Studies on the complex behavior of optical phonon modes in wurtzite (ZnO)1−x (Cr2O3) x

This paper reports on the use of phonon spectra obtained with laser Raman spectroscopy for the uncertainty concerned to the optical phonon modes in pure and composite ZnO_1?x (Cr₂O₃) _x . Particularly, in previous literature, the two modes at 514 and 640 cm^?1 have been assigned to ZnO are not found for pure ZnO in our present study. The systems investigated for the typical behavior of phonon modes with 442 nm as excitation wavelength are the representative semiconductor (ZnO)_1?x (Cr₂O₃) _x (x = 0, 5, 10 and 15 %). Room temperature Raman spectroscopy has been demonstrated polycrystalline wurtzite structure of ZnO with no structural transition from wurtzite to cubic with Cr₂O₃. The incorporation of Cr³⁺ at most likely on the Zn sub-lattice sites is confirmed. The uncertainty of complex phonon bands is explained by disorder-activated Raman scattering due to the relaxation of Raman selection rules produced by the breakdown of translational symmetry of the crystal lattice and dopant material. The energy of the E ₂ (high) peak located at energy 53.90 meV (435 cm^?1) due to phonon–phonon anharmonic interaction increases to 54.55 meV (441 cm^?1). A clear picture of the dopant-induced phonon modes along with the B ₁ silent mode of ZnO is presented and has been explained explicitly. Moreover, anharmonic line width and effect of dislocation density on these phonon modes have also been illustrated for the system. The study will have a significant impact on the application where thermal conductivity and electrical properties of the materials are more pronounced.     

Study of potassium-zinc borophosphate glasses

Potassium-zinc borophosphate glasses were prepared and studied in two compositional series xK₂O-(50−x)ZnO-10B₂O₃-40P₂O₅ and xK₂O-(50−x)ZnO-20B₂O₃-30P₂O₅ with x=0, 10, 20, 30, 40 and 50 mol% K₂O. The replacement of zinc by potassium decreases the density and increases the molar volume of these glasses, whereas glass transition temperature and chemical durability decrease with increasing potassium content. Structural changes were studied by ¹¹B and ³¹P MAS NMR and Raman spectroscopy. The observed changes in the spectra and the properties of the studied glasses can be ascribed to several reasons and namely to the differences in the space occupied by cations Zn²⁺ and 2K⁺, the differences in the electronegativity of zinc and potassium and a large difference in the field strength of Zn²⁺ and K⁺ cations and thus higher ionicity of K-O bonds in comparison with Zn-O bonds.     

Infrared,Raman and neutron scattering studies of β- and β″-alumina: a static and dynamical structure analysis

Infrared and Raman data on β-aluminates with different compositions (x)=0.0, 0.25, 0.66) containing Na⁺, Ag⁺, K⁺, Tl⁺, H₃O⁺, NH⁺₄ ions are reviewed. β-gallates and β″-aluminates are also considered. Assignments of the cation modes are discussed in terms of oscillations of single cations and of normal modes of clusters (pairs and triangles formed by the conducting cations). The assignment of the spinel block spectra is proposed in the light of recent normal mode calculation and comparison of different experimental results in the literature. Potential barriers and activation energies obtained from these techniques are compared with calculation results and with data obtained from different sources. Inelastic and recent quasi elastic neutron scattering results are reviewed and compared with those deduced from the vibrational study.     

Laser-induced Raman scattering in calcium titanate

The Raman spectrum of polycrystalline calcium titanate prepared by a liquid mix technique and heated to 800°C has been recorded at room temperature using an argon-ion laser as exciter. The observed spectrum was interpreted on the basis of factor-group C_2V. Not all of the Raman active modes predicted by factor group analysis were observed and this could be due to: over-lapping of bands, or very low polarizabilities of some of the modes or masking of the weak bands by intense bands. The band at 639 cm^?1 is tentatively assigned to the TiO symmetric stretching vibration (γ₁) and the bands at 495 and 471 cm^?1 to torsional modes. The bands in the region 180–340 cm^?1 are assigned to the OTiO bending modes and the 155 cm^?1 band to the Ca(TiO₃) lattice mode. The observed Raman bands are compared with the available infrared absorption data and, as expected, some coincidences in frequencies are seen for this compound with a noncentrosymmetric structure.     

