Factor Group Splitting Research Articles

Raman spectroscopic study of the layer-dependent Davydov splitting and thermal conductivity of chemically vapor deposited two-dimensional MoSe2

Atomically thin MoSe2 is of interest from the perspective of estimating the layer-dependent material properties necessary for the translation of two-dimensional materials into devices. This work presents Raman spectroscopic protocols to determine a multitude of material parameters of two-dimensional MoSe2 films, including the layer thickness as well as the layer-dependent thermal conductivity, interlayer interactions, and anharmonicity. The Davydov splitting (factor-group splitting) observed in an out-of-plane A1g Raman mode, being layer-dependent in both the number and the peak positions, provides a method for estimating the number of layers. Furthermore, this work demonstrates the determination of the thermal conductivity (K) from the temperature-dependent Davydov split Raman modes of the multi-layers. The measurement of K by conventional methods is otherwise challenging for the micrometer sizes of the two-dimensional materials. The value of K thus determined increases significantly from 9 W m−1 K−1 for a four-layer thick MoSe2 film to 52 W m−1 K−1 for a monolayer. The diminishing effect of anharmonicity observed in the monolayer as compared to multi-layer MoSe2 supports the layer-dependent trend in the thermal conductivity. Overall, the findings are relevant for the applications of 2D MoSe2 in low power electronic, optoelectronic, and thermoelectric devices.

IR spectra of [formula omitted] in Ar and [formula omitted] cryomatrices: Evidence of unusual band splitting in [formula omitted] matrix

Matrix isolation IR spectra of CH2I2 in N2 matrix are analysed using IR spectra obtained in the Ar matrix, quantum chemical calculations, and molecular point group symmetry to determine the origin of the unusual splitting observed in the CH2 antisymmetric stretching (ν6), CH2 rocking (ν7), CH2 wagging (ν8), and CI2 antisymmetric stretching (ν9) modes of CH2I2 only in N2 matrix and not in Ar matrix. The ν6, ν7, ν8, and ν9 vibrational modes belong to either B1 or B2 irreducible representations under C2V point group symmetry. IR spectra in N2 matrix is reported for the first time. IR spectra recorded in Ar matrix are consistent with previous reports. Electronic structure calculations have been performed to obtain simulated IR spectra of CH2I2 with C2V and C2 point group symmetries, (CH2I2)2 conformers, [CH2I2-H2O], and [CH2I2-(N2)4]. IR spectra obtained in Ar and N2 matrices originate from CH2I2, (CH2I2)2, and splitting of IR peaks in N2 matrix. Splitting of the IR peaks of CH2I2 in solid state is described by Davydov splitting or factor group splitting. The observed splitting of IR peaks in N2 matrix is due to the lowering of symmetry of CH2I2 from C2V to one of its sub groups due to the perturbation of the rigid N2 matrix that possess quadrupole moment.

Synthesis, characterization, vibrational spectroscopy, and factor group analysis of partially metal-doped phosphate materials

A simple precipitating method was used to synthesize effectively a partially metal-doped phosphate hydrate (Mn0.9Mg0.1HPO4·3H2O), whereas the thermal decomposition process of the above hydrate precursor was used to obtain Mn1.8Mg0.2P2O7 and LiMn0.9Mg0.1PO4 compounds under different conditions. To separate the overlapping thermal decomposition peak, a deconvolution technique was used, and the separated peak was applied to calculate the water content. The factor group splitting analysis was used to exemplify their vibrational spectra obtained from normal vibrations of HPO42−, H2O, P2O74− and PO43− functional groups. Further, the deconvoluted bending mode of water was clearly observed. Mn0.9Mg0.1HPO4·3H2O was observed in the orthorhombic crystal system with the space group of Pbca (D2h15). The formula units per unit cell were found to be eight (Z = 8), and the site symmetric type of HPO42− was observed as Cs. For the HPO42− unit, the correlation filed splitting analysis of type C3v − Cs − D2h15 was calculated and had 96 internal modes, whereas H2O in the above hydrate was symbolized as C2v − Cs − D2h15 and had 24 modes. The symbol C2v − Cs − C2h3 was used for the correlation filed splitting analysis of P2O74− in Mn1.8Mg0.2P2O7 (monoclinic, C2/m (C2h3), Z = 2, and 42 modes). Finally, the symbol Td − Cs − D2h16 was used for the correlation filed splitting analysis of PO43− in LiMn0.9Mg0.1PO4 (orthorhombic, Pnma (D2h16), Z = 4, and 36 modes).

