Resonant Raman scattering of graphite intercalation compounds KC8, KC24, and KC36

Graphite intercalation compounds, due to charge transfer between layers of graphite and intercalants, have a strongly shifted Fermi level. Potassium is known to give its electron leading to a large charge transfer f_c close to for stage 1 (KC₈) and for stage 2 (KC₂₄). The question is more subtle in stage 3 (KC₃₆) for which the graphene layers are not equivalent. For stage 3, two Raman G bands are clearly visible, corresponding to the interior layer and the boundary layers, respectively. By varying the excitation energy from UV to infrared, we observe that the intensity of the boundary layers G band versus that of the interior layer is maximum at 2.5 eV, leading to a sharp resonance profile at room temperature. Using first‐principle calculation, we associate this transition to π → π^∗ of the bounding layers. Copyright © 2014 John Wiley & Sons, Ltd.     

Raman light scattering,infrared absorption and DSC studies of the phase transition and vibrational and reorientational dynamics of H2O ligands and ClO4– anions in [Ba(H2O)3](ClO4)2

[Ba(H₂O)₃](ClO₄)₂ between 90 and 300 K possesses two solid phases. One phase transition of the first‐order type at: = 211.3 K (on heating) and = 204.6 K (on cooling) was determined by differential scanning calorimetry. The entropy change value (ΔS ≈ 15 Jmol^–1 K^–1), associated with the observed phase transition, indicates a moderate degree of molecular dynamical disorder. Both, vibrational and reorientational motions of H₂O ligands and ClO₄^– anions, in the high‐temperature and low‐temperature phases, were investigated by Fourier transform far‐infrared and middle‐infrared and Raman light scattering spectroscopies. The temperature dependences of the full‐width at half‐maximum values of the bands associated with ρ_w(H₂O) mode, in both infrared (~570 cm^–1) and Raman light scattering (~535 cm^–1) spectra, suggest that the observed phase transition is not associated with a sudden change of a speed of the H₂O reorientational motions. Ligands reorient fast, with correlation time of the order of several picoseconds, with a mean activation energy value E_a = 5.1 kJ mol^–1 in both high and low temperature phases. On the other hand, measurements of temperature dependences of full‐width at half‐maximum values of the infrared band at ~460 cm^–1, associated with δ_d(OClO)E mode, and Raman band at ~1105 cm^–1, associated with ν_as(ClO)F₂ mode, revealed the existence of a fast ClO₄^– reorientation in phase I and in phase II, with the E_a(I) and E_a(II) values equal to 8.0 and 6.5 kJ mol^–1, respectively. These reorientational motions of ClO₄^– are slightly distorted at the T_C. Fourier transform far‐infrared and middle‐infrared spectra with decreasing of temperature indicated characteristic changes at the vicinity of PT at T_C, which suggested lowering of the crystal structure symmetry. All these experimental facts suggest that the discovered phase transition is associated with small change of H₂O ligands and somewhat major change of ClO₄^– anions reorientational dynamics, and with insignificant change of the crystal structure, too. Copyright © 2012 John Wiley & Sons, Ltd.     

Phonon anomalies and structural transition in spin ice Dy2Ti2O7: a simultaneous pressure‐dependent and temperature‐dependent Raman study

We revisit the assignment of Raman phonons of rare‐earth titanates by performing Raman measurements on single crystals of O¹⁸ isotope‐rich spin ice and nonmagnetic pyrochlores and compare the results with their O¹⁶ counterparts. We show that the low‐wavenumber Raman modes below 250 cm⁻¹ are not due to oxygen vibrations. A mode near 200 cm⁻¹, commonly assigned as F_2g phonon, which shows highly anomalous temperature dependence, is now assigned to a disorder‐induced Raman active mode involving Ti⁴⁺ vibrations. Moreover, we address here the origin of the ‘new’ Raman mode, observed below T_C ~ 110 K in Dy₂Ti₂O₇, through a simultaneous pressure‐dependent and temperature‐dependent Raman study. Our study confirms the ‘new’ mode to be a phonon mode. We find that dT_C/dP = + 5.9 K/GPa. Temperature dependence of other phonons has also been studied at various pressures up to ~8 GPa. We find that pressure suppresses the anomalous temperature dependence. The role of the inherent vacant sites present in the pyrochlore structure in the anomalous temperature dependence is also discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Vibrational and reorientational dynamics,crystal structure and solid–solid phase transition studies in [Ca(H2O)6]Cl2 supported by theoretical (DFT) calculations

