正磷酸盐晶体Ba3(PO4)2和Sr3(PO4)2高温拉曼光谱研究

测量了碱土金属正磷酸盐Ba₃(PO₄)₂和Sr₃(PO₄)₂常温及高温拉曼光谱, 对拉曼振动模式进行指认, 并分析了晶体拉曼振动光谱及晶体结构在高温下的变化. 在温度升高的过程中, 拉曼振动频率向低频移动且振动峰宽度展宽, 晶体中的P-O平均键长随温度升高而变长, 但O-P-O的键角并未发生变化. 晶体在900 ℃以下无结构相变发生. 关键词： 3(PO₄)₂和Sr₃(PO₄)₂')" href="#">Ba₃(PO₄)₂和Sr₃(PO₄)₂ 高温拉曼光谱 振动模式 高温结构     

Raman and FTIR studies of the structural aspects of Nasicon-type crystals; AFeTi(PO4)3 [A=Ca, Cd]

Raman and FTIR spectra of CaFeTi(PO₄)₃ and CdFeTi(PO₄)₃ are recorded and analyzed. The observed bands are assigned in terms of vibrations of TiO₆ octahedra and PO₄ tetrahedra. The symmetry of TiO₆ octrahedra and PO₄ tetrahedra is lowered from their free ion symmetry. The presence of Fe³⁺ ion disrupts the Ti-O-P-O-Ti chain and leads to the distortion of TiO₆ octrahedra and PO₄ tetrahedra. The PO₄³⁻ tetrahedra in both crystals are linearly distorted. The covalency bonding factor of PO₄³⁻ polyanion of both the crystals are calculated from the Raman spectra and compared to that of other Nasicon-type systems. The numerical values of covalency bonding factor indicates that there is a reduction in redox energy and cell voltage and is attributed to strong covalency of PO₄³⁻ polyanionin.     

Detailed infrared spectroscopic study of thermal transitions in new hybrid [(CH3)2CHNH3]4Cd3Cl10 crystal

Phase transitions of tetra(isopropylammonium)decachlorotricadmate(II) [(CH₃)₂CHNH₃]₄Cd₃Cl₁₀ crystal have been studied by infrared, far infrared and Raman measurements in wide temperature range, between 11 K and 388 K. The temperature changes of wavenumber, center of gravity, width and intensity of the bands were analyzed to clarify cationic and anionic contributions to the phase transitions mechanism. The results of investigation showed earlier by differential scanning calorimetry (DSC), thermal expansion and dielectric measurements clearly confirmed the sequence of phase transitions at T₁=353 K, T₂=294 K and T₃=260 K. The current results derived from DSC and infrared measurements revealed additional phase transition at T₄=120 K.     

High pressure Raman spectroscopic study of spinel MgCr2O4

An in situ Raman spectroscopic study was conducted to investigate the pressure induced phase transformation of MgCr₂O₄ spinel up to pressures of 76.4 GPa. Results indicate that MgCr₂O₄ spinel undergoes a phase transformation to the CaFe₂O₄ (or CaTi₂O₄) structure at 14.2 GPa, and this transition is complete at 30.1 GPa. The coexistence of two phases over a wide range of pressure implies a sluggish transition mechanism. No evidence was observed to support the pressure-induced dissociation of MgCr₂O₄ at 5.7-18.8 GPa, predicted by the theoretical simulation. This high pressure MgCr₂O₄ polymorphism remains stable upon release of pressure, but at ambient conditions, it transforms to the spinel phase.     

Raman spectroscopic analysis of sulphur-doped TiO2 by co-grinding with TiS2

Doping sulphur into titania has been tried using TiS₂ as a doper based on the mechanically induced solid-state reaction between TiO₂ and TiS₂. The prepared samples have been characterized by X-ray diffraction (XRD), Raman spectroscopy and UV-Vis reflectance spectroscopy. Raman analysis, particularly has been proved to be effective in assessing the sulphur doping by correlating the oxygen deficiency of the doped oxide with the change of active Eg mode of rutile phase.     