Structure and Vibrational Spectra of 2,5-Diiodothiophene: A Model for Polythiophene

The structure and vibrational spectroscopy of 2,5-diiodothiophene have been described for the first time. The structure is remarkable in that, despite the presence of eight molecules in the unit cell, there is almost no evidence for any interaction between them. This is reflected in the spectra that show coincident modes in the infrared, Raman, and inelastic neutron scattering (INS) spectra and almost resolution-limited line widths in the INS spectrum, showing that both the factor group splitting and the phonon dispersion are very small. This property has led to the use of the compound as a calibrant in INS spectroscopy. The deductions from the spectra are confirmed by a periodic density functional theory (DFT) calculation that allows a complete spectral assignment to be made. Comparison to the INS and Raman spectra of polythiophene (for which 2,5-diiodothiophene may be used as a monomer) shows a marked similarity for the former and a marked difference to the latter. The INS spectrum of polythiophene is d...

Vibrational spectroscopy of the double complex salt Pd(NH3)4(ReO4)2, a bimetallic catalyst precursor

Tetraamminepalladium(II) perrhenate, a double complex salt, has significant utility in PdRe catalyst preparation; however, the vibrational spectra of this readily prepared compound have not been described in the literature. Herein, we present the infrared (IR) and Raman spectra of tetraamminepalladium(II) perrhenate and several related compounds. The experimental spectra are complemented by an analysis of normal vibrational modes that compares the experimentally obtained spectra with spectra calculated using DFT (DMol3). The spectra are dominated by features due to the ammine groups and the ReO stretch in Td ReO4−; lattice vibrations due to the D4h Pd(NH3)42+ are also observed in the Raman spectrum. Generally, we observe good agreement between ab initio calculations and experimental spectra. The calculated IR spectrum closely matches experimental results for peak positions and their relative intensities. The methods for calculating resonance Raman intensities are implemented using the time correlator formalism using two methods to obtain the excited state displacements and electron-vibration coupling constants, which are the needed inputs in addition to the normal mode wave numbers. Calculated excited state energy surfaces of Raman-active modes correctly predict relative intensities of the peaks and Franck-Condon activity; however, the position of Raman bands are predicted at lower frequencies than observed. Factor group splitting of Raman peaks observed in spectra of pure compounds is not predicted by DFT.

Diffusion and Crystallization of Dichloromethane within the Pores of Amorphous Solid Water

Dichloromethane (CH2Cl2) thin films deposited on Ru(0001) at low temperatures (∼80 K or lower) undergo a phase transition at ∼95 K, manifested by the splitting of its wagging mode at 1265 cm–1, due to factor group splitting. This splitting occurs at relatively higher temperatures (∼100 K) when amorphous solid water (ASW) is deposited over it, with a significant reduction in intensity of the high-wavenumber component (of the split peaks). Control experiments showed that the intensity of the higher wavenumber peak is dependent on the thickness of the water overlayer. It is proposed that diffusion of CH2Cl2 into ASW occurs and it crystallizes within the pores of ASW, which increases the transition temperature. However, the dimensions of the CH2Cl2 crystallites get smaller with increasing thickness of ASW with concomitant change in the intensity of the factor group split peak. Control experiments support this suggestion. We propose that the peak intensities can be correlated with the porosity of the ice film....

