Lattice dynamics of MF3 crystals (M = Al, Ga, and In)

The phonon spectra, Born effective charges, and dielectric constants ε_∞ for the □AlF₃, □GaF₃, and □InF₃ crystals (where □ is a vacancy) have been calculated in terms of the generalized Gordon-Kim method. The calculated spectra of lattice vibrations contain no imaginary vibrational frequencies. This suggests the stability of the cubic phase of these compounds but contradicts the observable structural transition from cubic to rhombohedral phase. It is assumed that such a transition in the □AlF₃, □GaF₃, and □InF₃ crystals is brought about by structural defects. The calculated spectrum of lattice vibrations of the “completely defective” crystal M□F₃ (M = Al, Ga, and In) indicates a strong instability of the cubic phase. Within the mean crystal approximation, the cubic phase of M _x M _1?x F₃ crystals appears to be unstable at small x≤0.05.     

Physical properties of multicomponent fluoride glasses for photonic and superionic applications

Heavy metal fluoride glasses are promising materials for ultra-low loss mid-infrared optical fibers. The fibers are applied in remote spectroscopy, laser surgery, and thermal imaging. Upon doping with rare earth ions, heavy metal fluoride fibers are suitable for a development of high power laser materials, up-conversion lasers, and optical amplifiers for telecommunications systems. As heavy metal fluorides are prospective fast fluoride ion conductors, fluoride glasses based on ZrF₄, BaF₂, LaF₃, AlF₃ and NaF (ZBLAN), PbF₂, InF₃, BaF₂, AlF₃, LaF₃ (PIBAL) or ZnF₂, BaF₂, InF₃, SrF₂, AlF₃, NaF (ZBISAN) are interesting for a development of glassy or fibrous ionic conductors. In this paper, the ionic conductivity and dielectric response of the abovementioned multicomponent fluoride glasses is studied. The influence of the glass composition on the glass transition temperature (T_g) and on the crystallization temperature (T_cr) is also reported. The optimum composition and drawing temperature for fluoride glass fibers is specified. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Exploration of fluoride glasses with faster fluoride-ion conduction

Electrical conductivity measurements by ac methods were made of various fluoride glasses to explore glasses with faster fluoride-ion conduction. One of the measurements was made on ZrF₄BaF₂CsF glasses in which some of the fluoride-ions were substituted by the chloride, bromide, iodide and oxide ions. All of these substitutions resulted in a decrease in conductivity. The magnitude of the decrease was in proportion to the substituted fluoride-ion concentration, regardless of the substituent species. This may be explained by a blocking effect of the introduced anions of the fluoride-ion motion. Other electrical conductivity measurements were made of ternary and/or quaternary glasses of the ZrF₄⁻, HfF₄⁻, ZnF₂⁻, ScF₃⁻, MnF₂⁻ GaF₃⁻, FeF₃⁻ and InF₃⁻based fluoride systems. Remarkably high conductivities were observed in the InF₃⁻, FeF₃⁻ and GaF₃⁻based glasses containing appreciable amounts of PbF₂ as one of the glass constituents. Among these highly conductive glasses 35InF₃·30SnF₂·35PbF₂ glass exhibited extremely high conductivity, e.g., 6.3×10⁻⁴ S cm⁻¹ at 150 °C. This conductivity value is about 10² times the highest conductivity value thus far reported for fluorozirconate glasses and is higher than that of a crystalline material with fast fluoride-ion condition, β-PbF₂.     

