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We report on the microwave, terahertz (THz), infrared and Raman spectroscopic studies of BiMn7O12 ceramics, shedding more light into the nature of two structural phase transitions and their possible relation with ferroelectricity in this compound. We observed a softening of one polar phonon in the THz range on cooling towards 460 and 300 K, i.e., temperatures at which BiMn7O12 undergoes subsequent structural phase transitions from monoclinic I2/m to polar monoclinic Im and triclinic P1 phases. The soft phonon causes dielectric anomalies typical for displacive ferroelectric phase transitions. Microwave measurements performed at 5.8 GHz up to 400 K qualitatively confirmed not only the dielectric anomaly at 300 K, but also revealed two other weak dielectric anomalies near the magnetic phase transitions at 60 K and 28 K. This evidences the multiferroic nature of the low-temperature phases, although the relatively high conductivity in the kHz and Hz spectral range prevented us from directly measuring the permittivity and ferroelectric polarization. Some Raman modes sense the magnetic phase transitions occurring near 60 and 25 K, showing that spin-phonon coupling is relevant in this compound and in this temperature range. The deviation of the Mn-O stretching mode frequency from the anharmonic temperature behavior was successfully explained by the spin correlation function calculated from the magnetic contribution to the specific heat.

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Keyword: BiMn7O12, perovskites, multiferroic, ferroelectric transition

Date published: 2024-04-25

Publisher: American Physical Society (APS)

Journal:

• Physical Review B (ISSN: 24699950) vol. 109 issue. 13 134111

Funding:

• Vedecká Grantová Agentúra MŠVVaŠ SR a SAV 2/0137/19
• Grantová Agentura České Republiky 21–06802S

Manuscript type: Publisher's version (Version of record)

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First published URL: https://doi.org/10.1103/physrevb.109.134111

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Updated at: 2024-05-02 16:30:12 +0900

Published on MDR: 2024-05-02 16:30:13 +0900