Electric-field-induced tetragonal phase in [Pb(Mg1/3Nb2/3)O3]0.68–[PbTiO3]0.32 by Raman scattering

ABSTRACT

The paper describes investigation of the electric-field-induced phase transition in PMN-xPT single crystals with x = 0.32 by means of detailed polarized Raman scattering measurements. Field-induced tetragonal state was reached by applying an electric field up to 30 kV/cm along pseudo-cubic direction at a temperature slightly below that of the zero-field rhombohedral–tetragonal phase transition (T_RT). In this field-induced tetragonal state, the angular dependencies of the polarized Raman spectra were recorded and compared with the similar data available for the rhombohedral single-domain and multidomain states.     

Investigation of thermostability and phonon–phonon interactions in Mo6S3I6 nanowires by Raman scattering spectroscopy

Detailed Raman scattering measurements were performed on molybdenum–sulfur–iodine nanowires (Mo₆S₃I₆). At room temperature, 21 well‐resolved Raman modes were experimentally observed for the first time in this new compound. The phase stability and vibrational properties of the nanowires were investigated by different temperature treatments. High‐temperature Raman spectra showed that the phase separation of Mo₆S₃I₆ nanowires took place between 573 and 673 K, followed by appearance of a new mode at 819 cm⁻¹ characteristic of the MoO₃ phase. Low‐temperature Raman scattering spectra showed a significant difference in phonon–phonon interactions between internal and external Raman modes of Mo₆S₃I₆ nanowires. These interesting vibrational properties can give new insights for improved material preparation and achievement of higher conductivity and other functional properties of these otherwise interesting materials. Copyright © 2009 John Wiley & Sons, Ltd.     

Vibrational spectra of disordered oxyfluoride (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>WO<Subscript>3</Subscript>F<Subscript>3</Subscript>

Comparative analysis of the IR absorption and Raman scattering spectra of a polycrystalline sample of perovskite-type oxyfluoride (NH₄)₃WO₃F₃ has been performed in the frequency range 370–4000 cm^?1 at temperatures from 92 to 303 K, including the transition between the orientationally disordered cubic and low-symmetry phases. The conformation of WO₃F₃ octahedral groups is established and transitional anomalies of the internal modes of these groups and ammonium ions are revealed. Comparative analysis of the IR and Raman spectra suggests that the phase transition under study is mainly related to the ordering of octahedral groups and formation of W-O…H-N hydrogen bonds.     

A Raman scattering study of the phase transition in the (NH4)3WO3F3 oxyfluoride

The Raman scattering spectra of polycrystalline samples of the (NH₄)₃WO₃F₃ perovskite-like oxyfluoride were measured at frequencies of 70–3600 cm^?1 in the temperature range 93–323 K, including the transition from the orientationally disordered cubic phase to the low-symmetry phase. Transitional anomalies in the spectral parameters were revealed in the frequency ranges of internal vibrations of the ammonium ions and WO₃F₃ octahedral groups. An analysis of the experimental results allowed the conclusion that the phase transition under study is associated primarily with the ordering of the octahedral groups.     

Probing the ferroelectric phase transition in sol–gel‐derived polycrystalline bismuth ferrite thin films

Polycrystalline BiFeO₃ (BFO) thin films were successfully grown on Pt/Ti/SiO₂/Si(100) and SrTiO₃ (STO) (100) substrates using the chemical solution deposition (CSD) technique. X‐ray diffraction (XRD) patterns indicate the polycrystalline nature of the films with rhombohedrally distorted perovskite crystal structure. Differential thermal analysis (DTA) was performed on the sol–gel‐derived powder to countercheck the crystal structure, ferroelectric (FE) to paraelectric (PE) phase transition, and melting point of bismuth ferrite. We observed a significant exothermic peak at 840 °C in DTA graphs, which corresponds to an FE–PE phase transition. Raman spectroscopy studies were carried out on BFO thin films prepared on both the substrates over a wide range of temperature. The room‐temperature unpolarized Raman spectra of BFO thin films indicate the presence of 13 Raman active modes, of which five strong modes were in the low‐wavenumber region and eight weak Raman active modes above 250 cm⁻¹. We observed slight shifts in the lower wavenumbers towards lower values with increase in temperature. The temperature‐dependent Raman spectra indicate a complete disappearance of all Raman active modes at 840 °C corresponding to the FE–PE phase transitions. There is no evidence of soft mode phonons. Copyright © 2008 John Wiley & Sons, Ltd.     