[Ca(H₂O)₆]Cl₂ between 93 and 300 K possesses two solid phases. One phase transition (PT) of the first‐order type at = 218.0 K (on heating) and = 208.0 K (on cooling) was determined by differential scanning calorimetry. Thermal hysteresis of this PT (10 K), as well as the heat flow anomaly sharpness, suggests that the detected PT is a first‐order one. The entropy change value [ΔS ≈ 8.5 J mol⁻¹ K⁻¹ ≈ Rln(2.8)] associated with the observed PT suggests a moderate degree of molecular dynamical disorder of the high‐temperature phase. The temperature dependencies of the full width at half maximum values of the infrared band are due to ρ(H₂O)A₂ mode (at 205 cm⁻¹), and two Raman bands are arising from τ(H₂O)E and τ(H₂O)A₁ modes (at ca. 410 and 682 cm⁻¹, respectively), suggesting that the observed PT is associated with a sudden change of speed of the H₂O reorientational motions. The estimated mean value of activation energy for the reorientation of the H₂O ligands in the high‐temperature phase is ca. 11.4 kJ mol⁻¹ from Raman spectroscopy and 11.9 kJ mol⁻¹ from infrared spectroscopy. X‐ray single‐crystal diffraction measurement and spectroscopic studies (infrared, Raman and inelastic neutron scattering) also confirm that [Ca(H₂O)₆]Cl₂ includes two sets of differently bonded H₂O molecules. Ab initio calculations of the complete unit cell of one molecule of calcium chloride with a different number of water molecules (2, 4 and 6) have also been carried out. A comparison of Fourier Transform Infrared (FT‐IR), Fourier Transform Raman Scattering (FT‐RS) and inelastic neutron scattering spectroscopies results with periodic density functional theory calculations was used to provide a complete assignment of the vibrational spectra of [Ca(H₂O)₆]Cl₂. Copyright © 2016 John Wiley & Sons, Ltd.     

DFT‐assisted interpretation of the Raman spectra of hydrogen‐ordered ice XV

The vibrational spectra of the condensed phases of water often show broad and strongly overlapping spectral features which can make spectroscopic interpretations and peak assignments difficult. The Raman spectra of hydrogen‐ordered H₂O and D₂O ice XV are reported here, and it is shown that the spectra can be fully interpreted in terms of assigning normal modes to the various spectral features by using density functional theory (DFT) calculations. The calculated lattice‐vibration spectrum of the experimental antiferroelectric structure is in good agreement with the experimental data whereas the spectrum of a ferroelectric Cc structure, which computational studies have suggested as the crystal structure of ice XV, differs substantially. Moreover, the calculated coupled O–H stretch spectrum also seems in better agreement with the experiment than the calculated spectrum for the Cc structure. Both the hydrogen bonds as well as the covalent bonds appear to be stronger in hydrogen‐ordered ice XV than in the hydrogen‐disordered counterpart ice VI. A new type of stretching mode is identified, and it is speculated that this kind of mode might be relevant for other condensed water phases as well. Furthermore, the ice XV spectra are compared to the spectra of ice VIII which is the only other high‐pressure phase of ice for which detailed spectroscopic assignments have been made so far. In summary, we have established a link between crystallographic data and spectroscopic information in the case of ice XV by using DFT‐calculated spectra. Such correlations may eventually help interpreting the vibrational spectra of more structurally‐disordered aqueous systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Hyper‐Raman and Raman scattering in paratellurite TeO2

Raman spectra of the tetragonal structure of paratellurite TeO₂ have been revisited avoiding anomalous polarization‐selection‐rules violations previously observed and due to optical activity. We present a complementary hyper‐Raman scattering study of paratellurite. Wavenumber and symmetry assignments are given for all expected 21 Raman active optical branches, except one LO component (out of the eight expected TO–LO pairs) of the polar doublet E modes. Also, the four expected hyper‐Raman active A₂ (TO) modes have been observed. Moreover, we have observed a strong Kleinman‐disallowed hyper‐Rayleigh signal, which is tentatively assigned as a first evidence of hyper‐Rayleigh optical activity. Copyright © 2013 John Wiley & Sons, Ltd.     

