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Phonon softening in paramagnetic bcc Fe and its relationship to the pressure-induced phase transition

机译:顺磁性bcc铁中的声子软化及其与压力诱导相变的关系

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摘要

The structural stability of paramagnetic (PM) body-centered-cubic (bcc) Fe under pressure is investigated based on first-principles phonon calculations. Spin configurations of the PM phase are approximated using a binary special quasirandom structure with a supercell approach. The behavior of phonon modes can be associated with pressure-induced phase transitions to the face-centered-cubic (fcc) and hexagonal-close-packed (hcp) structures as follows: For the PM phase, it is found that the low-frequency transverse mode at the N point (N_4~- mode), which corresponds to a bcc-hcp phase transition pathway, exhibits strong softening under isotropic volume compression. The frequency of this mode becomes zero by a 2% volume decrease within the harmonic approximation. This result is not consistent with the experimental fact that the phase transition from the PM bcc to hcp phases does not occur under volume compression. The seeming contradiction can be explained only when the anharmonic behavior of the N_4~- mode is taken into consideration; A potential energy curve along the N_4~-mode becomes closer to a double-well shape for the PM phase under volume compression. On the other hand, softening of the longitudinal mode at the 2/3[111 ] point under volume compression is also found for the PM phase, which indicates the pressure-induced bcc-fcc phase transition along this mode. Such behaviors are not seen in ferromagnetic bcc Fe, implying that the magnetic structure plays an essential role on the phase transition mechanism.
机译:基于第一性原理声子计算,研究了顺磁性(PM)体心立方(bcc)Fe在压力下的结构稳定性。 PM相的自旋构型是使用具有超级单元方法的二进制特殊准随机结构来近似的。声子模式的行为可以与压力引起的到面心立方(fcc)和六方密堆积(hcp)结构的相变有关:对于PM相,发现低频N点的横向模式(N_4〜-模式)对应于bcc-hcp相变路径,在各向同性体积压缩下表现出较强的软化作用。在谐波近似范围内,此模式的频率通过减少2%的体积而变为零。该结果与在体积压缩下不会发生从PM bcc到hcp的相变的实验事实不一致。只有考虑到N_4〜-模式的非谐行为,才能解释表面上的矛盾。在体积压缩下,PM相沿N_4〜模式的势能曲线更接近双井形状。另一方面,对于PM相,在体积压缩下的2/3 [111]点处的纵向模式的软化也被发现,这表明压力诱导的bcc-fcc相沿该模式转变。铁磁性bcc Fe中未见此类行为,这表明磁性结构在相变机理中起着至关重要的作用。

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  • 来源
    《Physical review》 |2014年第13期|134106.1-134106.7|共7页
  • 作者单位

    Center for Elements Strategy Initiative for Structure Materials (ESISM), Kyoto University, Kyoto 606-8501, Japan;

    Center for Elements Strategy Initiative for Structure Materials (ESISM), Kyoto University, Kyoto 606-8501, Japan,Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan;

    Center for Elements Strategy Initiative for Structure Materials (ESISM), Kyoto University, Kyoto 606-8501, Japan;

    Center for Elements Strategy Initiative for Structure Materials (ESISM), Kyoto University, Kyoto 606-8501, Japan,Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan,Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456-8587, Japan;

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  • 正文语种 eng
  • 中图分类
  • 关键词

    density functional theory, local density approximation, gradient and other corrections; fe and its alloys;

    机译:密度泛函理论;局部密度近似;梯度和其他校正;铁及其合金;

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