Aluminium-doped LiFePO4 single crystals. Part II. Ionic conductivity, diffusivity and defect model

We report on lithium ion conductivity and diffusivity along major crystallographic directions of Al-doped LiFePO4 single crystals. Impedance spectroscopy as well as galvanostatic polarization measurements have been carried out on the electronically blocking symmetric cell LiAl/LiI/LiFe(Al)PO4/LiI/LiAl. Neither ionic conductivity nor lithium diffusivity show anisotropy in the bc planes within the experimental error, but much lower values in the a-direction. Similar features were observed earlier by us for the pure single-crystal and the Si-doped single crystal. On Al-doping the ionic conductivity has increased while the electronic conductivity has decreased compared to undoped LiFePO4. Not only this donor doping effect but also the temperature dependence of ionic conductivity and of lithium-diffusivity are successfully interpreted in terms of lithium vacancies, holes and associates in the framework of a detailed defect chemical analysis. Ion-electron as well as ion-ion associates play a significant role in this system.     

Growth and characterization of 2-amino-4-picolinium 4-aminobenzoate single crystals

The organic material of 2-amino-4-picolinium 4-aminobenzoate (C₆H₉N₂⁺·C₇H₆NO₂^?) was synthesized and grown as single crystals at room temperature by slow evaporation solution growth technique in the constant temperature bath (±0.01 °C) using ethanol as solvent. The grown crystals were characterized by XRD and FT-IR spectral analyses. The melting point, density, UV–visible spectral studies were carried out for the grown crystals. The theoretical factor group analysis predicts 372 internal modes of vibration and optical modes in the grown title compound. The second harmonic generation (SHG) output of 2-amino-4-picolinium 4-aminobenzoate was recorded by Kurtz–Perry powder technique and it is found to be 355 mV at a given pulse energy of 1.45 mJ/s and for urea the SHG output was 525 mV. The dielectric behaviour of 2-amino-4-picolinium 4-aminobenzoate was investigated with different frequencies and temperatures.     

Phase boundary propagation in large LiFePO4 single crystals on delithiation

Large single crystals of LiFePO(4) have been chemically delithiated. The relevance of chemical oxidation in comparison with electrochemical delithiation is discussed. Analyses of the Li content and profiles were done by electron energy loss spectroscopy and secondary ion mass spectrometry. The propagation of the FePO(4) phase growing on the surface of the large single crystal was followed by in situ optical microscopy as a function of time. The kinetics were evaluated in terms of linear irreversible thermodynamics and found to be characterized by an induction period followed by parabolic growth behavior of the FePO(4) phase indicating transport control. The growth rate was shown to depend on the crystallographic orientation. Scanning electron microscopy images showed cracks and a high porosity of the FePO(4) layer due to the significant changes in the molar volumes. The transport was found to be greatly enhanced by the porosity and crack formation and hence greatly enhanced over pure bulk transport, a result which is supposed to be very relevant for battery research if coarse-grained powder is used.     

Growth, spectral and thermal characterization of vanillin semicarbazone (VNSC) single crystals

Vanillin semicarbazone (VNSC) has been synthesized from 4-hydroxy 3-methoxy benzaldehyde and semicarbazide hydrochloride using sodium acetate as catalyst. Good quality single crystals of VNSC were successfully grown by slow evaporation method at room temperature using DMF as solvent. The grown crystals have been characterized using melting point, elemental analysis, FT-IR, UV–Visible, ¹H NMR, ¹³C NMR, and X-Ray diffraction analysis. The compound crystallizes into an orthorhombic Pca21 space group. Intermolecular hydrogen bonding interactions facilitate unit cell packing in the crystal lattice. The UV–Visible spectrum confirmed the transparency of the compound between the wavelengths 420 and 1,200 nm, which is characteristic to property of an NLO material. The thermal decomposition of the compound under static air atmosphere was investigated by simultaneous TG/DTG at a heating rate of 10 °C/min.     