A comparison between ab initio calculated and measured Raman spectrum of triclinic albite (NaAlSi3O8)

Albite is one of the most common minerals in the Earth's crust, and its polymorphs can be found in rocks with different cooling histories. The characteristic spectrum of vibration of the albite mineral reflects its structural Si/Al ordering. In this study, we report on the comparison between the Raman spectra measured on a natural and fully ordered (as deduced on the basis of single‐crystal X‐ray diffraction data) ‘low albite’, NaAlSi₃O₈, and those calculated at the hybrid Hartree–Fock/density functional theory level by employing the WC1LYP Hamiltonian, which has proven to give excellent agreement between calculated and experimentally measured vibrational wavenumbers in silicate minerals. All the 39 expected A_g modes are identified in the Raman spectra, and their wavenumbers and intensities, in different scattering configurations, correspond well to the calculated ones. The average absolute discrepancy is ~3.4 cm⁻¹, being the maximum discrepancy |Δv|_max ~ 10.3 cm⁻¹. The very good quality of the WC1LYP results allows for reliable assignments of the Raman features to specific patterns of atomic vibrational motion. Copyright © 2015 John Wiley & Sons, Ltd.     

Investigation of the vibrational modes of ZnO grown by MOCVD on different orientation planes

Wurtzite ZnO thin films were prepared on sapphire substrate by metal organic chemical vapor deposition (MOCVD). Raman scattering studies on different crystallographic textures were performed in the backscattering geometry, and polarization effect is investigated in different configurations and . ZnO Raman modes are investigated in each texture. In the case of ZnO thin film deposed on r‐() sapphire plane and using backscattering geometry, new Raman line was observed at 390 cm⁻¹ because this mode has not been noticed in this geometry. It is shown that the frequencies of the quasi‐phonon modes of the examined thin film are in good agreement with the theoretical values calculated within the framework of Loudon model. Copyright © 2014 John Wiley & Sons, Ltd.     

Real‐time Raman spectroscopy measurements to study the uniaxial tension of isotactic polypropylene: a global overview of microstructural deformation mechanisms

Micro‐Raman spectroscopy was used to investigate the main deformation micromechanisms of isotactic polypropylene uniaxially stretched at constant temperature (T = 30 °C) under a constant true strain rate ( = 5.10⁻³ s⁻¹). To accurate measurements namely to be free of the recovering phenomenon which causes in most of the cases interference during post‐mortem analysis, we introduced a new experimental setup combining a Raman spectrometer with a tensile machine piloted by the VidéoTraction™ system. Microstructure is described by essential parameters such as the crystallinity index, the macromolecular orientation both in the crystalline and the amorphous phase, and distribution of the internal stress at the chemical bonds scale. For each, a well‐tried Raman spectral criterion was used. Cross‐checking of these results, obtained with a minimum of tensile tests, allows a more complete understanding of the deformation micromechanisms of semi‐crystalline polymer. Copyright © 2013 John Wiley & Sons, Ltd.     

Abnormal Raman spectra in Er‐doped BaTiO3 ceramics

Greatly enhanced and abnormal Raman spectra were discovered in the nominal (Ba_{1 − x}Er_x)Ti_{1 − x/4}O₃ (x = 0.01) (BET) ceramic for the first time and investigated in relation to the site occupations of Er³⁺ ions. BaTiO₃ doped with Ti‐site Er³⁺ mainly exhibited the common Raman phonon modes of the tetragonal BaTiO₃. Er³⁺ ions substituted for Ba sites are responsible for the abnormal Raman spectra, but the formation of defect complexes will decrease spectral intensity. A large increase in intensity showed a hundredfold selectivity for Ba‐site Er³⁺ ions over Ti‐site Er³⁺ ions. A strong EPR signal at g = 1.974 associated with ionized Ba vacancy defects appeared in BET, and the defect chemistry study indicated that the real formula of BET is expressed by (Ba_{1 − x}Er_3x/4)(Ti_{1 − x/4}Er_x/4)O₃. These abnormal Raman signals were verified to originate from a fluorescent effect corresponding to ⁴S_3/2→⁴I_15/2 transition of Ba‐site Er³⁺ ions. The fluorescent signals were so intense that they overwhelmed the traditional Raman spectra of BaTiO₃. The significance is that the abnormal Raman spectra may act as a probe for the Ba‐site Er³⁺ occupation in BaTiO₃ co‐doped with Er³⁺ and other dopants. A new broad EPR signal at g = 2.23 was discovered, which originated from Er³⁺ Kramers ions. Copyright © 2014 John Wiley & Sons, Ltd.     

