Polarized Raman study on the lattice structure of BiFeO3 films prepared by pulsed laser deposition

Polarized Raman spectroscopy was used to study the lattice structure of BiFeO₃ films on different substrates prepared by pulsed laser deposition. Interestingly, the Raman spectra of BiFeO₃ films exhibit distinct polarization dependences. The symmetries of the fundamental Raman modes in 50–700 cm⁻¹ were identified based on group theory. The symmetries of the high order Raman modes in 900–1500 cm⁻¹ of BiFeO₃ are determined for the first time, which can provide strong clarifications to the symmetry of the fundamental peaks in 400–700 cm⁻¹ in return. Moreover, the lattice structures of BiFeO₃ films are identified consequently on the basis of Raman spectroscopy. BiFeO₃ films on SrRuO₃ coated SrTiO₃ (0 0 1) substrate, CaRuO₃ coated SrTiO₃ (0 0 1) substrate and tin-doped indium oxide substrate are found to be in the rhombohedral structure, while BiFeO₃ film on SrRuO₃ coated Nb: SrTiO₃ (0 0 1) substrate is in the monoclinic structure. Our results suggest that polarized Raman spectroscopy would be a feasible tool to study the lattice structure of BiFeO₃ films.     

Impact of substrate on some physical properties of PrFe0.5Ni0.5O3 thin films

We carried out the structural, morphological and transport study of PrFe_0.5Ni_0.5O₃ thin films prepared by pulsed laser deposition (PLD) over various substrates. Different substrates like LaAlO₃ (001),GaAs(001) and Si(001) were used for deposition to understand effect of lattice mismatch on various physical properties. The film deposited on LaAlO₃ was of best quality with well (001)-oriented and having good crystalline properties. Whereas, film deposited on GaAs(001) is well textured. Both films shows semiconducting behavior and resistance of the film deposited on GaAs(001) shows larger than that of film deposited on LAO. However, film deposited on Si, also shows polycrystalline growth with unusual metallic behavior. We tried to correlate this behavior with strain-induced growth of these films. Other possibilities for this unusual trend is also explored.     

Fabrication and electron transport properties of epitaxial films of electron-doped 12CaO·7Al2O3 and 12SrO·7Al2O3

Epitaxial growth and electron doping of 12CaO·7Al₂O₃ (C12A7) and 12SrO·7Al₂O₃ (S12A7) are reported. The C12A7 films were prepared on Y₃Al₅O₁₂ (YAG) single-crystal substrates by pulsed laser deposition at room temperature and subsequent thermal crystallization. X-ray diffraction patterns revealed the films were grown epitaxially with the orientation relationship of (001)[100] C12A7 || (001)[100] YAG. For S12A7, pseudo-homoepitaxial growth was attained on the C12A7 epitaxial layer. Upon electron doping, metallic conduction was achieved in the C12A7 film and the S12A7/C12A7 double-layered films. Analyses of optical absorption spectra for the S12A7/C12A7 films provided the densities of free electrons in each layer separately. Hall measurements exhibited larger electron mobility in the S12A7/C12A7 film than those in C12A7 and S12A7 films, suggesting free electrons may be accumulated at the S12A7/C12A7 interface due presumably to a discontinuity of the cage conduction bands.     

Substrate-induced anisotropy of c-axis textured NaxCoO2 thin films

Na_xCoO₂ [x = 0.51, 0.54, and 0.59] thin films have been grown on SrTiO₃ (100)-oriented single crystals with a 5° vicinal cut towards [010] by pulsed laser deposition. We analysed the films by X-ray diffractometry, atomic force microscopy (AFM), and dc-transport measurements. X-ray diffraction patterns of the films show single phase and c-axis textured growth with the film plane closely aligned to the [001]-direction of 5° miscut SrTiO₃ (001) substrates. In addition to the structural analysis of these films we performed transport measurements along and perpendicular to the substrate tilt direction and determined the resistivity anisotropy as a function of temperature. The results enable the development of a strategy for the fabrication of Na_xCoO₂ based thermoelectric thin film devices.     

A multiconfigurational approach of the symmetry breaking problem in the cyclic C3H radical

The ground state of the cyclic C₃H radical is prone to symmetry breaking due to a peculiar structure that require to distribute three electrons on two centers which are equivalent by symmetry. A pseudo Jahn–Teller effect is also observed due to the non-adiabatic interaction with the first excited state. The multiconfigurational self-consistent-field method is used to calculate wavefunctions free from symmetry breaking. The weakening of the pseudo Jahn–Teller effect is observed when the correlation energy is increased. The equilibrium geometry and harmonic vibrational frequencies are calculated with the multireference configuration interaction method and a good agreement with experimental results is obtained. The present work confirms that the ground state of the cyclic C₃H has the C_2v symmetry.     

