Jens Kreisel

Phonon anomalies and the ferroelectric phase transition in multiferroic BiFeO3

We report a temperature-dependent Raman and neutron scattering investigation of the multiferroic material bismuth ferrite BiFeO3 (BFO).

Large bulk polarization and regular domain structure in ceramic BiFeO3

Regularly twinned domain structures are observed by scanning piezoforce microscopy on single crystalline grains of BiFeO3 ceramics being grown by a special low temperature sintering process. The domains are considerably larger than those observed in thin films. Their spontaneous polarization comes close to that predicted theoretically and overcomes restrictions hitherto being set to bulk single crystals. The observed ferroelastic twin domain structure resembles that of classic T domains in rhombohedrally distorted NiO, but is additionally superimposed by ferroelectric twin domain patterns.

A structural study of the (Na 1− x K x ) 0.5 Bi 0.5 TiO 3 perovskite series as a function of substitution ( x ) and temperature

Rietveld neutron powder profile analysis of the (Na 1− x K x ) 0.5 Bi 0.5 TiO 3 (NKBT) series ( x =0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) is reported over the temperature range 293–993 K. A detailed characterization of the structures and phase transitions occurring across this series as a function of temperature has been made. Room-temperature refinements have revealed a rhombohedral phase, space group R 3 c for x =0, 0.2, and 0.4, which exhibits an antiphase, a − a − a − oxygen tilt system with parallel cation displacements along [111] p . An intermediate zero-tilt rhombohedral phase, space group R 3 m possessing cation displacements along [111] p , has been established for x =0.5 and 0.6. At the potassium-rich end of the series at x =0.8 and 1.0, a tetragonal phase, space group P 4 mm is observed possessing cation displacements along [001]. At the sodium-rich end of the series for x =0.2, the unusual tetragonal structure with space group P 4 bm is seen for Na 0.5 Bi 0.5 TiO 3 which possesses a combination of in-phase a 0 a 0 c + tilts and antiparallel cation displacements along the polar axis. Temperature-induced phase transitions are reported and structural modifications are discussed.

EXAFS study of lead-free relaxor ferroelectric BaTi(1-x)Zr(x)O3 at the Zr K-edge

Extended X-ray absorption fine structure (EXAFS) experiments at the Zr K-edge were carried out on perovskite relaxor ferroelectrics BaTi(1-x)Zr(x)O3 (BTZ) (x = 0.25, 0.30, 0.35), and on BaZrO3 for comparison. Structural information up to 4.5 A around the Zr atoms is obtained, revealing that the local structure differs notably from the average Pm-3m cubic structure deduced from X-ray diffraction. In particular, our results show that the distance between Zr atoms and their first oxygen neighbors is independent of the Zr substitution rate x and equal to that measured in BaZrO3, while the X-ray cubic cell parameter increases linearly with x. Furthermore, we show that the Zr atoms tend to segregate in Zr-rich regions. We propose that the relaxor behavior in BTZ is linked to random elastic fields generated by this particular chemical arrangement, rather than to random electric fields as is the case in most relaxors.

Heteroepitaxially enhanced magnetic anisotropy in BaTiO3–CoFe2O4 nanostructures

The authors have studied the magnetic properties of BaTiO3–CoFe2O4 nanostructures, which were prepared using pulsed laser deposition. Such nanostructures show a large uniaxial magnetic anisotropy with an easy axis along the pillar long direction. As the growth temperature decreases, the magnetic anisotropy increases. Careful analyses reveal that heteroepitaxial strain is the primary contribution to the magnetic anisotropy.

Raman scattering of the model multiferroic oxide BiFeO3: effect of temperature, pressure and stress

The perovskite Bismuth ferrite BiFeO3 (BFO) is considered to be a model multiferroic and offers a rare multiferroelectric robustness since it presents a coexistence of ferroelectric and antiferromagnetic order up to unusually high temperatures. Perovskite-type materials are known for their common structural instabilities which can be driven by diverse external parameters like temperature, pressure, stress etc. Such instabilities and the associated structural distortions are often very subtle and difficult to detect by techniques probing the average structure such as X-ray diffraction. Here we present an investigation of the BFO phonon spectrum by the local probe Raman spectroscopy as a function of polarization, temperature and pressure. We review a recent temperature-dependent Raman investigation which illustrates a first-order structural phase transition at the ferroelectric Curie temperature. Our temperature dependence results further indicate a phonon-anomaly around the magnetic Néel temperature, which is discussed in the light of multiferroicity. We will further illustrate that BFO presents important pressure-induced structural instabilities and we discuss the role of such instabilities for the understanding of strained thin film BFO which show distinct properties compared to the bulk.

