temperature and high-pressure.