40 research outputs found
Peculiar Bi-ion dynamics in Na1/2Bi1/2TiO3 from terahertz and microwave dielectric spectroscopy
Dynamics of the main dielectric anomaly in Na1/2Bi1/2TiO3 (NBT) was studied
by time-domain THz and microwave spectroscopy, using also previously published
data and their new overall fits. Above the dielectric maximum temperature Tm ~
600 K, the response consists of coupled sub-THz oscillator and a relaxation
mode, assigned to strongly anharmonic Bi-ion vibrations and hopping, whose
slowing down explains the paraelectric-like permittivity increase to Tm. Below
Tm, the main relaxation continues slowing down and additional relaxation,
assigned to quasi-Debye losses, appears in the 10^11 Hz range. The oscillator
hardens on cooling and takes over the whole oscillator strength. The
permittivity decrease below Tm is caused by the reduced strength of the
relaxations due to dominance of the rhombohedral phase within the coexistence
region with the tetragonal phase. The anharmonic dynamics of Bi is supported by
previous structural studies. NBT represents a hybrid between standard and
relaxor ferroelectric behaviour
Magnetic and dielectric properties of multiferroic Eu0.5Ba0.25Sr0.25TiO3 ceramics
Dielectric and magnetic properties of Eu0.5Ba0.25Sr0.25TiO3 are investigated
between 10 K and 300 K in the frequency range from 10 Hz to 100 THz. A peak in
permittivity revealed near 130 K and observed ferroelectric hysteresis loops
prove the ferroelectric order below thistemperature. The peak in permittivity
is given mainly by softening of the lowest frequency polar phonon (soft mode
revealed in THz and IR spectra) that demonstrates displacive character of the
phase transition. Room-temperature X-ray diffraction analysis reveals cubic
structure, but the IR reflectivity spectra give evidence of a lower crystal
structure, presumably tetragonal I4/mcm with tilted oxygen octahedra as it has
been observed in EuTiO3. The magnetic measurements show that the
antiferromagnetic order occurs below 1.8 K. Eu0.5Ba0.25Sr0.25TiO3 has three
times lower coercive field than Eu0.5Ba0.5TiO3, therefore we propose this
system for measurements of electric dipole moment of electron.Comment: Phase Transitions, in pres
A new electron diffraction approach for structure refinement applied to Ca3Mn2O7
The digital large-angle convergent-beam electron diffraction (D-LACBED) technique is applied to Ca3Mn2O7 for a range of temperatures. Bloch-wave simulations are used to examine the effects that changes in different parameters have on the intensity in D-LACBED patterns, and atomic coordinates, thermal atomic displacement parameters and apparent occupancy are refined to achieve a good fit between simulation and experiment. The sensitivity of the technique to subtle changes in structure is demonstrated. Refined structures are in good agreement with previous determinations of Ca3Mn2O7 and show the decay of anti-phase oxygen octahedral tilts perpendicular to the c axis of the A21am unit cell with increasing temperature, as well as the robustness of oxygen octahedral tilts about the c axis up to ∼400°C. The technique samples only the zero-order Laue zone and is therefore insensitive to atom displacements along the electron-beam direction. For this reason it is not possible to distinguish between in-phase and anti-phase oxygen octahedral tilting about the c axis using the [110] data collected in this study