Low-temperature vibrational properties of KGd(WO4)2: (Er, Yb) single crystals studied by Raman spectroscopy

Temperature-dependent polarized Raman spectra of KGd(WO₄)₂: (Er, Yb) single crystals have been analyzed over the 77-292 K temperature range. The A_g and B_g spectra obtained are discussed in terms of factor group analysis. The spectra have been found to reveal the bands related to internal and external vibrations of WO₄²⁻, WOW and WOOW molecular groups. Strong depolarization of the majority of the Raman bands has been observed in the whole temperature range. Some anomalies in the spectral parameters of selected Raman bands below 175 K have been discussed in terms of the local distortion of WO₄²⁻ ions in KGd(WO₄)₂: (Er, Yb) crystals.     

High pressure X-ray diffraction study of CdAl2Se4 and Raman study of AAl2Se4 (A=Hg, Zn) and CdAl2X4 (X=Se, S)

We report the results of an X-ray diffraction study of CdAl₂Se₄ and of Raman studies of HgAl₂Se₄ and ZnAl₂Se₄ at room temperature, and of CdAl₂S₄ and CdAl₂Se₄ at 80 K at high pressure. The ambient pressure phase of CdAl₂Se₄ is stable up to a pressure of 9.1 GPa above which a phase transition to a disordered rock salt phase is observed. A fit of the volume pressure data to a Birch-Murnaghan type equation of state yields a bulk modulus of 52.1 GPa. The relative volume change at the phase transition at ∼9 GPa is about 10%. The analysis of the Raman data of HgAl₂Se₄ and ZnAl₂Se₄ reveals a general trend observed for different defect chalcopyrite materials. The line widths of the Raman peaks change at intermediate pressures between 4 and 6 GPa as an indication of the pressure induced two stage order-disorder transition observed in these materials. In addition, we include results of a low temperature Raman study of CdAl₂S₄ and CdAl₂Se₄, which shows a very weak temperature dependence of the Raman-active phonon modes.     

In situ pressure-induced solid-state amorphization in Sm2(MoO4)3, Eu2(MoO4)3 and Gd2(MoO4)3 crystals: chemical decomposition scenario

The effect of pressure on the phase transformations in Sm₂(MoO₄)₃, Gd₂(MoO₄)₃ and Eu₂(MoO₄)₃ crystals has been studied in situ using synchrotron radiation. All three isostructural compounds undergo a structural phase transition at 2.2-2.8 GPa to a new phase, which is interpreted as a possible precursor of amorphization. Amorphization in these crystals occurs irreversibly over a wide pressure range, and its mechanism, interpreted as a chemical decomposition, is found to be weakly affected by the degree of hydrostaticity.     

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.     

Phase transitions and molecular motions in [Zn(NH3)4](BF4)2 studied by nuclear magnetic resonance, infrared and Raman spectroscopy

Middle infrared absorption, Raman scattering and proton magnetic resonance relaxation measurements were performed for [Zn(NH₃)₄](BF₄) in order to establish relationship between the observed phase transitions and reorientational motions of the NH₃ ligands and BF₄⁻ anions. The temperature dependence of spin-lattice relaxation time (T₁(¹H)) and of the full width at half maximum (FWHM) of the bands connected with ρ_r(NH₃), ν₂(BF₄) and ν₄(BF₄) modes in the infrared and in the Raman spectra have shown that in the high temperature phase of [Zn(NH₃)₄](BF₄)₂ all molecular groups perform the following stochastic reorientational motions: fast (τ_R≈10⁻¹² s) 120° flips of NH₃ ligands about three-fold axis, fast isotropic reorientation of BF₄⁻ anions and slow (τ_R≈10⁻⁴ s) isotropic reorientation (“tumbling”) of the whole [Zn(NH₃)₄]²⁺ cation. Mean values of the activation energies for uniaxial reorientation of NH₃ and isotropic reorientation of BF₄⁻ at phases I and II are ca. 3 kJ mol⁻¹ and ca. 5 kJ mol⁻¹, respectively. At phases III and IV the activation energies values for uniaxial reorientation of both NH₃ and of BF₄⁻ equal to ca. 7 kJ mol⁻¹. Nearly the same values of the activation energies, as well as of the reorientational correlation times, at phases III and IV well explain existence of the coupling between reorientational motions of NH₃ and BF₄⁻. Splitting some of the infrared bands at T_C2=117 K suggests reducing of crystal symmetry at this phase transition. Sudden narrowing of the bands connected with ν₂(BF₄), ν₄(BF₄) and ρ_r(NH₃) modes at T_C3=101 K implies slowing down (τ_R?10⁻¹⁰ s) of the fast uniaxial reorientational motions of the BF₄⁻ anions and NH₃ ligands at this phase transition.     