Effect of substitution of K ions on the structural and electrical properties of nanocrystalline MgAl2O4 spinel oxide

Potassium substituted nanosized magnesium aluminates having a nominal composition Mg_1−xK_xAl₂O₄ where x=0.0, 0.25, 0.5, 0.75, 1.0 have been synthesized by the chemical co-precipitation method. The samples have been characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), and dc electrical resistivity measurements. The XRD results reveal that the samples are spinel single phase cubic close packed crystalline materials. The calculated crystallite size ranges between 6 and 8 nm. The behaviour of the lattice constant seems to deviate from the Vegard's law. While X-ray density clearly increases, the bulk density and consequently, the percentage porosity do not exhibit a significant change on increasing the K⁺ content. The SEM micrographs suggest homogeneous distribution of the nanocrystallites in the samples. The dc electrical resistivity exhibits a typical semiconducting behaviour. Substitution of a Mg²⁺ ion by a K⁺ ion provides an extra hole to the system, which forms small polaron. Thermally activated hopping of these small polarons is believed to be the conduction mechanism in the Mg_1−xK_xAl₂O₄. The activation energy of hopping of small polarons has been calculated and found K⁺ ions content dependent.     

Raman study of KMnF3 perovskite crystals doped by Na+

A study of the influence of cationic Na⁺ substitution in the archetype KMnF₃ perovskite crystal was performed using the Raman method. The Raman spectra of mixed K_1-xNa_xMnF₃ crystals with x = 0.029, 0.048 and 0.065 were recorded versus temperature and fully interpreted in terms of a “one mode” behaviour. In addition to the soft mode not completely vanishing close to Tc, attention was especially paid to evidence of static and dynamical disorder. From this point of view the behaviour of the hard Raman modes versus temperature has been studied together with two unexpected Raman bands in the cubic phase. The interpretation has been made within the more general framework of structural disorder existing in such perovskites with anisotropic interactions.     

Scattering of light in a ferroelectric with incommensurate phase: K2ZnCl4

We presented and analyzed spectra of Raman scattering in various phase states of K₂ZnCl₄ crystals, including the incommensurate phase. On the basis of this analysis we determined the parameters of modes associated with internal and external motions of ions. We found that the phase mode reveals itself in the Raman scattering spectra; this mode is responsible for the instability of the crystal lattice in the vicinity of the incommensurate phase transition.     

Fast ion conduction in the defect pyrochlore system KBxW2−xO6(B = Al,Ti and Ta)

K⁺ ionic conductivity has been measured by the complex impedence method for the defect pyrochlore type compounds of the general formula K(B^y+_xW⁶⁺_2?x)O₆, (x=1/(6?y)) where B=Al³⁺, Ti⁴⁺ and Ta⁵⁺. Thermogravimetric analysis showed the compounds to be hydrated at room temperature with from ∽0.2 mole for the Al compound to ∽1 mole of water for the Ta substituted phase. There is a correlation between the lattice parameter and amount of water present in the cavities. The ionic conductivity of 5.2×10^?3ω^?1cm^-1 at 300°C for KAl_0.33W_1.67O₆ is found to be best among known KB₂O₆ type pyrochlores. High ionic conductivity has been explained on the basis of occupancy of K⁺ ions in the available sites and polarization of the B₂X₆ network.     

Raman studies of (Fe,V)3S4 solid solution

Raman scattering of Fe_xV_3-xS₄ with 0?x?2 has been studied. The observed spectra of V₃S₄, one of the end members of this solid solution, have been assigned to the Raman active lattice modes based on the C_2h³ space group symmetry. On the basis of the results obtained for V₃S₄, the nature of the metal-metal interactions and the site distributions of Fe atoms in (Fe, V)₃S₄ have been discussed.     

Vibrational spectroscopy of the Ga1−x AlxP epitaxial layers grown on GaP(111) substrate by the liquid-phase epitaxial technique

The paper reports the results of measurements of the lattice IR reflection and Raman scattering spectra for the Ga_1?x Al_xP (x=0–0.8) films grown on the GaP(111) substrate by the liquid-phase epitaxy technique. The dispersion analysis of the experimental spectra has demonstrated that, for the studied system of the Ga_1?x Al_xP alloy, the vibrational spectra of the alloys with different compositions exhibit three modes of the Ga-P vibrations and one mode of the Al-P vibrations. The frequencies of modes only slightly depend on the composition x of the Ga_1?x Al_xP alloy, but the composition considerably affects the oscillator strengths of these modes.     