Ion–polymer and ion–ion interaction in PEO‐based polymer electrolytes having complexing salt LiClO4 and/or ionic liquid, [BMIM][PF6]

Ion–polymer and ion–ion association in polymer electrolyte films of PEO complexed with salt LiClO₄, ionic liquid (1‐butyl‐3‐methylimidazolium hexafluorophosphate, BMIMPF₆) and (LiClO₄ + BMIMPF₆) have been studied by laser Raman spectroscopy. The cations (Li⁺ and/or BMIM⁺) of the dopant salt/IL are shown to complex with the ether oxygen of the polymer backbone (i.e. C O C bond of PEO). The polymer–cation complexation results in the appearance of an additional peak at ∼1131 cm⁻¹ apart from the C O C stretching vibrations of PEO at ∼1062 and 1141 cm⁻¹. This peak due to polymer–cation complexation is relatively strong for LiClO₄ than BMIMPF₆, indicating stronger interaction for the former. In the PEO:LiClO₄ and PEO:BMIMPF₆ spectra, Raman peaks at 937 and 747 cm⁻¹, respectively related to Li⁺· ClO and BMIM⁺· PF ‘contact ion pairs’, have also been observed as a result of ion–ion association. In the polymer electrolyte PEO:LiClO₄ + BMIMPF₆ which contained two different anions, viz. ClO and PF, an interesting observation of the formation of ‘cross contact ion pairs’ viz. Li⁺· PF and BMIM⁺· ClO is also reported. Copyright © 2011 John Wiley & Sons, Ltd.     

Beryllium Silicate,Be2SiO4 (phenakite) – a Novel Trigonal SRS‐Active Crystal

In single crystals of the beryllium silicate Be₂SiO₄ with trigonal symmetry , known also as the mineral phenakite, χ⁽³⁾‐nonlinear lasing by stimulated Raman scattering (SRS) is investigated. All observed Stokes and anti‐Stokes lasing components are identified and ascribed to a single SRS‐promoting vibration mode with ω_SRS ≈876 cm⁻¹. With picosecond single‐wavelength pumping at one micrometer the generation of an octave‐spanning Stokes and anti‐Stokes comb is observed.     

Layered MoS2 grown on c ‐sapphire by pulsed laser deposition

Layered growth of molybdenum disulphide (MoS₂) was successfully achieved by pulsed laser deposition (PLD) method on c ‐plane sapphire substrate. Growth of monolayer to a few monolayer MoS₂, dependent on the pulsed number of excimer laser in PLD is demonstrated, indicating the promising controllability of layer growth. Among the samples with various pulse number deposition, the frequency difference (A_1g–E¹_2g) in Raman analysis of the 70 pulse sample is estimated as 20.11 cm^–1, suggesting a monolayer MoS₂ was obtained. Two‐dimensional (2D) layer growth of MoS₂ is confirmed by the streaky reflection high energy electron diffraction (RHEED) patterns during growth and the cross‐sectional view of transmission electron microscopy (TEM). The in‐plane relationship, (0006) sapphire//(0002)MoS₂and sapphire//MoS₂ is determined. The results imply that PLD is suitable for layered MoS₂ growth. Additionally, the oxide states of Mo 3d core level spectra of PLD grown MoS₂, analysed by X‐ray photoelectron spectroscopy (XPS), can be effectively reduced by adopting a post sulfurization process. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Infrared and Raman spectroscopic studies of the charge localization in one‐dimensional organic metals (DMtTTF)2X (X = ReO4, ClO4) with regular organic stacks