A Raman study of multiferroic LuMnO3

Magnetic and dielectric measurements confirm the multiferroic nature of LuMnO₃. Raman spectra of LuMnO₃ have been recorded in the 77–800 K range covering both the antiferromagnetic transition at 90 K and the ferroelectric–paraelectric transition at 750 K. The changes in the phonon modes frequencies and band-widths indicate the presence of phonon–spin coupling in the antiferromagnetically ordered phase. The ferroelectric–paraelectric transition is accompanied by the broadening and disappearance of many of the phonon modes. Some of the phonon modes also show anomalies at the ferroelectric transition.     

High-resolution transmission electron microscopy investigation of nanostructures in SnO2 thin films prepared by pulsed laser deposition

Pulsed laser deposition (PLD) was used to grow nanocrystalline SnO₂ thin films onto glass substrates. The nanocrystallites and microstructures in SnO₂ thin films grown by PLD techniques have been investigated in detail by using X-ray diffraction and high-resolution transmission electron microscopy (HRTEM). The PLD process was carried out at room temperature under a working pressure of about 2×10⁻⁶ mbar. Experimental results indicate that thin films are composed of a polycrystalline SnO₂ and an amorphous SnO phase. In particular, the presence of such an amorphous SnO phase in the thin films greatly limits their practical use as gas-sensing devices. HRTEM observations revealed that SnO₂ nanocrystallites with tetragonal rutile structure embed in an amorphous SnO matrix, which are approximatively equiaxed. These approximatively equiaxed SnO₂ nanocrystallites contain a high density of defects, such as twin boundaries and edge dislocations. The grain growth of SnO₂ thin films may be discussed in terms of the coalescent particle growth mechanism.     

Raman, XRD and microscopic investigations on CeO2-Lu2O3 and CeO2-Sc2O3 systems: A sub-solidus phase evolution study

A sub-solidus phase evolution study was done in CeO₂-Sc₂O₃ and CeO₂-Lu₂O₃ systems under slow-cooled conditions from 1400 °C. Long-range order probing of X-ray diffraction technique is utilized in conjunction with the ability of Raman spectroscopy to detect the changes in local co-ordination. Lu₂O₃ showed solubility of 30 mol% in CeO₂, thus forming an anion deficient fluorite-type (F-type) solid solution, whereas Sc₂O₃ did not show any discernible solubility. A biphasic region (F+C) was unequivocally detected by Raman spectroscopy in Ce_1−xLu_xO_2−x/2 (0.4?x?0.9) and in Ce_1−xSc_xO_2−x/2 (0.1?x?0.9) systems. Raman spectroscopy was valuable in studying these systems since oxygen vacancies are created on doping RE₂O₃ into ceria and Raman spectroscopy is very much sensitive to oxygen polarizability and local coordination. Back scattered images collected on representative compositions support the above-mentioned results.     

High pressure behavior of α-NaVO3: A Raman scattering study

The reported pressure-induced amorphization in α-NaVO₃ has been re-investigated using Raman spectroscopy. Discontinuous changes are noted in the Raman spectrum above 5.6 GPa implying large structural changes across the transition. The decrease in frequency of the V-O stretching mode across the transition suggests that the vanadium atom may be in octahedral coordination in the high pressure phase. Excessive broadening of the internal modes is observed above 6 GPa. New peaks characteristic of a crystalline phase gain in intensity at higher pressures in the bending modes region; however, the transformation is not complete even at 13 GPa. Co-existence of phases is noted over a significant pressure range above the onset of transition. Pressure released spectrum is found to be a mixture of crystalline α-phase, traces of crystalline β-phase and highly disordered phase consisting of V-O units in five- and six-fold coordination.     

Li2Si3O7: Crystal structure and Raman spectroscopy

The crystal structure of metastable Li₂Si₃O₇ was determined from single crystal X-ray diffraction data. The orthorhombic crystals were found to adopt space group Pmca with unit cell parameters of , and . The content of the cell is Z=4. The obtained structural model was refined to a R-value of 0.035. The structure exhibits silicate sheets, which can be classified as [Si₆O₁₄] using the silicate nomenclature of Liebau. The layers are build up from zweier single chains running parallel to c. Raman spectra are presented and compared with other silicates. Furthermore, the structure is discussed versus Na₂Si₃O₇.     

