B1g Mode Research Articles

Dielectric and Raman spectroscopy study of structural phase transformation of Sr-doped La2CoMnO6 double perovskite

Doping at rare earth site by divalent alkaline-earth ions in the double pervoskite system have noticed a variety of magnetic and electronic orders with spatially correlated charge, spin and orbital degree of freedom. In present report, we aim to study the structural, dielectric and magnetic properties of La2−xSrxCoMnO6 $$\left( {x=0,{\text{ }}0.1,{\text{ }}0.3,{\text{ }}0.5} \right)$$ samples synthesized by the solid-state reaction technique. Using powder X-ray diffraction and data analysis with Rietveld refinement a structural identification has been carried out. From high-temperature Raman spectra, a clear structural phase transformation from monoclinic with P21/n symmetry to a rhombohedral phase with R-3 symmetry was evidenced from the softening and an anomalous change in peak widths of both stretching (A1g) and anti-stretching (B1g) modes around 365K. The dielectric measurement is carried out for all the synthesized samples, a sudden change in dielectric constant (e′) and corresponding dielectric loss (tanδ) around 365 K in x = 0, x = 0.1, and x = 0.5 composition samples reflects a remarkable change in the tilts in oxygen octahedra which is because of the structural phase transformation. The susceptibility measurement was also performed to observe the ferromagnetic transitions. In partially ordered undoped and Sr-doped ceramic samples, relaxor-like behavior combined with a large dielectric constant (> 103) is observed. In this paper, successful attempts have been made to observe the structural phase transition through dielectric properties.

Influence of Al doping concentration on the opto-electronic chattels of SnS thin films readied by NSP

Herein, we limelight the key features of SnS thin films coated by spray pyrolysis process with different Al doping concentrations. The SnS thin films coated with different Al concentration (0, 2, 4, 6 at wt%) were characterized by various techniques. Structural parameters studied by XRD data revealed that all the prepared films were orthorhombic crystal structure with (111) as a preferential direction. Raman spectroscopy showed the presence of Ag, B1g, B2g, and B3g vibration modes at their corresponding wave numbers. Concentration dependent granular change of Al:SnS films were shown by AFM images. The presence of Sn, S and Al element without any other impurities was confirmed through EDAX picture. UV studies revealed the shrinkage of band gap from 1.97 to 1.72 eV on increasing the Al doping concentration from 0% to 4 at wt%. Room temperature PL showed a high intense red shift, as emission observed at 722 nm for 4% of Al doped SnS thin film. Hall measurements exhibited p-type conducting nature of undoped and Al doped SnS films. The films showed a lowest resistivity of 2.695×10−1 Ω cm for 4 at wt% Al doping concentration. 0.093% efficiency was obtained for the solar cell device fabricated by 4 at wt% of Al doped SnS thin film.

Understanding the Structure and Ground State of the Prototype CuF2 Compound Not Due to the Jahn-Teller Effect.

Insulating CuF2 is considered a prototype compound displaying a Jahn-Teller effect (JTE) which gives rise to elongated CuF64- units. By means of first-principles calculations together with an analysis of experimental data of both CuF2 and Cu2+-doped ZnF2, we demonstrate that such an idea is not correct. For ZnF2:Cu2+, we find that CuF64- units are compressed always along the Z local axis with a hole essentially in a 3 z2- r2 antibonding orbital, in agreement with experimental EPR data that already underline the absence of a JTE. The structure of the monoclinic CuF2 crystal also comes from compressed CuF64- complexes, although hidden by an additional orthorhombic instability due to a negative force constant of b2g and b3g local modes. The associated distortion, similar to that involved in K2CuF4 and other layered Cu2+ compounds, is also shown to be developed for ZnF2:Cu2+ upon increasing the copper concentration. The origin of this cooperative effect is discussed together with the differences between non-Jahn-Teller systems like ZnF2:Cu2+ and CuF2 and true Jahn-Teller systems like KZnF3:Cu2+. From present results and those on layered compounds, the usual assumption of a JTE for explaining the properties of d9 ions in low-symmetry lattices can hardly be right.