Structural and electrical properties of high-energy ball-milled NASICON type Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 ceramics

Nano-crystallites of Li_1.3Ti_1.7Al_0.3(PO₄)_2.9(VO₄)_0.1 NASICON type material are prepared by means of solid-state reaction of a stoichiometric mixture after milling it for 22 and 55 h. The milling reduces the average crystallite size of the ceramic to 80 and 60 nm, respectively. Mechanical milling changes structural parameters and the strain induced at the grain-boundaries plays a major role in improving electrical conductivity. An order of magnitude increase in electrical conductivity is observed in the material milled for 55 h compared to the unmilled material, which is also reflected in permittivity loss. Modulus and permittivity representations substantiate the constriction effect of grain-boundaries observed in the complex impedance representation.     

Spectroscopic and magnetic properties of Fe3Fe4(AsO4)6

The infrared (IR) and ⁵⁷Fe-Mössbauer spectra of Fe₃^IIFe₄^III(AsO₄)₆ were recorded and analyzed on the basis of its structural characteristics. The IR spectrum presents a high complexity, showing an important number of bands and splittings, as a consequence of the presence of three structurally independent AsO₄³⁻ groups. The analysis of the four quadrupole signals shown by the Mössbauer spectrum allowed to attain a detailed insight into the cation distribution over the available crystallographic sites. The alternating current susceptibility measurements indicate a paramagnetic to ferrimagnetic transition in the material at about 59 K.     

Recombination luminescence mechanisms in Ba3(PO4)2

The effect of hole self-trapping in Ba₃(PO₄)₂ at low temperatures has been studied. The TSL peak at 135 K is due to hole delocalization and diffusion by thermally activated hopping between perfect lattice sites, resulting in a composite uv band, corresponding to the tunneling recombination of holes with localized electrons.     

Synthesis of high tap density Li3V2(PO4)3 cathode materials using mixed lithium precursors

High tap density Li₃V₂(PO₄)₃ cathode materials were synthesized using mixed LiF and LiNO₃ as lithium precursors, LiNO₃ was used as the sintering agent. Rietveld refinement results show that no impurities phases are detected in products. Particle size distribution and tap density measurement results show that particle size and tap density of products can be increased by the addition of LiNO₃. Electrochemical characterization results show that electrochemical performance of products is declined with the increase in contents of LiNO₃ in the lithium precursors. Only a small amount of LiNO₃ added in the lithium precursors (mole ratio of LiNO₃ to LiF is 1:9) can increase the tap density and also retain the good performance of products. Scanning electron microscopy (SEM) images indicate that the samples prepared by mixed lithium precursors present particles agglomerate, and the particle size increased with increase in contents of LiNO₃. Large amount of LiNO₃ added in the lithium precursors induces the particles to become spheric and smooth, which worsens the performance. The particles obtained with the mole ratio of LiNO₃ to LiF in 1:9 show a flake-like shape with a high specific surface area, which leads to good electrochemical performance.     

Structural and luminescence characterization of synthetic Cr-doped Ni3(PO4)2

Ni_3–xCr_2x/3(PO₄)₂ (x=0 and 0.02) microcrystalline powders were obtained as single phases via a modified sol–gel Pechini-type in situ polymerizable complex method. The samples were characterized using scanning electron microscopy, X-ray diffraction, cathodoluminescence (CL), and thermoluminescence (TL) techniques. We found that Cr³⁺ doping modified the average particle and distribution. The mean particle size was 0.441 μm for Ni₃(PO₄)₂ and 0.267 μm for Ni_2.98Cr_0.013(PO₄)₂. The results also reveal that Cr³⁺ doping notably enhanced the CL and TL UV-blue emission.     

