Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA.
J Phys Condens Matter. 2010 Mar 10;22(9):096006. doi: 10.1088/0953-8984/22/9/096006. Epub 2010 Feb 17.
The 0 K pressure-induced magnetic phase transformations of face-centered cubic (FCC) and hexagonal close packed (HCP) Co have been examined using first-principles calculations. Issues of fitting an equation of state to the first-principles energy versus volume data points containing a magnetic transformation and comparing to experimental phase equilibria are discussed. It is found that a fitting scheme employing only data where the magnetic moment decreases linearly with volume offers a physically meaningful behavior for the equation of state at metastable volumes. From this fitting, the ferromagnetic to nonmagnetic transformations with increasing pressure at 0 K are at 77 GPa and 123 GPa for FCC and HCP, respectively, and are first order and second order, respectively, on the basis of an unambiguous measure proposed in the paper. In addition to the HCP/FCC structure transformation at 99 GPa, another transformation at negative pressures is predicted, at - 31 GPa. These results are shown to be consistent with the extrapolations of the experimental pressure-temperature phase diagram to 0 K.
采用第一性原理计算研究了面心立方（FCC）和六方密堆积（HCP）Co 的 0 K 压力诱导的磁相转变。讨论了将状态方程拟合到包含磁转变的第一性原理能量与体积数据点以及与实验相平衡进行比较的问题。结果发现，仅使用磁矩随体积线性减小的数据的拟合方案为亚稳体积的状态方程提供了物理上有意义的行为。根据本文提出的明确测量方法，从该拟合中得出，在 0 K 下，随着压力的增加，FCC 和 HCP 的铁磁到非磁转变分别在 77 GPa 和 123 GPa 处发生，分别为一级和二级转变。此外，还预测了在 99 GPa 处的 HCP/FCC 结构转变之外，在负压下还会发生另一种转变，在-31 GPa 处发生。这些结果与实验压力-温度相图向 0 K 的外推结果一致。