Structural, optical and gas sensing properties of Cu2O/CuO mixed phase: effect of the number of coated layers and (Cr + S) co-Doping

ABSTRACTThin films of Cu2O/CuO mixed phase have been deposited on pre-cleaned glass substrate by a spin-coating technique. The influence of Cr-doping, (Cr + S) co-doping and the number of coated layers on the structures and optical behaviors of the films were investigated by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman, and UV-Visible spectroscopies. From XRD, FTIR and Raman results; the films are composed of polycrystalline monoclinic CuO and cubic Cu2O phases with crystallite sizes ranged from 10.05 to 23.08 nm. Increasing the thickness improves the films’ crystallinity and decreases the defects level in the films. Cr and S incorporation encourages the growth of CuO phase at the expense of Cu2O one and affects the preferred growth direction. The doping with Cr and S blue shifted the Bg mode and multi-phonon transitions. The direct and indirect optical band gaps decreased from 2.25 eV and 1.60 eV to 2.10 eV and 1.20 eV by growing the number of deposited layers from 2 to 8 layers. The film sensitivity towards CO2 at different gas flow rate was studied and compared with those of similar systems. Also; response time, recovery time, detection limit, and limit of quantification are estimated.

Explaining the optical spectrum of CrF2and CuF2model materials: role of the tetragonal to monoclinic instability

The properties of MF2 (M = Cr, Cu) model compounds are usually interpreted assuming a Jahn-Teller effect leading to elongated MF64- units. By means of the analysis of experimental data and first-principles calculations on both the monoclinic P21/c structure and the parent rutile structure (tetragonal P42/mnm space group), we prove that such an assumption is not correct. It is shown that in MF2 compounds, the MF64- complexes are actually compressed in the parent phase but along a different direction, a situation that is however hidden by an additional orthorhombic instability due to a negative force constant of b2g and b3g modes of the cell. This distortion plays a key role in understanding the high experimental value of the lowest d-d transition energy, E1 = 1.23 and 0.93 eV for CrF2 and CuF2, respectively, when compared to the value E1 = 0.40 eV derived for the Jahn-Teller system of KZnF3:Cu2+. Aside from reproducing reasonably the experimental values of spin allowed d-d transitions of both compounds, our first-principles calculations show the existence of an accidental degeneracy involving the yz and xy levels in the final P21/c structure. Moreover, the internal electric field of CrF2 and CuF2 is found to be much less anisotropic than in layered compounds like K2CuF4 and thus it has little influence on the d-d transition energy. The influence of the (3z2 - r2) - (x2 - y2) hybridization, caused by the orthorhombic distortion, on the electronic density and the magnetic coupling between layers is also briefly discussed. The present results stress that the interpretation of experimental data using simple parameterized models can lead to wrong conclusions.

Temperature-Dependent Lattice Vibration of Magnesium Hydride

The thermodynamic stability of magnesium hydride hinders its application as a hydrogen storage medium. Efforts to destabilize MgH2 demonstrate the undesired reduction of entropy concomitant with the decrement of enthalpy. Lattice vibration contributes to entropy as well as thermal properties, whereas its temperature dependence is rarely reported for MgH2. Here, vibrational spectra of MgH2 are detected by means of Raman scattering between room temperature and the critical temperature of significant dehydrogenation. The laser power is changed, and the localized sample temperature in the irradiated region is evaluated according to the bandshift of the incorporated carbon with MgH2. With increasing temperature, the frequency for the B1g mode increases, while phonon softening is exhibited for both Eg and A1g modes. It is found that quasiharmonicity decides the thermal frequency shifts for Raman-active vibrations. Phonon–phonon interactions result in significant phonon broadenings for B1g and Eg modes. By contr...

Quantitative Tracking of the Oxidation of Black Phosphorus in the Few-Layer Regime.