Study of the P-T phase diagram of pyridinium perchlorate by X-ray diffraction and Raman spectroscopy

The crystal structure and vibrational spectra of deuterated pyridinium perchlorate (d ₅PyH)ClO₄ (C₅D₅NHClO₄) are studied by means of neutron diffraction in ambient conditions, X-ray diffraction at high pressures up to 3.5 GPa in the temperature range 297–420 K, and Raman spectroscopy at high pressures up to 5.7 GPa. Deuterated pyridinium perchlorate at ambient conditions has rhombohedral structure with the R3m symmetry (paraelectric phase I). Over the pressure range of 0.5–1.2 GPa, the phase II with monoclinic symmetry Cm exists. At pressure P ～ 1.2 GPa, the phase transition to monoclinic phase III with the Pm symmetry is observed at ambient temperature. The lattice parameters, unit cell volume, and frequencies of internal vibrational modes as functions of pressure are obtained for different phases of deuterated pyridinium perchlorate. The P-T phase diagram of (d ₅PyH)ClO₄ over the extended pressure and temperature range is discussed.     

Electron-phonon coupling through the orthorhombic to rhombohedral phase transition in La2/3(Ca1−xSrx)1/3MnO3 manganites

We present a systematic study of the Raman modes of a ferromagnetic series of manganites, La_2/3(Ca_1?xSr_x)_1/3MnO₃ with 0?x?1, that presents two different regimes for the variation of the ferromagnetic order temperature, T_C, with Sr doping. This change occurs for x～0.5 when the temperature of the orthorhombic, Pbnm, to rhombohedral, R-3c, structural phase transition, T_t, coincides with T_C. We have analyzed the evolution of the frequencies and widths of the observed Raman modes as a function of Sr doping and temperature. At room temperature, features of the Pbnm structure are detected for compounds up to x=0.6, over the orthorhombic phase limit x=0.45 at 300 K. Octahedra bending modes behave as expected with doping while the tilt mode, which is related to Mn–O–Mn angles and therefore to electronic conduction mechanisms, presents different behaviors in both structures. The tilt frequency is much less sensitive to Mn–O–Mn angle in R-3c than in Pbnm structure indicating a reduction of electron-phonon coupling. Its width presents an anomalous behavior both as a function of doping and temperature with unexpectedly large width in the rhombohedral phase.     

A Raman and dielectric study of a diffuse phase transition in (Pb<Subscript>1-x</Subscript>Ca<Subscript>x</Subscript>)TiO<Subscript>3</Subscript> thin films

Dielectric and Raman scattering experiments were performed on polycrystalline Pb_1-xCa_xTiO₃ thin films (x=0.10, 0.20, 0.30, and 0.40) as a function of temperature. The results showed no shift in the dielectric constant (K) maxima, a broadening with frequency, and a linear dependence of the transition temperature on increasing Ca²⁺ content. On the other hand, a diffuse-type phase transition was observed upon transforming from the cubic paraelectric to the tetragonal ferroelectric phase in all thin films. The temperature dependence of Raman scattering spectra was investigated through the ferroelectric phase transition. The temperature dependence of the phonon frequencies was used to characterize the phase transitions. Raman modes persisted above the tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive. The origin of these modes was interpreted in terms of a breakdown of the local cubic symmetry due to chemical disorder. The lack of a well-defined transition temperature and the presence of broad bands in some temperature interval above the FE–PE phase transition temperature suggested a diffuse-type phase transition. This result corroborates the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in these thin films. PACS 77.80.Bh; 77.55.+f; 78.30.-j; 77.80.-e; 68.55.-a     

Raman spectra of single-crystal C60

We have analysed the Raman spectra of C₆₀ single crystals between room temperature and 10K and studied the temperature-induced phase transition in this material. The spectra show crystal field splitting of the internal Raman modes but no evidence for a line shift near the phase transition. The photo-induced transformation of the crystals and its implication on the interpretation of the Raman spectra is discussed. In the low temperature phase we observed two lines at 30 cm^–1 and 41 cm^–1 which we assign to the librational modes of the crystal.     

Crystal → nematic phase transition in the liquid crystalline system 1‐isothiocyanato‐4‐(trans‐4‐propylcyclohexyl) benzene (3CHBT) probed by temperature‐dependent micro‐Raman study and DFT calculations