Characterization of polycrystalline (Na,K)β/β′- alumina ceramics

K β′-alumina is unstable at >1300°C. Mixed alkali β′-alumina has a variable stability depending on the alkali ratio, [K⁺]/([Na⁺]+[K⁺]). For f(β)<[K⁺]/([Na⁺]+[K⁺]), the β′-Al₂O₃ phase decomposes to Kβ-Al₂O₃ 0997 0815 V 3 and a solid solution of Na β′-Al₂O₃ and K β′-Al₂O₃. For f(β)=[K⁺]/([Na⁺]+[K⁺], the ceramic consists of K β-Al₂O₃ and Naβ′-Al₂O₃ and for f(β)>[K⁺]/([Na⁺]+[K⁺]), the excess Na⁺ after Na β′-Al₂O₃ dissolves in the β phase, giving Na β-Al₂O₃/K β-Al₂O₃ solid solution and Na β′-Al₂O₃. These sequences were confirmed by measuring the dependence of the c-axis lattice parameters of β- and β′-Al₂O₃ phases on the f(β), and the change of these parameters during the ion-exchange of Na⁺ and K⁺ ions.     

插层化合物钨青铜单晶的物理研究

本工作利用X射线衍射分析、电导测量、红外、喇曼散射等手段,对用熔盐电解法生长的K_xWO₃,Na_yWO₃和K_xNa_yWO₃单晶进行了深入的研究,得出K_xNa_yWO₃是钠离子插入到K_xWO₃的结论,提出了K_xNa_{y关键词：}     

Variations of ionic binding energies and the distribution of charge carriers in orthorhombic La2−x Sr x CuO4

The significance of heterovalent, substitutional disorder for the distribution of charge carriers in La_2?x Sr _x CuO₄ has been investigated. Disorder is shown to cause strong variations of binding energies of the ions ranging to some eV for Sr contentsx=0.1. Balancing the energy for a hole transport, Cu³⁺+O^2?→Cu²⁺+O^?, and taking binding energy variations into account, the process is realized to become possible without consuming energy for a subset Θ for allx Cu³⁺ in one formula unit of La_2?x Sr _x CuO₄. The functions Θ(x) are presented for hole transports to apex and in-plane oxygens, respectively. The delocalization of charge carriers is interpreted to be caused by valency disorder on metal lattice sites.     

Mode dispersion in the optical-phonon spectra of mixed crystals ZnS1−x Sex

The IR reflection and the Raman spectra of ZnS_1?x Se_x crystals (0≤ x ≤ 1) are measured. The mode dispersion of the solid solutions is found to deviate from that calculated using an isodisplacement model. The reasons behind this deviation are discussed. Two additional modes are found in the frequency range between the ZnS-like TO and LO modes. It is assumed that one of them is a line of the second-order spectrum amplified by the Fermi resonance and the other is linked to the resonance (additional local) mode of Se impurity atoms. The latter conclusion is confirmed by calculating the spectrum of a Se impurity in a ZnS crystal in terms of the microscopic lattice dynamics theory in the low impurity concentration approximation. The oscillator strengths of the main and additional optical phonons in ZnS_1?x Se_x solid solutions are discussed.     

Raman scattering investigations on Co-doped ZnO epitaxial films: Local vibration modes and defect associated ferromagnetism

Raman scattering spectroscopy has been performed on high quality Co-doped ZnO epitaxial films, which were grown on Al₂O₃ (0001) by oxygen-plasma assisted molecular beam epitaxy. Raman measurements revealed two local vibration modes (LVMs) at 723 and 699 cm⁻¹ due to the substitution of Co²⁺ in wurtzite ZnO lattice. The LVM at 723 cm⁻¹ is found to be an elemental sensitive vibration mode for Co substitution. The LVM at 699 cm⁻¹ can be attributed to enrichment of Co²⁺ bound with oxygen vacancy, the cobalt–oxygen vacancy–cobalt complexes, in Zn_1−xCo_xO films associated with ferromagnetism. The intensity of LVM at 699 cm⁻¹, as well as saturated magnetization, enhanced after the vacuum annealing and depressed after oxygen annealing.     

Softening of charge density wave excitations at the superstructure transition in 2H-TaSe2

Raman scattering measurements performed between 5 K and 300 K on 2H-TaSe₂ reveal new modes which are assigned to the modes of the charge density wave, observed in light scattering due to the Fermi surface induced distortion. The mode at 49 cm^?1 of E_2g symmetry softens (with concurrent line-width broadening) towards 122 K, the transition temperature from the incommensurate distorted to the undistorted phase. The mode at 82 cm^?1 of A_1g symmetry appears to be connected with the transition at 90 K from the commensurate to the incommensurate superstructure. The mode at 24.5 cm^?1 of E_2g shows no temperature dependence and is clearly due to the rigid-layer vibration.     