Previous theoretical reports have described the oxidation of few-layer black phosphorus and its effects on the electronic properties. Theoretically, native oxide layers bring opportunities for band gap engineering, but the detection of the different types of oxides is still a challenge at the experimental level. In this work, we uncover a correlation between thermal processes and Raman shift for the Ag1, B2g, and Ag2 vibrational modes. The thermal expansion coefficients (temperature range, 290–485 K) for the Ag1, B2g, and Ag2 were −0.015, −0.027, and −0.028 cm–1 K–1, respectively. Differential scanning calorimetry analysis shows an endothermic process centered at 528 K, and it was related with a mass increase according to thermogravimetric analysis. Raman shift temperature dependence was correlated to theoretical lattice thermal expansion, and a significant deviation was detected in the stacking direction at 500 K.

Polarized Raman spectra of β-FeSi2 epitaxial film grown by molecular beam epitaxy

Polarized Raman spectra of a β-FeSi2(100)//Si(001) epitaxial film grown by molecular beam epitaxy were measured to identify the Raman mode of the observed Raman active lines. Twelve of the observed 18 Raman lines showed a clear dependence of the Raman intensity on the crystal rotation angle. By factor group analysis using the orthorhombic symmetry D2h18 of β-FeSi2, five Raman lines (193, 200, 249, 401, 494 cm-1) and seven lines (175, 277, 284, 298, 327, 410, 442 cm-1) were completely assigned to the Ag and B3g modes, respectively. The depolarization ratio of Raman scattering intensities was obtained from polarized Raman spectra measured in two polarization configurations. The values of the depolarization ratio also support the assignment of the Ag and B3g modes in β-FeSi2.

Phonon characteristics, crystal structures and intrinsic properties of non-stoichiometric Ba1+xWO4 ceramics

Scheelite-type non-stoichiometric Ba1+xWO4 (0 ≤ x ≤ 0.04) ceramics were synthesized by a conventional solid-state reaction method. X-ray diffraction patterns showed that the main phase is a tetragonal scheelite structure with space group I41/a. The vibrational modes were obtained from Raman spectra, which were assigned and illustrated. The intrinsic properties were obtained by Clausius-Mosotti equation (C–M) and the damping equation, and there is a positive relationship between the FWHM values of the Bg(Ba) modes and the dielectric losses (calculated by the damping equation), as well as a negative correlation between the Raman shifts of the Eg (near 355 cm−1) and the dielectric constants (deduced by C–M equation). The far-infrared reflectivity spectra were analyzed by the four-parameter semi-quantum (FPSQ) model to calculate the intrinsic properties of the samples, which agreed well with those values obtained by the measured data.

Polarized Raman spectroscopy and lattice dynamics of potassic-magnesio-arfvedsonite

We report polarized Raman spectra from potassic-magnesio-arfvedsonite in all informative scattering configurations. On the basis of the polarization selection rules, several Ag vibrational modes have been identified. The Bg modes, however, are below the detection limits of the Raman spectrometer. The OH stretching band is situated between 3630 and 3750 cm−1, and its spectral shape is typical of amphiboles with high occupancy of the A site. It is composed of seven overlapping but resolvable subbands, which stem from occupied A-site configurations M(1)M(1)M(3)–OH–A(K/Na)–WOH and M(1)M(1)M(3)–OH–A(K/Na)–WF, as well as from vacant A-site configurations M(1)M(1)M(3)–OH–A□–WOH, with different Mg and Fe occupancy of the M(1) and M(3) sites. The experimental Raman spectra are compared with the results of theoretical calculations based on a shell-model force-field and a bond polarizability model. The simulated partial Raman spectra allowed us to assign many low-frequency Raman bands to stretching vibrations involving specific cation-oxygen bonds, as well as the higher-frequency modes of the Si–O skeleton. On the basis of our calculations we hypothesize that the Raman bands at 467, 540 and 589 cm−1 are related to a superposition of M(2)Fe3+–O bond stretching and Si–O–Si bending vibrations.