Polymer single crystals of poly(4-hydroxybenzoate). I. Morphology

Poly(4-hydroxybenzoate) was prepared from 4-acetoxybenzoic acid, 4-pivaloyloxybenzoic acid, and 4-trimethylsiloxy benzoyl chloride by condensation without the addition of a catalyst. Although various reaction conditions were used the products were always crystalline. The morphology of the as-polymerized crystals has been studied by transmission electron microscopy as a function of molecular weight and end-group type. It was possible to detect morphological features during the course of reaction from oligomers to polymer which enabled us to determine the mechanisms that are likely to control chain growth for different regimes of polymerization kinetics.     

Regeneration and characterization of air-oxidized LiFePO4

Phase-pure LiFePO₄ as a cathode material for lithium ion batteries was prepared in argon, then exposed to hot air and, at last, regenerated with polypyrrole (PPy) as a reductant. Mössbauer spectroscopy and Raman spectroscopy were used to characterize the chemistry of LiFePO₄ before and after regeneration. Electrochemical tests were carried out to evaluate the impacts of hot-air oxidation and regeneration on the properties of LiFePO₄.     

Growth and characterization of sodium pentaborate [Na(H4B5O10)] single crystals

The bulk single crystals of sodium pentaborate [Na(H4B5O10)] were grown by slow evaporation solution growth technique using deionized water as solvent. The grown crystal was confirmed by single crystal X-ray diffraction studies. The structural perfection of the grown crystals has been analyzed by high resolution X-ray diffraction (HRXRD) studies by recording rocking curve. The photoluminescence (PL), UV-vis spectral studies were performed and the optical bandgap of the material was calculated. The presence of the functional groups was identified by FT-IR measurement. The factor group analysis was done on Na(H4B5O10) to reveal the vibrational optical modes. The thermal and mechanical properties of the grown crystal were studied by TG-DTA and Vickers microhardness tester, respectively. The dielectric behavior of Na(H4B5O10) was investigated with different frequencies and temperatures.     

Growth and characterization studies on glycine barium dichloride single crystals for NLO applications

The novel nonlinear optical single crystal of glycine barium chloride has been successfully synthesized by taking the appropriate amount of glycine and barium dichloride and single crystals have been grown by slow evaporation solution growth technique at room temperature. The grown single crystals have been analyzed with XRD, FT-IR, TG/DTA measurements. Its mechanical behavior has been assessed by Vickers Microhardness measurements. Its nonlinear optical property has been tested by Kurtz powder technique. Its optical behavior was examined by UV-vis, and found that the crystal is transparent in the region between 250 and 1200 nm.     

Growth and characterization of naphthalene single crystals grown by modified vertical Bridgman method

The large size 20 mm diameter naphthalene single crystals were grown by modified vertical Bridgman method. The optical quality of the grown crystal was analyzed by photoluminescence and ultraviolet visible absorption spectral measurements. Photoluminescence spectrum indicated violet emission at 410 nm. The UV–vis absorption spectrum shows the maximum at 219.32 nm. The microhardness measurements and differential thermal analysis performed to analyze the mechanical and thermal stability of the as grown crystals. The dielectric measurements were carried out at five frequencies, viz., 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz and at various temperatures, ranging from 308 K to 348 K indicate an increase of the dielectric parameters with the increase of temperature at all the five frequencies. The conductivity of the material is understood to be more of electronic in nature. Also, the increase in the dielectric parameters with that of temperature is due to the temperature variation of electronic polarizability.     

Halogenation of polyethylene. I. Bromination of single crystals

It has been demonstrated that the fold surfaces of polymers can be specific towards chemical attack, if the reaction is mild and nondestructive of the fold. Bromination of suspensions of single crystals of polyethylene in carbon tetrachloride has been shown to be such a system. This chemical modification of a fold surface is a powerful means of extending the applications of the physical methods available. Several methods were used, among them DTA, DSC, infrared spectroscopy and small-angle x-ray diffraction. Experimental results from these methods lead to the following conclusions. (a) Bromination takes place preferentially at the folds and is consistent with a regular adjacent reentry fold model. (b) Annealing of these brominated crystals demonstrates the major role played by the crystal surface in this process. (c) The preparation and properties of a novel copolymer system has been demonstrated. It is felt that this copolymer system may prove a useful addition to those systems presently available.     