Influence of methyl and nitro group substitutions on the structure and vibrational characteristics of the hydrazo-bridge in 6,6′-dimethyl-3,3′,5,5′-tetranitro-2,2′-hydrazobipyridine

The crystal and molecular structures of 6,6′-dimethyl-3,3′,5,5′-tetranitro-2,2′-hydrazobipyridine have been determined by X-ray diffraction and DFT quantum chemical methods. The B3LYP/6-311G(2d,2p) approach has been used in the theoretical calculations. The studied compound crystallizes in the monoclinic P21/n space group (Z=2) with one-half molecule in the asymmetric unit. In the molecular structure of the studied compound the intramolecular NH⋯O hydrogen bonds play a crucial role. These molecules also interact in the unit cell by means of weak intermolecular CH⋯O contacts. IR and Raman spectra have been measured and simulated from the optimized geometry. In the calculations the monomeric unit has been taken into account because the intermolecular interactions between the components of the unit cell are weak and the factor group splitting is not observed in the experimental spectra. The structural and vibrational properties of the hydrazo bonds have been described with their relation to the intramolecular NH⋯O interaction existing in the structure of the monomeric form.

長鎖アルカンの高圧相転移挙動

The effect of pressure on the phase transition behavior of tridecane, pentadecane and heptadecane has been investigated up to 489, 220 and 387 MPa, respectively, using Fourier transform infrared spectroscopy at 25℃. Short range positional order in the rotator phase is discussed on the basis of factor group splitting of the P1+P3 band of the methylene rocking mode. A large proportion of alkane molecules is found to possess herringbone-type short-range positional order in the high pressure rotator phase while a smaller proportion of alkane molecules is considered to possess herringbone-type short-range positional order in the high temperature rotator phase.

Second-order many-body perturbation study of solid hydrogen fluoride under pressure

A linear-scaling, embedded-fragment, second-order many-body perturbation (MP2) method with basis sets up to aug-cc-pVTZ is applied to the antiparallel structure of solid hydrogen fluoride and deuterium fluoride under 0-20 GPa of ambient pressure. The optimized structures, including the lattice parameters and molar volume, and phonon dispersion as well as phonon density of states (DOS), are determined as a function of pressure. The basis-set superposition errors are removed by the counterpoise correction. The structural parameters at 0 GPa calculated by MP2 agree accurately with the observed, making the predicted values at higher pressures a useful pilot for future experiments. The corresponding values obtained by the Hartree-Fock method have large, systematic errors. The MP2/aug-cc-pVDZ frequencies of the infrared- and Raman-active vibrations of the three-dimensional solids are in good agreement with the observed and also justify previous vibrational analyses based on one-dimensional chain models; the non-coincidence of the infrared and Raman mode pairs can be explained as factor-group (Davydov) splitting. The exceptions are one pair of modes in the librational region, for which band assignments based on a one-dimensional chain model need to be revised, as well as the five pseudo-translational modes that exist only in a three-dimensional treatment. The observed pressure dependence of Raman bands in the stretching region, which red-shift with pressure, is accounted for by theory only qualitatively, while that in the pseudo-translational region is reproduced with quantitative accuracy. The present calculation proves to be limited in explaining the complex pressure dependence of the librational modes. The hydrogen-amplitude-weighted phonon DOS at 0 GPa is much less structured than the DOS obtained from one-dimensional models and may be more realistic in view of the also broad, structureless observed inelastic neutron scattering spectra. All major observed peaks can be straightforwardly assigned to the calculated peaks in the DOS. With increasing pressure, MP2 predicts further broadening of bands and breach of the demarcation between the pseudo-translational and librational bands.

Low-temperature (293-60 K) Raman spectra of the crystalline tert-butylisocyanide complexes, Cr(CO)5(CNBu-t) and cis-Cr(CO)4(CNBu-t)2

Low-temperature (293–60 K) Raman spectra have been recorded in the 3500–50 cm–1 region for the crystalline tert-butylisocyanide (CNBu-t) complexes, Cr(CO)5(CNBu-t) (I) and cis-Cr(CO)4(CNBu-t)2 (II). Several of the factor group splittings that are predicted for the νCO, νCN, νCrC, νCrCN and δCrCO modes on the basis of the P21/a (C2h5) (I) and Pccn (D2h10) (II) crystallographic symmetries can be detected at very low temperatures. No phase transitions are observed for either complex throughout the temperature range investigated.

