BiFeO3 ceramics were investigated by means of infrared reflectivity and time domain THz transmission spectroscopy at temperatures 20 -950 K and the magnetodielectric effect was studied at 10 -300 K with the magnetic field up to 9 T. Below 175 K, the sum of polar phonon contributions into the permittivity corresponds to the value of measured permittivity below 1 MHz. At higher temperatures, a giant low-frequency permittivity was observed, obviously due to the enhanced conductivity and possible Maxwell-Wagner contribution. Above 200 K the observed magnetodielectric effect is caused essentially through the combination of magnetoresistance and the Maxwell-Wagner effect, as recently predicted by Catalan (Appl. Phys. Lett. 88, 102902 (2006)). Since the magnetodielectric effect does not occur due to a coupling of polarization and magnetization as expected in magnetoferroelectrics, we call it improper magnetodielectric effect. Below 175 K the magnetodielectric effect is by several orders of magnitude lower due to the decreased conductivity. Several phonons exhibit gradual softening with increasing temperature, which explains the previously observed highfrequency permittivity increase on heating. The observed non-complete phonon softening seems to be the consequence of the first-order nature of the ferroelectric transition.

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Pr 1−x Na x MnO 3 solid solutions up to x=0.25 were prepared. Irrespective of magnetic order all samples are electrically insulating. The Curie temperature is maximum at TC=125 K for x=0.15; the samples with x⩾0.2 are charge-ordered antiferromagnets. The x=0.2 antiferromagnetic charge-ordered insulator (AFCO) can be switched to the ferromagnetic metallic state (FMM) by application of a relatively weak external magnetic field. This is accompanied by a huge CMR effect, irreversible at low temperatures. The neutron diffraction data, which show a decoupling of charge order (TCO=215 K) and long range AF order of the “pseudo” CE (TN=175 K), are supported by specific heat measurement. The specific heat at 14 T confirmed the transformation of the x=0.2 sample to FMM state and the nature of the AFCO–FMM transition was probed by magnetic moment and electrical resistivity relaxation measurements. This comprehensive characterization of the sample led to a complete determination of the magnetic phase diagram which demonstrated the near degeneracy of the ferro- and antiferromagnetic ground states in Pr0.8Na0.2MnO3.