High-temperature conductivity relaxation in undoped CdS and CdSe single crystals

The high-temperature conductivity (HTC) relaxation in CdS and CdSe single crystals, caused by a steep change of cadmium vapor pressure P_Cd, was measured. For obtaining a steep change of P_Cd improved van Doorn equipment was used. Conductivity measurements were carried out using the Van der Pauw method. It was shown that the Cd diffusion from gaseous phase into CdS and CdSe platelets leads to a conductivity time-dependence function σ(t) in the form of a sum of decreasing exponents, indicating that the rate of HTC relaxation is limited by the chemical diffusion of cadmium. A correlation between chemical and tracer-diffusion coefficients was found. Values of activation energies and preexponential factors for both types of diffusion were determined. The results are discussed using a model for transformation of Cd^··_i into V^··_S,Se previously proposed by Chern and Kröger for chemical self-diffusion in CdTe.     

Growth, optical, thermal, and conductivity behavior of nonlinear optical single crystals of CdHg(SCN)4(CH3OC2H5O)

In this study, organometallic nonlinear optical single crystal of Cadmium mercury thiocyanate glycol monomethyl ether was grown by slow evaporation technique at room temperature. The grown crystal was confirmed by single crystal XRD and FT-IR analysis. Optical and thermal properties of the sample were investigated by UV–Vis–NIR, TG–DTA and DSC technique. Using Tauc’s relation, the band gap of the crystal was estimated as 3.99 eV. The thermal studies confirm that the CMTG is thermally stable up to 105 °C. Photoluminescence and ac & dc conductivity measurements of the sample were also carried out and reported for the first time. The PL analysis confirms the suitability of the material for application in the blue–green region. The temperature dependences of ac & dc conductivities of CMTG were investigated, and the activation energies were found to be 0.098 and 0.045 eV, respectively. The SHG efficiency measurement confirms that the NLO efficiency of CMTG is nearly three times more efficient than urea.     

Growth, structural, spectral, mechanical, thermal and dielectric characterization of phosphoric acid admixtured L-alanine (PLA) single crystals

Phosphoric acid admixtured L-alanine (PLA) single crystals were grown successfully by solution method with slow evaporation technique at room temperature. Crystals of size 18 mm×12 mm×8 mm have been obtained in 28 days. The grown crystals were colorless and transparent. The solubility of the grown samples has been found out at various temperatures. The lattice parameters of the grown crystals were determined by X-ray diffraction technique. The reflection planes of the sample were confirmed by the powder X-ray diffraction study and diffraction peaks were indexed. Fourier transform infrared (FTIR) studies were used to confirm the presence of various functional groups in the crystals. UV-visible transmittance spectrum was recorded to study the optical transparency of grown crystal. The nonlinear optical (NLO) property of the grown crystal was confirmed by Kurtz-Perry powder technique and a study of its second harmonic generation efficiency in comparison with potassium dihydrogen phosphate (KDP) has been made. The mechanical strength of the crystal was estimated by Vickers hardness test. The grown crystals were subjected to thermo gravimetric and differential thermal analysis (TG/DTA). The dielectric behavior of the sample was also studied.     

The electrical conductivity of single crystals of Ag2HgI4

Electrical conductivity measurements are reported for single crystal samples of Ag₂HgI₄ in the temperature range between ?17°C and 72°C. The results are interpreted in terms of two distinct regions of linear log conductivity versus reciprocal temperature behavior for β-Ag₂HgI₄, a phase transition region, and an α-Ag₂HgI₄ region. The data reaffirm the anomalously high pre-exponential and activation energy terms in the conductivity equation for α-Ag₂HgI₄ as compared with other AgI-type conductors. Thermo-emf measurements on polycrystalline Ag₂HgI₄ samples are also reported.     