A novel selective synthesis of the unsymmetrically substituted tetrathiafulvalene dimethyltrimethylene‐tetrathiafulvalene (DMtTTF) is described together with its electrocrystallization to the known conducting mixed‐valence ClO₄^– and ReO₄^– salts. Infrared (IR) and Raman spectra of the two isostructural quasi‐one‐dimensional cation radical salts (DMtTTF)₂X (X = ReO₄^–, ClO₄^–) are investigated as a function of temperature (T = 5–300 K). At ambient temperature, these salts show metallic‐like properties and below T_ρ = 100–150 K, they undergo a smeared transition to semiconducting state. To study this charge localization, we measured temperature dependence of polarized IR reflectance spectra (700–16 000 cm^–1) and Raman spectra (150–3500 cm^–1, excitation λ = 632.8 nm) of single crystals. For both compounds, the Raman data and especially the bands related to the C=C stretching vibration of the DMtTTF molecule show that the charge distribution on molecules is uniform down to the lowest temperatures. Similarly, IR data confirm that down to the lowest temperatures, there is neither charge ordering nor important modification of the electronic structure. However, the temperature dependence of Raman spectra of both salts reveals a regime change at about 150 K. Additionally, using Density Functional Theory (DFT) methods, the normal vibrational modes of the neutral DMtTTF⁰ and cationic DMtTTF⁺ species and also their theoretical IR and Raman spectra were calculated. The theoretical data were compared with the experimental IR and Raman spectra of neutral DMtTTF⁰ molecule. Copyright © 2013 John Wiley & Sons, Ltd.     

Elastic properties study of single crystal NH3 up to 26 GPa

Single crystal Brillouin and Raman scattering measurements on NH₃ in a diamond anvil cell have been performed under pressures up to 26 GPa at room temperature. The pressure dependencies of acoustic velocity, adiabatic elastic constants, and bulk moduli of ammonia from liquid to solid III and solid IV phase have been determined. All the nine elastic constants in orthorhombic structure phase IV were presented for the first time, each elastic constant grows monotonously with pressure and a crossover of the off‐diagonal moduli C₁₂ and C₁₃ was observed at around 12 GPa because of their different pressure derivative values. We also performed ab initio simulations to calculate the bulk elastic moduli for orthorhombic ammonia, the calculated bulk moduli agree well with experimental results. In Raman spectra the very weak bending modes ν₂ and ν₄ for orthorhombic ammonia are both observed at room temperature, a transition point near 12 GPa is also found from the pressure evolution of the Raman bands. Copyright © 2011 John Wiley & Sons, Ltd.     

Towards a metallic quasi‐d9 system without copper: AgO at high pressure

AgO is a prototypical mixed‐valence compound, with markedly different coordination environment of dumbbell Ag(1+) and low‐spin square‐planar Ag(3+) which render it a narrow band gap semiconductor. The hybrid HSE06 functional reproduces fairly well the band gap of its P 2₁/c form at ambient conditions (ΔE_exp = 1.0–1.1 eV, ΔE_theor = 0.94 eV) and suggest progressive band gap decrease with external pressure with metallization at 50 GPa via band overlap. Dynamic (phonon) instability appears at the onset of metallization leading to a structural phase transition to a more stable but still metallic $$P\stackrel{?}{1}$$ form. The density of electronic states at the Fermi level of the $$P\stackrel{?}{1}$$ polymorph is small and a pseudo‐gap at the Fermi level is preserved.

Illustration of the predicted P2₁/c → $$P\stackrel{?}{1}$$ transformation.     

Kinetics and mechanisms of gas‐phase decarbonylation of α‐methyl‐trans‐cinamaldehyde and E‐2‐methyl‐2‐pentenal under homogeneous catalysis of hydrogen chloride

The kinetics of the gas‐phase elimination of α‐methyl‐trans‐cinamaldehyde catalyzed by HCl in the temperature range of 399.0–438.7 °C, and the pressure range of 38–165 Torr is a homogeneous, molecular, pseudo first‐order process and undergoing a parallel reaction to produce via (A) α‐methylstyrene and CO gas and via (B) β‐methylstyrene and CO gas. The decomposition of substrate E‐2‐methyl‐2‐pentenal was performed in the temperature range of 370.0–410.0 °C and the pressure range of 44–150 Torr also undergoing a molecular, pseudo first‐order reaction gives E‐2‐pentene and CO gas. These reactions were carried out in a static system seasoned reactions vessels and in the presence of toluene free radical inhibitor. The rate coefficients are given by the following Arrhenius expressions:

• Products formation from α‐methyl‐trans‐cinamaldehyde
• α‐methylstyrene :
• β‐methylstyrene :
• Products formation from E‐2‐methyl‐2‐pentenal
• E‐2‐pentene :

The kinetic and thermodynamic parameters for the thermal decomposition of α‐methyl‐trans‐cinamaldehyde suggest that via (A) proceeds through a bicyclic transition state type of mechanism to yield α‐methylstyrene and carbon monoxide, whereas via (B) through a five‐membered cyclic transition state to give β‐methylstyrene and carbon monoxide. However, the elimination of E‐2‐methyl‐2‐pentenal occurs by way of a concerted cyclic five‐membered transition state mechanism producing E‐2‐pentene and carbon monoxide. The present results support that uncatalyzed α‐β‐unsaturated aldehydes decarbonylate through a three‐membered cyclic transition state type of mechanism. Copyright © 2014 John Wiley & Sons, Ltd.     

Raman spectroscopic investigations on transition‐metal dichalcogenides MX2 (M = Mo,W; X = S,Se) at high pressures and low temperature

Transition‐metal dichalcogenides have been investigated using Raman spectroscopy both being off‐resonance and in resonance. The first‐order Raman spectra of MoS₂, MoSe₂, WS₂ and WSe₂ single crystal synthesized by vapor transport technique have been studied as a function of hydrostatic pressure (0–20 GPa) and temperature (80–300 K). Isobaric and isothermal mode‐Grüneisen parameters have been determined from the temperature and pressure‐dependent Raman spectra. The pressure dependence of the chalcogen–chalcogen and metal–chalcogen force constant has been obtained using a central force model. Separation of the temperature dependence of Raman mode wavenumbers into quasi‐harmonic and purely anharmonic contributions using measured high‐pressure Raman data allows us to extract the changes in the phonon wavenumbers arising exclusively due to anharmonic interactions. Copyright © 2014 John Wiley & Sons, Ltd.     

Raman spectra and phase composition of MnGeO3 crystals

MnGeO₃ single‐crystal samples have been synthesized by optical zonal melting and spontaneous crystallization. X‐ray crystal analysis showed the first sample to be a two‐phase one with phase ratio as follows: 17% – monoclinic C2/c, and 83% – orthorhombic Pbca; the phase ratio of the second sample was unknown. Raman spectra have been produced for these samples. Lattice dynamics has been simulated and polarization dependencies of lines' intensities have been analyzed to interpret experimental Raman spectra and to attribute lines to the spectra of monoclinic and orthorhombic phases. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis,crystal structure and dielectric properties of C6H18N2SbCl5

The result of the X‐ray diffraction, differential scanning calorimetry and dielectric studies on a new crystal material C₆H₁₈N₂SbCl₅ is presented. The new organic–inorganic compound has been synthesized and characterized by the X‐ray diffraction method at 296(2) K. It crystallizes in the monoclinic P21/n space group. The cell dimensions are: a = 5.8617(1) Å, b = 15.7069(2) Å, c = 16.6693(2) Å, β = 97.627(1)° and Z = 4. The crystal structure consists of a discrete ionic layer of (C₆H₁₈N₂)²⁺ cations and [SbCl₅]^2? anions linked via simple and bifurcated N―H · · · Cl hydrogen bonds. DSC analysis shows that this compound undergoes a phase transition at about (384 ± 2) K. AC and DC conductivities, complex dielectric permittivity ε*(ω) and complex electrical modulus M*(ω) were respectively studied as temperature and frequency functions. The combined data support each other and confirm the existence of a structural phase transition at about 384 K. Moreover, the temperature dependence of the DC conductivity and relaxation frequency followed the Arrhenius relation. The frequency dependence of the real part of the AC conductivity in both phases follows the Jonscher's universal dynamic law: . The behavior of s(T) with temperature suggests that the hopping over barrier model (CBH) and the small polaron tunneling mechanism (SPTM) prevail in phases I and II, respectively. Copyright © 2016 John Wiley & Sons, Ltd.