Kawano Jun, Miyake Akira, Shimobayashi Norimasa, Kitamura Masao
Department of Earth and Planetary Science, Faculty of Science, Kyoto University, Kyoto 606-8502, Japan.
J Phys Condens Matter. 2009 Jul 8;21(27):275403. doi: 10.1088/0953-8984/21/27/275403. Epub 2009 Jun 10.
Molecular dynamics (MD) simulation of calcium carbonate at high pressure was performed to understand the phase transition between calcite [Formula: see text] and CaCO(3)-II (P 2(1)/c). In the 300-800 K temperature range, the transition of calcite to CaCO(3)-II was reproduced at a pressure of around 8 GPa. This transition is of first order and reversible in the MD calculations except for runs at 300 K where a small hysteresis exists. The slope of the dP/dT curve at the phase boundary between calcite and CaCO(3)-II is negative at 300 K and turns positive at around 600 K, which was confirmed by analyzing the enthalpy change. Just below the transition pressure, the P 2(1)/c structure appears and its orientation switches among three positions with time, resulting in the maintenance of the [Formula: see text] structure as a whole. The P 2(1)/c structure resembles the structure of CaCO(3)-II on an increase of temperature. It can be suggested that the existence of the P 2(1)/c structure and the switching of its orientation just below the transition pressure are responsible for the change of the slope of the dP/dT curve at the boundary from negative to positive on an increase of temperature, because the switching increases entropy and results in an expansion of the stability field of calcite.
为了理解方解石[化学式：见正文]与CaCO₃-II（P 2₁/c）之间的相变，进行了碳酸钙在高压下的分子动力学（MD）模拟。在300 - 800K的温度范围内，在约8GPa的压力下再现了方解石向CaCO₃-II的转变。除了在300K运行时存在小的滞后现象外，这种转变在MD计算中是一级且可逆的。通过分析焓变证实，方解石与CaCO₃-II相界处的dP/dT曲线斜率在300K时为负，在约600K时变为正。就在转变压力以下，P 2₁/c结构出现，其取向随时间在三个位置之间切换，从而整体上维持了[化学式：见正文]结构。随着温度升高，P 2₁/c结构类似于CaCO₃-II的结构。可以认为，P 2₁/c结构的存在及其在转变压力以下取向的切换导致了边界处dP/dT曲线斜率随温度升高从负变正，因为这种切换增加了熵并导致方解石稳定场的扩展。