Phase and structural characterization of Sr2Nb2O7 and SrNbO3 thin films grown via pulsed laser ablation in O2 or N2 atmospheres

The influence of substrate temperature, process gas, deposition pressure, and substrate type on the phase selection, orientation/epitaxy, and growth morphology of thin films in the SrNbO_y (y≈3.0 or 3.5) family was investigated. Pulsed laser deposited films (from a Sr₂Nb₂O₇ target) obtained in both oxygen and nitrogen atmospheres upon various substrates were characterized with X-ray diffraction, energy dispersive spectroscopy, atomic force microscopy, and transmission electron microscopy. In oxygen atmospheres, films adopted the (110)-layered perovskite structure of the target. Higher temperatures, lower pressures of oxygen, and use of (110)-oriented SrTiO₃ substrates lead to highly crystalline, epitaxial films of Sr₂Nb₂O₇. The use of nitrogen atmospheres resulted in cubic perovskite SrNbO₃ formation: epitaxial, textured, or polycrystalline films were obtained depending on the substrate; no nitrogen incorporation could be observed on the anion sublattice. On SrTiO₃, the cubic perovskite films followed a cube-on-cube epitaxy and planar defects were observed to occur on the (110) perovskite planes.     

Comparative study of LiMn2O4 thin film cathode grown at high, medium and low temperatures by pulsed laser deposition

LiMn₂O₄ thin films with different crystallizations were respectively grown at high, medium and low temperatures by pulsed laser deposition (PLD). Structures, morphologies and electrochemical properties of these three types of thin films were comparatively studied. Films grown at high temperature (?873 K) possessed flat and smooth surfaces and were highly crystallized with different textures and crystal sizes depending on the deposition pressure of oxygen. However, films deposited at low temperature (473 K) had rough surfaces with amorphous characteristics. At medium temperature (673 K), the film was found to consist mainly of nano-crystals less than 100 nm with relatively loose and rough surfaces, but very dense as observed from the cross-section. The film deposited at 873 K and 100 mTorr of oxygen showed an initial discharge capacity of 54.3 μAh/cm² μm and decayed at 0.28% per cycle, while the amorphous film had an initial discharge capacity of 20.2 μAh/cm² μm and a loss rate of 0.29% per cycle. Compared with the highly crystallized and the amorphous films, nano-crystalline film exhibited higher potential, more capacity and much better cycling stability. As high as 61 μAh/cm² μm of discharge capacity can be achieved with an average decaying rate of only 0.032% per cycle up to 500 cycles. The excellent performance of nano-crystalline film was correlated to its microstructures in the present study.     

Pyrochlore “dynamic spin-ice” Pr2Sn2O7 and monoclinic Pr2Ti2O7: A comparative temperature-dependent Raman study

Here we report a temperature-dependent Raman study of the pyrochlore “dynamic spin-ice” compound Pr₂Sn₂O₇ and compare the results with its non-pyrochlore (monoclinic) counterpart Pr₂Ti₂O₇. In addition to phonon modes, we observe two bands associated with electronic Raman scattering involving crystal field transitions in Pr₂Sn₂O₇ at ∼135 and 460 cm⁻¹ which couple strongly to phonons. Anomalous temperature dependence of phonon frequencies that are observed in pyrochlore Pr₂Sn₂O₇ are absent in monoclinic Pr₂Ti₂O₇. This, therefore, confirms that the strong phonon-phonon anharmonic interactions, responsible for the temperature-dependent anomalous behavior of phonons, arise due to the inherent vacant sites in the pyrochlore structure.     

Lithium electrochemistry of NiSe2: A new kind of storage energy material

NiSe₂ thin film has been successfully fabricated by reactive pulsed laser deposition and was investigated for its electrochemistry with lithium for the first time. The reversible discharge capacities of NiSe₂/Li cells cycled between 1.0 V and 3.0 V were found in the range of 314.9–467.5 mA h g⁻¹ during the first 200 cycles. By using ex situ X-ray diffraction, transmission electron microscopy, and selected-area electron diffraction measurements, the intermediates of β-NiSe, and Ni₃Se₂ were identified during the reversible conversions of NiSe₂ into metal nickel and Li₂Se. Both cation (nickel) and anion (selenium) in NiSe₂ provide the redox active centers in its electrochemical reaction with lithium, indicating one of the features of its lithium electrochemistry. The high reversible capacity and good cycle ability of NiSe₂ electrode made it become a promising cathode material for future rechargeable lithium batteries.     