Features of the electrochemical lithium insertion into monophosphate tungsten bronzes (PO2)4(WO3)2m (m=9 and 10)

Electrochemical lithium insertion into (PO₂)₄(WO₃)_2m, where m=9 and 10, has allowed the determination of several phases Li_x(PO₂)₄(WO₃)_2m between 3.4 and 0.01 V vs Li⁺/Li⁰. After the first cycle the electrochemical system was unable to maintain the high specific capacity of the cells (540 Ah/kg) due to irreversible processes. Nevertheless at high voltage values, above 1.4 V vs Li⁺/Li⁰, the lithium insertion proceeded through a reversible mechanism. By means of X-ray diffraction experiments we have detected the nature of different phases Li_x(PO₂)₄(WO₃)_2m formed and we have established a correlation with the reversible/irreversible processes detected during the electrochemical insertion.     

Structural study by X-ray diffraction and Mössbauer spectroscopy of a new synthetic iron phosphate: K4MgFe3(PO4)5

A new iron phosphate K₄MgFe₃(PO₄)₅ has been synthesized by the flux method and characterized by single-crystal X-ray diffraction and Mössbauer spectroscopy. It crystallizes in the tetragonal system with the space group and the unit cell parameters a=9.714(3) Å and c=9.494(5) Å. The crystal structure is of a new type. It exhibits a three-dimensional framework built up from corner-sharing MO₅ (M=0.75Fe+0.25Mg) trigonal bipyramids and PO₄ tetrahedra. The K⁺ ions are occupying large eight-sided tunnels running along c. A room temperature Mössbauer study confirmed the +3 valence state of iron and its five-coordination.     

Doping-induced spin-manipulation in complex trimer system Ca3Cu3(PO4)4: A density functional investigation

Doping induced spin-manipulation with magnetic (Ni) and non-magnetic (Mg) dopants constitutes the experimental attempts to obtain a singlet ground state system from the linear chain Heisenberg antiferromagnetic Cu-based d⁹ spin-1/2 trimer compound Ca³Cu³(PO⁴)⁴ with doublet ground state. The present study is a density-functional investigation of the effects of such doping on the spin-exchange mechanism and electronic structure of the parent compound. Site-selective doping with zero-spin dopants like Mg is proved to be more efficient than an integral spin dopant Ni in obtaining a spin-gap system with singlet ground state, as also observed in the experimental studies. Doping induced dimerized state is found to be the lowest in ground-state energy. Calculated spin exchange couplings along various possible pathways are observed to attain good agreement with earlier experimental results with suitable optimization of Coulomb repulsion (U) and exchange (J) parameters.     

Structural characterization of dense reduced BaTiO3 and Ba0.95La0.05TiO3 nanoceramics showing colossal dielectric values

BaTiO_3−x and Ba_0.95La_0.05TiO_3−x nanoceramics showing colossal permittivity values have been characterized. While starting powders are of cubic symmetry, X-ray and Neutron Diffraction techniques and Raman Spectroscopy measurements show that the one-step processed ceramics obtained by Spark Plasma Sintering (SPS) contain cubic and tetragonal phases. Rather large oxygen deficiency determined in such ceramics by Electron Micro Probe analysis and Electron Energy Loss Spectroscopy analyzes is explained by the presence of Ti³⁺, as evidenced by X-ray Photoelectron Spectroscopy measurements. Transmission Electron Microscopy and High Resolution Transmission Electron Microscopy show that these ceramics contain 50-300 nm grains, which have single-domains, while grain boundaries are of nanometer scale. Colossal permittivity values measured in our dense nanoceramics are explained by a charge hopping mechanism and an interfacial polarization of a large number of polarons generated after sample reduction in SPS apparatus.     

Raman spectra of Ba6−3xSm8+2xTi18O54 solid solution

Raman spectra of Ba_6−3xSm_8+2xTi₁₈O₅₄ solid solution were investigated as the function of x and sintering time. Reasonable explanations were provided about the Raman shifts and their intensities at 1013, 590, 751, 280, 232 cm⁻¹. 1013 cm⁻¹ demonstrates the existence of BaCO₃ phase in solid solution, 590 cm⁻¹ is the symmetric stretching mode of the basal oxygens of the octahedral; 280 and 232 cm⁻¹ are the symmetric stretching modes resulted from the tilt of octahedral when large cation sites are Sm³⁺ and Ba²⁺. The shoulder peak appearing around 302 cm⁻¹ is related to the vacancy produced by the unequal valence of Sm³⁺ and Ba²⁺.