The effect of pressure on the phase transition behavior of tridecane, pentadecane, and heptadecane: A Fourier transform infrared spectroscopy study

The effect of pressure on the phase transition behavior of tridecane (C(13)), pentadecane (C(15)), and heptadecane (C(17)) has been investigated up to 489, 220, and 387 MPa, respectively, using Fourier transform infrared spectroscopy at 25 °C. The transition between the high pressure ordered (HPO) and high pressure rotator (HPR) phases has been observed in the pressure ranges of 270-220, 106-95, and 152-181 MPa for C(13), C(15), and C(17), respectively, and the transition between the HPR and liquid phases was observed in the pressure ranges of 171-112, 73-47, and 43-70 MPa for C(13), C(15), and C(17), respectively. The P(1)+P(3) band of the methylene rocking mode exhibits factor group splitting caused by intermolecular vibrational coupling. This was observed in both the HPO and HPR phases, while the P(1)+P(3) band did not split in the liquid phase. The separation of the peaks in the P(1)+P(3) band changed discontinuously at the HPO-HPR and HPR-liquid phase transitions, even though the separation is known to change continuously in the transition from the liquid to the high temperature rotator (HTR) phase. In the HPR phase, the ratio of the intensities of the higher and lower frequency components in the P(1)+P(3) doublet is roughly unity independent of pressure, while it is known to be much less than unity in the HTR phase. The separation of the P(1)+P(3) doublet in the HPR phase is found to be larger for longer alkanes. From the intensity ratio, a large proportion of alkane molecules is believed to participate in intermolecular vibrational coupling and possess herringbone-type short-range positional order in the HPR phase. Conversely, in the HTR phase only small proportion of alkane molecules participate in intermolecular vibrational coupling. From the pressure dependence of the separation of the doublet, intermolecular vibrational coupling and herringbone-type short-range positional order is considered to change discontinuously at the HPR-liquid phase transition, while they are reported to change continuously at the HTR-liquid phase transition. The HPR-liquid phase transition is governed by the effect of molecular packing while the HTR-liquid phase transition is predominantly governed by the difference in entropy between the herringbone-type and parallel-type packing.

Modelling the visible absorption spectra of copper(II) acetylacetonate by Density Functional Theory

Spin restricted open-shell Density Functional Theory calculations have been carried out by means of linear response theory to investigate the visible absorption spectrum of copper(II) acetylacetonate complex, Cu(acac) 2. The 3 d → 3 d transition energies and the influence of molecular structure and non-coordinating solvent on the spectra have been investigated. The obtained four 3 d → 3 d transition energies accord well with the experimental data in the crystal phase. The presented results indicate that the experimentally observed four band structure of Cu(acac) 2 is of molecular nature, and not caused by factor-group splitting in the crystal environment as previously suggested.

The Vibrational Spectrum of Parabanic Acid by Inelastic Neutron Scattering Spectroscopy and Simulation by Solid-State DFT

The incoherent inelastic neutron scattering spectrum of parabanic acid was measured and simulated using solid-state density functional theory (DFT). This molecule was previously the subject of low-temperature X-ray and neutron diffraction studies. While the simulated spectra from several density functionals account for relative intensities and factor group splitting regardless of functional choice, the hydrogen-bending vibrational energies for the out-of-plane modes are poorly described by all methods. The disagreement between calculated and observed out-of-plane hydrogen bending mode energies is examined along with geometry optimization differences of bond lengths, bond angles, and hydrogen-bonding interactions for different functionals. Neutron diffraction suggests nearly symmetric hydrogen atom positions in the crystalline solid for both heavy-atom and N-H bond distances but different hydrogen-bonding angles. The spectroscopic results suggest a significant factor group splitting for the out-of-plane bending motions associated with the hydrogen atoms (N-H) for both the symmetric and asymmetric bending modes, as is also supported by DFT simulations. The differences between the quality of the crystallographic and spectroscopic simulations by isolated-molecule DFT, cluster-based DFT (that account for only the hydrogen-bonding interactions around a single molecule), and solid-state DFT are considered in detail, with parabanic acid serving as an excellent case study due to its small size and the availability of high-quality structure data. These calculations show that hydrogen bonding results in a change in the bond distances and bond angles of parabanic acid from the free molecule values.