The high-pressure behaviour of Ba-doped Na1/2Bi1/2TiO3 investigated by Raman spectroscopy

We report high-pressure Raman measurements up to 20.2 GPa on the perovskite-type oxide (Na1/2Bi1/2)0.89Ba0.11TiO3 (NBT–BT0.11), which is of interest as a potential lead-free piezoelectric material. Distinct changes of the Raman spectra with increasing pressure illustrate that NBT–BT0.11 shows at least two phase transitions. First, the application of low pressure progressively reduces the tetragonal signature in NBT–BT0.11 and leads at around 1.8 GPa to a Raman spectrum reminiscent of rhombohedral NBT. Second, with further increase of pressure, the pressure-dependent behaviour of NBT–BT0.11 resembles that of NBT and transforms to a non-cubic high-pressure structure at around 9 GPa. These observations suggest that a progressive substitution of Na+/Bi3+ by Ba2+ pushes the pressure-induced structural instabilities of NBT towards higher pressure, corresponding to a Ba2+-induced negative (tensile) pressure within the structure.

Multiple high-pressure phase transitions in BiFeO3

We investigate the high-pressure phase transitions in BiFeO3 by single-crystal and powder x-ray diffraction, as well as single-crystal Raman spectroscopy. Six phase transitions are reported in the 0-60-GPa range. At low pressures, four transitions are evidenced at 4, 5, 7, and 11 GPa. In this range, the crystals display large unit cells and complex domain structures, which suggests a competition between complex tilt systems and possibly off-center cation displacements. The nonpolar Pnma phase remains stable over a large pressure range between 11 and 38 GPa, where the distortion (tilt angle) changes only little with pressure. The two high-pressure phase transitions at 38 and 48 GPa are marked by the occurrence of larger unit cells and an increase of the distortion away from the cubic parent perovskite cell.We find no evidence for a cubic phase at high pressure, nor indications that the structure tends to become cubic. The previously reported insulator-to-metal transition at 50 GPa appears to be symmetry breaking.

RAMAN STUDY OF SUBSTITUTED BARIUM FERRITE SINGLE CRYSTALS, BAFE12-2XMEXCOXO19 (ME = IR, TI)

This paper reports the first single-crystal Raman spectra of substituted barium hexaferrites, BaFe12−2x MexCoxO19 (Me=Ir, Ti). The spectra were investigated in the 200–900 cm−1 range and are discussed in comparison with unsubstituted barium hexaferrite. Emphasis was placed on the different factors influencing the Raman spectra of substituted barium hexaferrites, namely the mass of the substituting cation, the ionic radius and charge and the ordering on the five distinct iron sites. Copyright

Pressure-temperature phase diagram ofSrTiO3up to 53 GPa

We investigate the cubic to tetragonal phase transition in the pressure-temperature phase diagram of strontium titanate SrTiO 3 STO by means of Raman spectroscopy and x-ray diffraction on single-crystal samples. X-ray diffraction experiments are performed at room temperature, 381 and 467 K up to 53 GPa, 30 GPa, and 26 GPa, respectively. The observation of the superstructure reflections in the x-ray patterns provides evidence that the crystal undergoes at all investigated temperatures a pressure-induced transition from cubic to the tetragonal I4 / mcm phase, identical to the low-temperature phase. No other phase transition is observed at room temperature up to 53 GPa. Together with previously published data, our results allow us to propose a linear phase boundary in the pressure-temperature phase diagram. The data are analyzed in the framework of the Landau theory of phase transitions. With a revised value of the coupling coefficient between the order parameter and the volume spontaneous strain, the model built from pressure-independent coefficients reproduces satisfactorily the boundary in the phase diagram, but fails at reflecting the more pronounced second-order character of the pressure-induced phase transition as compared to the temperature-induced transition. We propose a Landau potential suitable for the description of the pressure-induced phase transition. Finally, we show that particular attention has to be paid to hydrostatic conditions in the study of the high-pressure phase transition in STO.