Raman spectra of 3CHBT in unoriented form were recorded at 14 different temperature measurements in the range 25–55 °C, which covers the crystal → nematic (N) phase transition, and the Raman signatures of the phase transition were identified. The wavenumber shifts and linewidth changes of Raman marker bands with varying temperature were determined. The assignments of important vibrational modes of 3CHBT were also made using the experimentally observed Raman and infrared spectra, calculated wavenumbers, and potential energy distribution. The DFT calculations using the B3LYP method employing 6‐31G functional were performed for geometry optimization and vibrational spectra of monomer and dimer of 3CHBT. The analysis of the vibrational bands, especially the variation of their peak position as a function of temperature in two different spectral regions, 1150–1275 cm⁻¹ and 1950–2300 cm⁻¹, is discussed in detail. Both the linewidth and peak position of the ( C H ) in‐plane bending and ν(NCS) modes, which give Raman signatures of the crystal → N phase transition, are discussed in detail. The molecular dynamics of this transition has also been discussed. We propose the co‐existence of two types of dimers, one in parallel and the other in antiparallel arrangement, while going to the nematic phase. The structure of the nematic phase in bulk has also been proposed in terms of these dimers. The red shift of the ν(NCS) band and blue shift of almost all other ring modes show increased intermolecular interaction between the aromatic rings and decreased intermolecular interaction between two  NCS groups in the nematic phase. Copyright © 2009 John Wiley & Sons, Ltd.     

Enhanced ferromagnetism at the rhombohedral–tetragonal phase boundary in Pr and Mn co-substituted BiFeO3 powders

The effects of Pr substitution on the structure and magnetism of Bi(Fe_0.95Mn_0.05)O₃ powders are investigated systemically. X-ray diffraction and Raman spectra results confirm a phase transition from rhombohedral to tetragonal structure with increasing Pr concentration and the phase boundary occurs at 14% Pr substitution. At the phase boundary composition, enhanced room temperature ferromagnetism has been observed with a remanent magnetization about 0.260 emu/g. The increased magnetization can be understood considering both the modification of the short-range canted G-type antiferromagnetic order at phase boundaries and the suppression of long-range incommensurate spin cycloid of BiFeO₃ by Pr substitution.     

Dielectric,thermal and Raman spectroscopy studies of lead-free (Na0.5Bi0.5)1−xSrxTiO3 (x = 0, 0.04 and 0.06) ceramics

ABSTRACT

Lead-free (Na_0.5Bi_0.5)_1?xSr_xTiO₃ (x = 0, 0.04 and 0.06) ceramics with relative densities above 97% were prepared by solid-state synthesis process. Their dielectric, thermal and Raman properties were studied. X-ray diffraction analysis shows perovskite structure with rhombohedral symmetry at room temperature. Sr doping of Na_0.5Bi_0.5TiO₃ (NBT) results in an increase of the dielectric permittivity, diffusing of the permittivity maximum and its shift toward lower temperatures. The temperature of the rhombohedral–tetragonal phase transition indicated by the differential scanning calorimetry (DSC) peak and relaxational dielectric anomaly near the depolarization temperature are also shifted toward lower temperatures. The observed increase and broadening of the permittivity maximum, enhancement of the dielectric relaxation near the depolarization temperature, broadening of the DSC anomaly related to the rhombohedral–tetragonal phase transition and broadening of the Raman bands with increasing Sr content are attributed to the increase of the degree of cationic disorder and evident enhancement of the relaxor-like features in NBT–xST. This enhancement could play a positive role in the improvement of the piezoelectric performance of NBT-based ceramics.     

Raman spectroscopic study of the lattice dynamics in the Rb2KMoO3F3 oxyfluoride

The lattice dynamics of the Rb₂KMoO₃F₃ oxyfluoride has been studied by Raman spectroscopy in the temperature range 7–400 K. A phase transition has been revealed at T ≈ 185 K with decreasing temperature. Anomalies of the frequencies and Raman line half-widths have been analyzed. No condensation of soft lattice modes has been found. The character of changes in the Raman spectra of the Rb₂KMoO₃F₃ oxyfluoride shows that the phase transition is related to variations in the [MoO₃F₃]^3? molecular octahedron.     

Raman scattering from soft to-phonon in IV–VI compound semiconductors

The soft TO-phonon modes were investigated below the cubic-rhombohedral transition temperature in Pb_1?xGe_xTe (x=0.05) and SnTe. We observed resolved Raman spectra from ordinary and extraordinary modes. The temperature dependence of the spectra was analyzed in terms of phonon-phonon interactions. We also discussed origins of extra Raman spectra which appear even in the high temperature phase. The energy gain of valence bonding electrons due to lattice distortions was estimated to be ≈10⁹erg/cm³ by analyzing anomalous temperature behavior of the optical dielectric constant. This is about two order of magnitude larger than the depth of the well of free energy.     