Vibrational spectroscopic studies of the structure of di‐amino acid peptides. Part II: cyclo(L‐Asp‐L‐Asp) in the solid state and in aqueous solution

AbstractSolid‐state protonated and N,O‐deuterated Fourier transform infrared (IR) and Raman scattering spectra together with the protonated and deuterated Raman spectra in aqueous solution of the cyclic di‐amino acid peptide cyclo(L‐Asp‐L‐Asp) are reported. Vibrational band assignments have been made on the basis of comparisons with previously cited literature values for diketopiperazine (DKP) derivatives and normal coordinate analyses for both the protonated and deuterated species based upon DFT calculations at the B3‐LYP/cc‐pVDZ level of the isolated molecule in the gas phase. The calculated minimum energy structure for cyclo(L‐Asp‐L‐Asp), assuming C2 symmetry, predicts a boat conformation for the DKP ring with both the two L‐aspartyl side chains being folded slightly above the ring. The CO stretching vibrations have been assigned for the side‐chain carboxylic acid group (e.g. at 1693 and 1670 cm−1 in the Raman spectrum) and the cis amide I bands (e.g. at 1660 cm−1 in the Raman spectrum). The presence of two bands for the carboxylic acid CO stretching modes in the solid‐state Raman spectrum can be accounted for by factor group splitting of the two nonequivalent molecules in a crystallographic unit cell. The cis amide II band is observed at 1489 cm−1 in the solid‐state Raman spectrum, which is in agreement with results for cyclic di‐amino acid peptide molecules examined previously in the solid state, where the DKP ring adopts a boat conformation. Additionally, it also appears that as the molecular mass of the substituent on the Cα atom is increased, the amide II band wavenumber decreases to below 1500 cm−1; this may be a consequence of increased strain on the DKP ring. The cis amide II Raman band is characterized by its relatively small deuterium shift (29 cm−1), which indicates that this band has a smaller NH bending contribution than the trans amide II vibrational band observed for linear peptides. Copyright © 2009 John Wiley & Sons, Ltd.

Charge ordered state and frustration of site charges in(ET)5Te2I6and(BETS)5Te2I6

We have carried out x-ray structural analysis and have studied the temperature dependence of vibrational spectra and electrical resistivity under uniaxial strain for ${(\mathrm{ET})}_{5}{\mathrm{Te}}_{2}{\mathrm{I}}_{6}$ and ${(\mathrm{BETS})}_{5}{\mathrm{Te}}_{2}{\mathrm{I}}_{6}$ [$\mathrm{ET}=\mathrm{bis}(\text{ethylenedithio})\text{tetrathiafulvalene}$ and $\mathrm{BETS}=\mathrm{bis}(\text{ethylenedithio})\text{tetraselenafulvalene}$]. In the low-temperature insulating phase for each salt, a charge-sensitive mode ${\ensuremath{\nu}}_{2}$ exhibits peak splitting, and a vibronic ${\ensuremath{\nu}}_{3}$ mode shows factor group splitting. This observation confirms the charge ordered state. The distribution of site charges is determined from the factor group analysis of the vibronic ${\ensuremath{\nu}}_{3}$ mode, which is in agreement with that determined from x-ray structural analysis of the ET salt. In the high-temperature conducting phase, the vibronic ${\ensuremath{\nu}}_{3}$ mode is smeared out whereas the frequencies of the two charge-sensitive modes are almost unchanged over the whole temperature range. We have proposed a model wherein the highly conducting state is ascribed to the frustration between the site-charge distributions in the insulating state and the other distribution, which contributes to a reduction in the intersite Coulomb interaction along the stacking and diagonal directions, respectively. Our conjecture is supported by the temperature dependence of the electrical resistivity under the uniaxial strain.