Non-stoichiometric molybdenum oxides as cathodes for lithium cells. part v. thermodynamic,kinetic and structural aspects of the behaviour of Mo8O23 and Mo18O52

A deeper insight in the electrochemical behaviour of Li cells based on two non-stoichiometric Mo oxides, Mo₈O₂₃ and Mo₁₈O₅₂, was obtained by determining the OCV's, the diffusion coefficients and the variations of lattice parameters as a function of depth of discharge. The first material is a monoclinic framework-structured compound endowed with large channels which provide easy paths for Li⁺. Occupation of sites in the cavities of this structure produces at first a shrinkage of the unit cell, followed by a moderate re-expansion. Mo₁₈O₅₂ is a triclinic step-layered material which markedly expands upon Li⁺ intercalation. Li⁺ diffuses in it relatively slowly for x<1, i.e. before increasing the interlayer distance, and for x>0.4 due to coulombic repulsion between Li⁺ ions.     

Spectroscopic investigation of [(NH4)xK1−x]2SnCl6 mixed crystals

Polycrystalline samples of the solid solution [(NH₄)_xK_1?x]₂SnCl₆(0?x? 1) have been investigated by DSC, X-ray diffraction and Raman scattering experiments. Substitution of K⁺ by NH⁺₄ depresses the phase transition temperature T₁. For 0? x ?0.05 a linear temperature coefficient $\text{d}\text{T}{}_1\text{dx=?5.16}\text{K/mol}$ % is obtained. The cubic lattice constant roughly obeys Vegard's law, whereas the linewidth of the SnCl₆^2?F_2g internal vibration displays a nonlinear dependence on composition.     

Rietveld refinements and vibrational spectroscopic studies of Na1−xKxPb4(PO4)3 lacunar apatites (0≤x≤1)

Synthesis of apatites, Na_1−xK_xPb₄(PO₄)₃ 0≤x≤1, with anion vacancy was carried out using solid state reactions. The solid solution of apatite-type structure crystallizes in the hexagonal system, space group P6₃/m (No 176). Rietveld refinements showed that 75% of Pb²⁺ cations are located in the (6h) sites; the ninefold coordination sites (4f) are equally occupied by the other 25% lead cations and the K⁺ and Na⁺ monovalent ions.The structure can be described as built up from [PO₄]³⁻ tetrahedra and Pb²⁺ of sixfold coordination cavities (6h positions), which delimit void hexagonal tunnels running along [0 0 1]. These tunnels are connected by cations of mixed sites (4f) half occupied by Pb²⁺ and half by Na⁺/K⁺ mixed cations. The assignment of the observed frequencies in the Raman and infrared spectra is discussed on the basis of a unit cell group analysis and by comparison with other apatites. The Raman modes of all the compositions show some linear shifts of the frequencies as a function of the composition toward lower values due the substitutions of Na⁺ by K⁺ with a larger radius.     

Raman fingerprint of the interaction of K+ with the COO− group of zwitterionic alanine

The interaction of K⁺ with the zwitterionic form of alanine (ZAla) is investigated using Raman spectroscopy and density functional theory calculations. The Raman spectra of an aqueous solution of Ala and its mixture with KOH at different molar concentrations [ZAla + xKOH, x = 1–5 M] have been recorded in the spectral region 400–1800 cm⁻¹. The wavenumber position of the band at ~529 cm⁻¹ shows a red shift of 14 cm⁻¹, while the Raman band at ~634 cm⁻¹ shows a blue shift of 10 cm⁻¹ with the increasing x from 1 to 5 M. The intensity ratio I₆₃₄/I₅₂₉ is increased with increasing x, and it could be because of the increase in concentration of the [ZAla + K⁺] complex in the solution. The new Raman band appeared at ~1079 cm⁻¹ in the Raman spectra of [ZAla + xKOH, x = 1–5] complex. To determine the most probable site for the interaction of K⁺ with ZAla, the structures of ZAla and the [ZAla + K⁺] were optimized at B3LYP/6‐311++G(d,p) level of theory. The electrostatic potential calculation carried out for ZAla reveals that the maximum density of electron is lying over COO⁻, and therefore, COO⁻ would be the most probable site for the interaction of K⁺ with ZAla. The theoretically calculated Raman spectra of ZAla, [ZAla + K⁺] and the [ZAla⁻ + K⁺] are in good agreement with experimentally observed Raman spectra. Thus, the Raman bands at ~529, 634, and 1079 cm⁻¹ may be used as the Raman fingerprint for the interaction of K⁺ with COO⁻ of the ZAla and ZAla⁻. Copyright © 2015 John Wiley & Sons, Ltd.