Lattice dynamics and Raman scattering spectrum of elpasolite Rb<Subscript>2</Subscript>KScF<Subscript>6</Subscript>: Comparative analysis

Raman scattering spectra of elpasolite Rb₂KScF₆ are studied in a wide temperature range including two phase transitions: from the cubic to the tetragonal phase and then to the monoclinic phase. The experimental Raman scattering spectrum is compared with the lattice vibration spectra of these phases calculated using an ab initio approach. A number of anomalies (caused by structural rearrangement during the phase transitions) are revealed and quantitatively analyzed in the ranges of both the intramolecular vibrations of the octahedron molecular ScF₆ ions and low-frequency intermolecular lattice vibrations. The interaction between low-frequency intramolecular vibrations and the intermolecular modes is found to be significant, and strong resonance interaction of the rotational soft modes (which are recovered below the phase transition points) with hard low-frequency vibrations of the rubidium ion sublattice is detected. These interactions are shown to substantially complicate the spectra.     

Polarized Raman scattering in single crystals of Nd0.7Sr0.3MnO3

We report polarized Raman scattering in single crystals of Nd_0.7Sr_0.3MnO₃. The temperature dependence of the MnO₆ octahedral bending and stretching modes observed in the XX spectra points to the existence of local lattice distortions, possibly polarons. The XY spectra have been analyzed using a collision-dominated model, which allows the extraction of the carrier scattering rate.     

Lattice dynamics and low‐temperature Raman spectroscopy studies of PMN–PT relaxors

In this work we present a Raman scattering study of a specific region of the morphotropic phase boundary (MPB) of the [Pb(Mg_1/3Nb_2/3)O₃]_1−x (PbTiO₃)_x relaxor system. We performed low‐temperature measurement for the x = 0.4 composition in the 20–300 K temperature range, and a detailed analysis of Raman spectra of x = 0.4 and x = 0.37 compositions at 180 K. The analysis of Raman spectra indicates a structural phase transition at around 170 K for x = 0.4. The comparison of Raman data from x = 0.4 and x = 0.37 compositions suggests different phases for these samples at 180 K. These results are in accordance with the tetragonal to monoclinic structural phase transition observed in the PMN–PT MPB and contribute to improve the knowledge of the MPB of this solid solution. Additionally, we have performed the lattice dynamics phonon calculation of the (1 − x) PMN–xPT relaxor in order to best understand its complex Raman spectral properties. The normal mode analyses (at q ∼ 0) were performed by considering tetragonal symmetry for the (1 − x) PMN–xPT system and using the rigid ion model and mean field approximation. Our calculated wavenumber values are in good agreement with experimental and calculated results reported for PbTiO₃ thus providing a reliable assignment of the various Raman modes. The low wavenumber modes are interpreted as arising from a lifting of the degeneracy of the vibrational modes related to Mg, Nb and Ti sites. Copyright © 2009 John Wiley & Sons, Ltd.     

Lacking Raman spectroscopic evidence for a structural phase transition in ZrTe5 at 141 K

The Raman spectra of ZrTe₅ are recorded at various temperatures between 10 and 500 K. The data give no evidence for a structural phase transition, in spite of appreciable temperature dependent effects in some of the Raman modes. The lack of Raman spectroscopic evidence for a low temperature structural phase transition in ZrTe₅ is also confirmed by powder X-ray and neutron diffraction data collected over 100 to 295 and 10 to 295 K, respectively. The present findings strongly suggest that the reported anomaly in the electrical resistivity at 141 K must reflect change(s) in the electronic band structure of ZrTe₅, and the temperature dependences of the Raman spectra could be caused by variations in the first order susceptibility.     

High temperature Raman scattering and phase transition in EuAlO3

Polarized Raman spectra of EuAlO₃ have been obtained at various temperatures from 77–1500 K. Three phonon modes are observed to soften in frequency with increasing temperature. Discontinuity in the frequency temperature dependence appears at 1420 K. This result is consistent with a first order phase transition. Analysis of the Raman data shows evidence of a coupling between the soft mode of Bg(XY) symmetry and one mode of same symmetry.     

Evolution of Raman and Mössbauer spectra at high pressures up to 75 GPa and a phase transition in CaSnO<Subscript>3</Subscript> perovskite

Raman and Mössbauer spectra from ¹¹⁹Sn nuclei in CaSnO₃ perovskite have been studied at high pressures up to 75 GPa. A linear increase in the frequency of the main Raman modes and a monotonic decrease in the isomer shift in Mössbauer spectra in the pressure range of 0–40 GPa are established. It is shown that the pressure-induced increase in Raman frequencies can be associated with the variation of the angle between the Sn–O–Sn bonds in chains of oxygen octahedra SnO₆ along the c axis. The sharp variation of the parameters of the Raman and Mössbauer spectra is observed in the pressure region of 40–55 GPa, indicating the structural phase transformations, which can be associated with the transition into the post-perovskite state. Raman spectra of CaSnO₃ samples with the ilmenite structure have been obtained for the first time.