Pressure-induced metastable phase transition in orthoenstatite (MgSiO3) at room temperature: a Raman spectroscopic study

Phase behavior of a synthetic orthoenstatite in a diamond-anvil cell has been studied up to ∼22 GPa by using Raman spectroscopy at room temperature. Under quasi-hydrostatic conditions, orthoenstatite undergoes a reversible phase transformation at an apparent transition pressure of ∼10 GPa for compression and ∼9.5 GPa for decompression. The 3d transition-metal cations, e.g., Fe²⁺ and Ni²⁺, show only a minor effect on the transition pressure within 10 wt% of addition. All the Raman frequencies in both orthoenstatite and its high-pressure phase increase monotonically with increasing pressure. The amount of forward or backward transition is fixed at a given pressure and forms a hysteresis loop in the transition %-pressure plan. The type for the present metastable phase transition is inferred to be of first order and the high-pressure polymorph may be the intermediate between orthoenstatite and the high-pressure clinoenstatite (i.e., the high-P C2/c phase). A mechanism based on Mnyukh's edgewise model of interface motion has been suggested to account for the observed phenomena.     

Pressure-induced amorphization in Y2(WO4)3: in situ X-ray diffraction and Raman studies

The high-pressure behavior of Y₂(WO₄)₃ has been investigated at room temperature by in situ X-ray diffraction and Raman scattering measurements. Both the studies show that beyond ∼3 GPa, this compound smoothly transforms from the ambient orthorhombic phase to a disordered phase. The structural modifications are found to be reversible up to ∼4 GPa but become irreversible at higher pressures. Low pressures of transformation imply that these changes are intrinsic and not due to non-hydrostatic stresses. In addition, the correlation between the stability range of orthorhombic phase and counter cation size supports that this compound has a large field of negative thermal expansion in this family of compounds.     

Effect of capping and particle size on Raman laser-induced degradation of γ-Fe2O3 nanoparticles

Diol capped γ-Fe₂O₃ nanoparticles are prepared from ferric nitrate by refluxing in 1,4-butanediol (9.5 nm) and 1,5-pentanediol (15 nm) and uncapped particles are prepared by refluxing in 1,2-propanediol followed by sintering the alkoxide formed. X-ray diffraction (XRD) shows that all the samples have the spinel phase. Raman spectroscopy shows that the samples prepared in 1,4-butanediol and 1,5-pentanediol and 1,2-propanediol (sintered at 573 and 673 K) are γ-Fe₂O₃ and the 773 K-sintered sample is Fe₃O₄. Raman laser studies carried out at various laser powers show that all the samples undergo laser-induced degradation to α-Fe₂O₃ at higher laser power. The capped samples are however, found more stable to degradation than the uncapped samples. The stability of γ-Fe₂O₃ sample with large particle size (15.4 nm) is more than the sample with small particle size (10.2 nm). Fe₃O₄ having a particle size of 48 nm is however less stable than the smaller γ-Fe₂O₃ nanoparticles.     

Raman spectroscopic study of K3H(SO4)2 (phases I and II) single crystals

A Raman study of K₃H(SO₄)₂ as a function of temperature reveals that this compound undergoes a phase transition at T_c = 483 K prior to the decomposition at 508 K.     

Order-disorder transition in Nd2−yGdyZr2O7 pyrochlore solid solution: An X-ray diffraction and Raman spectroscopic study

Powder X-ray diffraction (XRD) and Raman spectroscopic study of order-disorder-phase transition with increase in the content of Gd in Nd_2−yGd_yZr₂O₇ solid solution is being reported. It has been observed from Rietveld analysis that with increase in concentration of Gd in Nd_2−yGd_yZr₂O₇, the value of the x parameter of the 48f oxygen changes from 0.332(1) to 0.343(1) with a sudden change in the slope for y=1.8, which indicates that the structure is transforming from ordered pyrochlore to disordered pyrochlore. In addition to that a sudden and drastic change in the Raman spectra including changes in the position and width of several Raman modes beyond y?1.8 has also been observed which has been correlated with increasing disorder. Based on these studies, it is suggested that there is a discontinuous order-disorder transition from ‘perfect pyrochlore’ to ‘defect pyrochlore’ phase in Nd_2−yGd_yZr₂O₇ solid solution.     

Phonons in isostructural (ND,Yb):YxGd1−x(VO4) laser crystals: A Raman scattering study

The room temperature phonon modes of the isostructural (Nd,Yb):Y_xGd_1−x(VO₄) laser crystals were determined using the Raman scattering technique, and the observed wavenumbers follow the overall mode distribution expected for REVO₄ (RE=rare earth) compounds with the tetragonal zircon structure, . They were assigned according to the group theory in terms of the internal modes of the VO₄ tetrahedron and the external modes of the Y_xGd_1−x(VO₄) lattice. No appreciable changes in the phonon wavenumbers were observed for Yb:GdVO₄ (Yb=0.008, 0.015, 0.020, 0.025, and 0.035), indicating that the force fields in the GdVO₄ lattice are not strongly altered by Yb doping at the Gd site. However, most of the phonon wavenumbers in the systems (Nd,Yb):Y_xGd_1−x(VO₄) shifts upwards (one-phonon-like behavior) when Y replaces for Gd.