High Pressure Raman Spectroscopy of Single Crystals of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)

To gain insight into the high-pressure polymorphism of RDX, an energetic crystal, Raman spectroscopy results were obtained for hydrostatic (up to 15 GPa) and non-hydrostatic (up to 22 GPa) compressions. Several distinct changes in the spectra were found at 4.0 +/- 0.3 GPa, confirming the alpha-gamma phase transition previously observed in polycrystalline samples. Detailed analyses of pressure-induced changes in the internal and external (lattice) modes revealed several features above 4 GPa: (i) splitting of both the A' and A' ' internal modes, (ii) a significant increase in the pressure dependence of the Raman shift for NO2 modes, and (iii) no apparent change in the number of external modes. It is proposed that the alpha-gamma phase transition leads to a rearrangement between the RDX molecules, which in turn significantly changes the intermolecular interaction experienced by the N-O bonds. Symmetry correlation analyses indicate that the gamma-polymorph may assume one of the three orthorhombic structures: D2h, C2v, or D2. On the basis of the available X-ray data, the D2h factor group is favored over the other structures, and it is proposed that gamma-phase RDX has a space group isomorphous with a point group D2h with eight molecules occupying the C1 symmetry sites, similar to the alpha-phase. It is believed that the factor group splitting can account for the observed increase in the number of modes in the gamma-phase. Spatial mapping of Raman modes in a non-hydrostatically compressed crystal up to 22 GPa revealed a large difference in mode position indicating a pressure gradient across the crystal. No apparent irreversible changes in the Raman spectra were observed under non-hydrostatic compression.

High-resolution FTIR study of the para-terphenyl phase transition at high pressure

High-resolution infrared (IR) spectroscopy has been used to investigate the pressure-induced (0–11 kbar) polymorphic phase transition of crystalline para-terphenyl at low temperature (25 K). A number of doublet bands observed in low-pressure triclinic p-terphenyl were observed to coalesce in the high-pressure monoclinic phase. The coalescing of doublet bands was attributed to changes in factor group (Davydov) splittings associated with the transition from a low-pressure triclinic phase to a high-pressure monoclinic phase. The bands that ‘disappear’ also do not correlate with frequency changes associated with changes in molecular symmetry. Molecular dynamics (MD) simulations at low temperature (20 K) yield a non-planar average molecular structure for the high-pressure monoclinic phase, in contrast to the high-temperature monoclinic phase. The MD simulations also reveal a broadening of the distribution of ring torsion angles near the triclinic–monoclinic phase transition pressure.

The unit cell expansion of branched polyethylene as detected by Raman spectroscopy: an experimental and simulation approach

Raman spectroscopy has become a commonly used technique for the study of phase structure in semicrystalline polymers [1–8]. Among them, polyethylene (PE) is one of the more deeply explored materials. However, there are still some remaining questions concerning the effect molecular and structural features in the intensity and position of characteristic Raman signals. The Raman crystallinity band appearing in linear PE at 1415 cm is thought to arise from factor group splitting, where the two PE chains packed in the orthorhombic unit cell cause the CH2 bending vibrational mode to split with a second component appearing at 1440 cm [4, 9, 10]. The other characteristic band appearing at 1460 cm has been shown to arise from superposition of rocking combination modes, which interact with the crystal mode with symmetry B3g [11]. Changes in the crystalline phase, i.e. in the unit cell parameters for example as a result of temperature changes or the inclusion of short branches, could affect this phenomenon resulting on changes in the interchain energy interactions [8, 9]. The shift in the position of the 1415 cm band towards a higher wavenumber observed as crystallinity decreases has been associated with a lower effectiveness of the interchain interaction within the lattice, and hence to lateral disorder in the crystals. Lagaron has recently found a correlation between the shift of this crystalline band and the macroscopic density for a set of PE samples with different molecular architectures [8]. However, the variation did not result in a linear correlation, probably due to the molecular heterogeneity of the selected samples. In this work we carried out a careful analysis of the orthorhombic crystalline band observed by Raman spectroscopy, i.e., the CH2 bending 1415 cm –1 mode, as a function of crystal features obtained from X-Ray scattering (WAXS) and differential scanning calorimetry (DSC), for a series of model ethylene/1-hexene copolymers, with varying comonomer content, obtained in our laboratory from single-site catalyst polymerization (see Table 1). The materials are characterized by having a narrow molecular weight distribution (Mw/Mn=2) and a homogeneous comonomer distribution. The weight average molecular weight (Mw) of the samples ranges between 120 and 300 kg mol. We will interpret the experimental shift observed in terms of the variations in the crystalline orthorhombic structure found in the materials. Moreover, we will compare the experiments with computed results calculated at the local Density Functional Theory (DFT) level. For that purpose, normal modes of vibration were obtained for the orthorhombic cells with different parameters derived from the X-ray data. The splitting of the single-chain vibrational active modes into the bands at 1415 and 1440 cm, can be observed in Fig. 1a for the linear PE sample. The separation between these two bands has been obtained for all polymers studied and listed in Table 1. The exact J. Otegui J. F. Vega (&) S. Martin V. Cruz A. Flores J. Martinez-Salazar Departamento de Fisica Macromolecular, Instituto de Estructura de la Materia, CSIC Serrano 113 bis, Madrid 28006, Spain e-mail: imtv477@iem.cfmac.csic.es