Crystal structure characteristics, intrinsic properties, and vibrational spectra of non-stoichiometric Ca1+xWO4 ceramics

Non-stoichiometric Ca1+xWO4 ceramics were determined in a composition range of −0.04 ≤ x ≤ 0.04 by a conventional solid-state method. X-ray diffraction patterns show that the main phase is a tetragonal scheelite structure. Scanning electron microscopy images exhibited dense and uniform grains. The vibrational modes were assigned and illustrated in particular. A positive correlation exists between the full-width at half-maximum values of the Bg(Ca) Raman modes and the intrinsic losses simulated according to the damping factor and also a negative correlation between the Raman shifts of the ν4 (Bg) modes and the dielectric constants calculated by the Clausius-Mossotti (C-M) equation. The intrinsic properties of the samples were calculated by the four-parameter semi-quantum (FPSQ) model based on the far-infrared reflectivity spectra, which agreed well with those values obtained by the C-M and damping equations. At x = −0.01, the Ca0.99WO4 ceramic sintered at 1150 °C possesses the intrinsic properties of εr = 10.62 and tan δ = 4.62 × 10−4 based on C-M and damping equations and εr = 10.19 and tan δ = 2.35 × 10−4 deduced from the FPSQ model.

Phonon anharmonicity in thermoelectric palladium sulfide by Raman spectroscopy

Recent advances in the study of thermoelectric materials mainly focus on the developments or designs of methods to reduce thermal conductivities. The information of phonon scattering processes is the key to the understanding of the thermal transfer and transport of a material. Such information is essential for the understanding of the thermal conductivity of a material itself and for the further improvement to demand the requirements for technological applications. Recently, palladium sulfide has been examined as a potential thermoelectric material. However, the high thermal conductivity limits its thermoelectric performance and technological applications. Here, Raman scattering spectroscopy is used to investigate the thermal transport properties of this material over a wide range of temperatures. The nonlinear temperature-dependent frequencies and linewidths of the Raman modes illustrate the anharmonicity of phonon scattering for thermal transport in this material. Three-phonon scattering processes are found to account for the thermal transport in the temperature range of 10–620 K. The high-energy bands of the Bg mode related to the light atom (S) contribute most to the thermal transport properties. More phonon scattering processes including higher orders are seemingly needed to further reduce the thermal conductivity of this material.

Acetone improves the topographical homogeneity of liquid phase exfoliated few-layer black phosphorus flakes

Liquid phase exfoliation of 2D materials has issues related to the sorption of the solvent, the oxidation of the sample during storage, and the topographical inhomogeneity of the exfoliated material. N-methyl-2-pyrrolidone (NMP), a common solvent for black phosphorus (BP) exfoliation, has additional drawbacks like the formation of by-products during sonication and poor solvent volatility. Here we demonstrate an improvement in the topographical homogeneity (i.e. thickness and lateral dimensions) of NMP-exfoliated BP flakes after resuspension in acetone. The typical size of monolayers and bilayers stabilised in acetone was 99.8 ± 27.4 nm and 159.1 ± 57 nm, respectively. These standard deviations represent a threefold improvement over those of the NMP-exfoliated originals. Phosphorene can also be exfoliated directly in acetone by very long ultrasonication. The product suspension enjoys the same dimensional homogeneity benefits, which confirms that this effect is an intrinsic property of the acetone–BP system. The quality and stability of the exfoliated flakes was checked by XRD, TEM, electron diffraction and Raman spectroscopy. Thermal expansion coefficients of the B2g and Raman modes were calculated for drop-casted samples as −0.018 28 cm−1 K–1, −0.030 56 cm−1 K–1 and −0.032 19 cm−1 K–1, respectively. The flakes withstand 20 min in O2 flow at 373 K without lattice distortion.