Electrical conductivity in Ag2F single crystals

The magnetic circular dichroism spectra of the halogen molecules I₂, Br₂ and Cl₂ were investigated in solutions. The spectral region studied was 800-250 nm. The MCD spectra were interpreted successfully in terms of the transitions to the states 1_u(³II) and ¹II. The major result is the resolution of the overlapping contributions from O⁺_u(³II) and ¹II.     

Growth and characterization of 2-amino-4-picolinium toluene sulfonate single crystal

2-Amino-4-picolinium toluene sulfonate (2A4PTS), a new organic material, was synthesized and grown as single crystals in room temperature by slow evaporation solution growth technique using water as solvent. The crystal structure of 2A4PTS has been determined using single crystal X-ray diffraction studies. 2A4PTS belongs to monoclinic crystal system. The molecular arrangements in the crystal were studied. The structural perfection of the grown crystals has been analysed by high-resolution X-ray diffraction (HRXRD) rocking curve measurements. Fourier transform infrared (FTIR) spectral studies have been performed to identify the functional groups. The optical transmittance window and the lower cutoff wavelength of the 2A4PTS have been identified by UV–Vis–NIR studies. The nonlinear optical properties have been investigated by Z-scan method. The nonlinear refractive index and linear absorption coefficient of the 2A4PTS are found to be in the order of 10⁻⁸ cm²/W and 10⁻⁴ cm/W, respectively. The laser induced surface damage threshold for the grown crystal was measured using Nd:YAG laser. Thermal analysis carried out on the compound reveals that 2A4PTS is stable up to 133 °C. The microhardness test was carried out and the load dependent hardness was measured.     

Ferroelectric liquid crystals derived from isoleucine I. Synthesis and characterization

A series of ferroelectric liquid crystals (FLCs), 4 (3-methyl-2-halopentanoyloxy)- 4-hexyloxybiphenyls (3M2XPHOB, X=F for fluorine, C for chlorine, B for bromine) and their racemates (3M2XPHOB-R), were synthesized and characterized. The FLCs contain a chiral tail comprised of alpha -halo acids which are derived from L-isoleucine (DL-isoleucine for the racemates). The mesogens were characterized by high-resolution 1H and 13C NMR and their phase behaviour was studied by optical microscopy and differential scanning calorimetry. The chloro and bromo derivatives show both chiral smectic C (SmC*) phases and smectic A (SmA) phases, while the fluoro derivatives exhibit only a SmA phase. The spontaneous polarization of 3M2CPHOB and 3M2BPHOB were measured in the respective SmC* phases; that of the fluoro derivative was inferred by extrapolating its concentration dependent polarization in an achiral SmC solvent, a racemic mixture of 3M2BPHOB.     

Growth and characterization studies of 2-bromo-4′-chloroacetophenone (BCAP) crystals

Sintering of barium titanate by microwave heating with barium borate as a liquid-phase forming material was examined as a means of saving energy. A liquid phase was generated faster and over a wider temperature range by intermittent heating at constant power than by constant heating with variable power. When sufficient amount of the liquid-phase forming material were present, the samples were densified at temperatures below those at which the liquid-phase is normally formed. A sample containing 10 mol% of the liquid-phase forming material was densified efficiently. When too much liquid-phase forming material was present, densification of the sample was obstructed.     

Structural and morphological characterization of synthetic chrysotile single crystals

Stoichiometric chrysotile single crystals have been synthesized as a unique phase by hydrothermal reaction under controlled conditions; the synthesized monocrystals show a cylinder-in-cylinder morphology and can be used as a reference sample with definite chemical composition to investigate the factors responsible of the chrysotile cytotoxicities and carcinogenicities.