Infrared Spectrum of Poly(l-lactide): Application to Crystallinity Studies

The spectrum of PLLA is analyzed in order to investigate its crystallinity and crystalline morphologies. The carbonyl and ester bands of PLLA have been analyzed, and individual components have been successfully assigned. Nucleation always proceeds through curved lamellar crystals, this crystalline morphology being exclusive for low crystallization temperatures. At higher crystallization temperatures, a transition from curved crystals to flat lozenge-shaped lamellae is observed. Curved crystals with edge-on orientation and flat crystals with flat-on orientation affect the intensity of spectral bands. The total crystallinity has been obtained from a skeletal band at 955 cm-1. In addition, intensity changes observed in the CO stretching region during crystallization provide a simple procedure to obtain the relative population of the two crystalline morphologies. As crystallization temperature increases, the relative population of curved edge-on crystals is observed to decrease, but their population remains important even at the higher crystallization temperatures. The CO stretching region shows a complex profile that can be fully explained assuming intramolecular through bond coupling and factor group splitting. The latter is also affected by crystalline perfection; hence, the observed crystalline components strongly depend on the crystallization temperature. In the CO stretching region, perfectly flat crystals give two narrow components at 1767 and 1758 cm-1. Curved crystals obtained at low crystallization temperatures give a broader band located at 1760 cm-1 attributed to factor group splitting averaged over the different curvatures shown by this crystalline morphology. This contribution is expected to depend on crystallization temperature according to theoretical considerations (larger nuclei sizes). DSC melting shows a shoulder at lower temperature attributed to the presence of the less stable edge-on crystalline morphology. Finally, the ester C−O stretching region also shows factor group splitting in both the perpendicular (split ∼ 10 cm-1) and parallel (split ∼ 18 cm-1) components.

Vibrational spectroscopic investigation of structurally-related LiFePO 4, NaFePO 4, and FePO 4 compounds

Vibrational spectroscopy was utilized to investigate the local structure of LiFePO 4, NaFePO 4, and FePO 4. The factor group splitting of the intramolecular PO 4 3− vibrations is between 10 and 20 cm −1 less for NaFePO 4 than for LiFePO 4. This is because Li + ions have a higher charge density than Na + ions and can form stronger coordinative bonds with the PO 4 3− anions. Thus, the internal modes are more perturbed in LiFePO 4 and exhibit larger factor group splitting effects. The similarity of the factor group multiplets for both LiFePO 4 and NaFePO 4, particularly the PO 4 3− bending modes, strongly suggests that the 506 and 470 cm −1 bands of LiFePO 4 consist almost entirely of lithium translatory motion. There are marked differences between the vibrational spectrum of FePO 4 and those of LiMPO 4 (M = Mn, Fe, Co, or Ni) or NaFePO 4. The monovalent cations interact with the oxygen atoms of the phosphate groups, affecting the frequencies and intensities of the intramolecular PO 4 3− modes, in a manner that is absent in FePO 4.