Grain boundary-dominated electrical conduction and anomalous optical-phonon behaviour near the Neel temperature in YFeO3 ceramics

We investigated the anomalous behaviour in the dielectric properties, occurring nearly at room temperature and at elevated temperatures (near the Neel temperature TN) of the polycrystalline samples of YFeO3 (YFO) ceramics. On the prepared YFO ceramics, the magnetic measurements showed the Neel temperature of YFO to be 650 K, below which the compound exhibited the weak ferromagnetic behaviour. X-ray photoelectron spectroscopy (XPS) shows the presence of Fe ions (Fe2+ and Fe3+ states) and also revealed the formation of the oxygen vacancies. The frequency dependence of the complex dielectric constant within the frequency domain of 100 Hz–1 MHz shows the presence of grain dominated dielectric relaxation over the thermal window of 300–373 K. The activation energy Eact.ε=0.611eV extracted from the imaginary permittivity spectrum indicates the involvement of oxygen vacancies in the relaxation process. Above 493 K, the ac conductivity, complex impedance, and modulus studies revealed appreciable conduction and relaxation processes occurring in YFO ceramics with respective activation energies Eact.σ=1.362eV and Eact.Z=1.345eV, which suggests that the oxygen vacancies are also involved for the anomalous behaviour of the dielectric constant at elevated temperatures. The temperature dependent Raman spectroscopic measurements within the thermal window of 298–698 K showed anomalous variations of the line widths and frequencies of several Raman active modes above 473 K up to the vicinity of TN pointing towards the presence of admixtures of the electron-phonon and spin-phonon coupling in the system. A further study on the thermal variation of the B2g(4) mode frequency with [M(T)/MS]2 shows the occurrence of strong spin–phonon (s-p) coupling, while the line shape shows the presence of the Fano asymmetry, suggesting spin dependent electron-phonon (e-p) coupling in the system below TN.

Observation of iron d-orbitals modifications in superconducting FeSe by Raman spectra study

We have studied the Raman spectra of superconducting FeSe and non-superconducting Cu-doped FeSe crystals at different temperatures. The spectral range covers the phonon modes at low frequency regime to electronic Raman scattering at high frequency regime, up to 2000 cm−1. A hardening in frequency of B1g mode, which is related to the spin-phonon coupling is observed. At high frequency region, our results on superconducting FeSe crystal show that the dzx and dyz orbitals of Fe split at ∼130 K and then become closer to achieve balance as temperature decreases. This result indicates that a modification of d-orbital electron state of iron happens at temperature much higher than the structural transition, implicating a precursor order in iron-based superconductor. In comparison with non-superconducting Cu0.03Fe0.97Se crystal, our observations suggest that the orbital modification could be an important clue for understanding the mechanism of superconductivity in iron-based superconductors.

Phonon spectrum of single-crystalline FeSe probed by high-resolution electron energy-loss spectroscopy

Utilizing high-resolution electron energy-loss spectroscopy (HREELS) we measure the phonon frequencies of β–FeSe(001), cleaved under ultra-high vacuum conditions. At the zone center (Γ¯–point) three prominent loss features are observed at loss energies of about ≃ 20.5 and 25.6 and 40 meV. Based on the scattering selection rules we assign the observed loss features to the A1g, B1g, and A2u phonon modes of β–FeSe(001). The experimentally measured phonon frequencies do not agree with the results of density functional based calculations in which a nonmagnetic, a checkerboard or a strip antiferromagnetic order is assumed for β–FeSe(001). Our measurements suggest that, similar to the other Fe-based materials, magnetism has a profound impact on the lattice dynamics of β–FeSe(001).

