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

Abstract

Structural stability of paramagnetic (PM) body-centered cubic (bcc) Fe under\npressure is investigated based on first-principles phonon calculations. Spin\nconfigurations of the PM phase are approximated using a binary special\nquasi-random structure (SQS) with a supercell approach. The behavior of phonon\nmodes can be associated with pressure-induced phase transitions to the\nface-centered cubic (fcc) and hexagonal close-packed (hcp) structures as\nfollows: For the PM phase, it is found that the low-frequency transverse mode\nat the N point (N$_4^-$ mode), which corresponds to a bcc-hcp phase transition\npathway, exhibits strong softening under isotropic volume compression. The\nfrequency of this mode becomes zero by $2\\%$ volume decrease within the\nharmonic approximation. This result is not consistent with the experimental\nfact that phase transition from the PM bcc to hcp phases does not occur under\nvolume compression. The seeming contradiction can be explained only when\nanharmonic behavior of the N$_4^-$ mode is taken into consideration; a\npotential energy curve along the N$_4^-$ mode becomes closer to a double-well\nshape for the PM phase under the volume compression. On the other hand,\nsoftening of the longitudinal mode at the 2/3[111] point under the volume\ncompression is also found for the PM phase, which indicates the\npressure-induced bcc-fcc phase transition along this mode. Such behaviors are\nnot seen in ferromagnetic (FM) bcc Fe, implying that the magnetic structure\nplays essential roles on the phase transition mechanism.\n

Keywords

Affiliated Institutions

• Japan Fine Ceramics Center JP
• Kyoto University JP

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