Revelation of rutile phase by Raman scattering for enhanced photoelectrochemical performance of hydrothermally-grown anatase TiO2 film

Photoelectrochemical (PEC) performances of the anatase TiO2 films hydrothermally grown on the seeded fluorine-doped tin oxide (FTO) substrates are examined in this work. Structural characterizations of the TiO2 films were conducted using Raman scattering spectroscopy. Although there is no obvious rutile peak appearing, an asymmetrical peak centered at ∼399 cm−1 was observed in the Raman spectra of the TiO2 films deposited either on the low-temperature-formed seed layers or with low concentrations of Ti precursor. The asymmetrical Raman shift can be deconvoluted into the B1g mode of anatase and Eg mode of rutile TiO2 peaks centered at ∼399 cm−1 and ∼447 cm−1, respectively. Therefore, a minute quantity of rutile phase was inspected in the anatase film using Raman scattering spectroscopy. With the same light harvesting ability, we found that the PEC performance of the anatase TiO2 film was significantly enhanced as the minute quantity of rutile phase existing in the film. It is ascribed to the formation of the anatase/rutile heterojunction which is beneficial to the charge separation in the photoanode.

Spin–phonon coupling in HoCr1 − xFexO3 (x = 0 and 0.5) compounds

AbstractWe report on the temperature dependent Raman scattering studies of polycrystalline HoCr1 − xFexO3 (x = 0 and 0.5) compounds. No signature of phonon mode related to structural phase transition is evident in the investigated temperature range in both the compounds. The thermal evolution of phonon spectra indicates a deviation of B3g(3) mode from the expected anharmonic behaviour below magnetic ordering temperature TN in HoCrO3 compound. This behaviour is attributed to spin–phonon coupling, which is due to the modulation of super exchange integral by lattice vibration. On the other hand, the anomalous behaviour of Ag mode and its line‐width around magnetic ordering in HoCr0.5Fe0.5O3 compound suggest the spin–phonon coupling. Apart from that, a clear deviation of wavenumber pertinent to B2g(1) mode suggests the possibility to observe ferroelectricity in HoCr0.5Fe0.5O3 compound. We believe that the present results would be helpful in obtaining new materials with multiferroicity near room temperature.

Raman scattering study of the structural phase transition in single crystal KDy(MoO4)2

Raman scattering of light in single-crystal KDy(MoO4)2 is studied at frequencies of 3–1000 cm−1 for temperatures ranging from 2 to 300 K, including that of a structural phase transition of the cooperative Jahn-Teller type (TC ∼ 14.5 K). During the transition to the low-temperature phase, a series of additional phonon lines corresponding to the Ag, B1g, B2g, and B3g modes is observed which indicates a doubling of the unit cell during the phase transition. An analysis of the symmetry of the phonon modes shows that the low-temperature phase has a predominantly monoclinic symmetry with conservation of a second order axis along the crystallographic b direction, i.e., perpendicular to the layers. Excitations are discovered which correspond to low-energy electronic transitions between levels of the ground-state 6H15/2 multiplet of the Dy3+ ion, which is split in the crystal field with a C2 symmetry. In the vicinity of the first excited Kramers doublet of the Dy3+ ion in crystalline KDy(MoO4)2, the scattered spectrum contains four lines [16.5, 21.0, 24.9, and 29.1 cm−1 (2 K)] at low temperatures, instead of a single line [18.3 cm−1 (25 K)] above the phase transition temperature (14.5 K). This indicates the existence of four nonequivalent dysprosium ions in the low-temperature phase.

Polarized Raman scattering on single crystals of rare earth orthochromite RCrO3 (R = La, Pr, Nd, and Sm)

AbstractWe report on polarized Raman scattering on orthorhombic RCrO3 (R = La, Pr, Nd, and Sm) single crystals at room temperature. The analysis of polarized Raman spectra as a function of polarizer and analyzer angles with respect to crystallographic axes allowed to observe in these flux grown crystals phonon modes that were not detected yet. On the basis of both Raman selection rules and lattice dynamics calculations within density functional perturbations theory framework, the observed Ag, B1g, B2g, and B3g phonon modes were unambiguously assigned. Phonon mode wave number evolutions as a function of orthorhombic distortion, related to rare earth ionic radius, were investigated and highlighted a complex behavior for several low wave number modes allowed crossing. For the modes of same symmetry, a mode mixing behavior is observed, and numerical calculations based on two or three coupling systems were performed to fit experimental data and to extract the coupling constants.