LiF及AlF3对掺Tm3+氟磷玻璃光学、光谱性质、析晶性能及结构的影响

为了平衡掺铥氟磷玻璃对辐射寿命、发射截面和析晶稳定性的综合要求,研究了LiF及AlF₃对掺铥氟磷玻璃光学光谱性质和析晶性能的影响. 研究表明,高达30 mol%的LiF仍可保证氟磷玻璃具有较好的析晶稳定性能且其在提高Tm³⁺离子的辐射寿命上具有非常重要的作用;高AlF₃玻璃具有很好的析晶稳定性能,但Tm³⁺离子在其中的辐射寿命下降较快; 鉴于LiF和AlF₃含量的相对变化对发射截面的影响不大,因此 关键词： 氟磷玻璃 辐射寿命 发射截面 析晶稳定性     

Investigation of the Cr:LiSrAlF6 crystal by high-temperature Raman spectroscopy

In this paper, Cr-doped LiSrAlF₆ crystals are investigated using high-temperature Raman spectroscopy and the single-crystal Raman spectra of Cr:LiSrAlF₆ are analysed by factor group theory and comparison with other fluorides. The results indicate that Cr:LiSrAlF₆ is stable below its melting point; Raman peaks located at 561, 322 and 250 cm^-1 are assigned to the A_1g modes of AlF₆, SrF₆ and LiF₆ octachdra, respectively; with temperature increasing, Raman peaks associated with AlF₆ octahedra shift towards low frequencies, while LiF₆ and SrF₆ octahedra are temperature-insensitive; around the crystal melting point, three new Raman peaks occur, which are associated with the AlF₆ octahedral chain structure. Finally, the microstructural evolution of Cr:LiSrAlF₆ from room temperature to its melting point is discussed based on its Raman spectra.     

A quasi-classical trajectory study of the Cl + H<Subscript>2</Subscript> (D<Subscript>2</Subscript>) reaction on a new BW3 potential energy surface

Molecular reaction dynamics of Cl + H₂ (D₂) has been studied on the latest analytical potential energy surface called BW3 using the Monte Carlo quasi-classical trajectory method. Excitation functions, differential cross sections and angular distributions of HCl and DCl products have been calculated. The excitation functions of the Cl (²P_3/2) + n-H₂ and Cl(²P_3/2) + n-D₂ reactions are also studied. The results are compared with those of quasi-classical trajectory [M. Alagia et al.: Phys. Chem. Chem. Phys. 2 (2000); F. J. Aoiz et al.: J. Phys. Chem. 100 (1996)], quantum mechanical (QM) calculations [F. J. Aoiz et al.:J. Chem. Phys. 115 (2001)] and experimental data [S. H. Lee et al.: J. Chem. Phys. 110 (1999); F. Dong et al.: J. Chem. Phys. 115 (2001)]. Discussions are given to some new results.     

Transport numbers in the molten system NaF–KF–AlF3–Al2O3

Transport numbers in the molten system NaF–KF–AlF₃ (Al₂O₃, CaF₂) were investigated by the Hittorf method. It was confirmed that in molten cryolite, Na₃AlF₆, 1,010 °C, the current is transported almost exclusively by the Na⁺ cations (t(Na⁺)?=?0.99). When AlF₃ was added to a Na₃AlF₆ melt, the transport number of sodium cations decreased to 0.74 at the composition corresponding to NaAlF₄. In molten K₃AlF₆, the transport number of K⁺ cations equals 0.836 at 1,005 °C. In melts containing both Na⁺ and K⁺, the cations contribute to the charge transport approximately in the ratio of the squares of their ionic radii. When 5 mass % of CaF₂ was added to the molten NaF–KF–AlF₃ system, it remarkably influenced the transport numbers of potassium and fluoride anions.     

Compression mechanism of GaF3 and FeF3: a high-pressure X-ray diffraction study

The VF₃-type compounds MF₃ with M = Fe and Ga have been studied by high-pressure energy-dispersive X-ray diffraction. The compression mechanism was found to be highly anisotropic for both compounds, with the c-axis showing little pressure dependence. The volume reduction is mainly achieved through coupled rotations of the MF₆ octahedra around the c-axis, which reduces the length of the a-axis. The compression mechanism of both compounds is reasonably well described in terms of deformation of an 8/3/c2 sphere-packing model up to the pressures where the fluorine atoms become hexagonally close-packed. It is proposed that both compounds enter a phase with the fluorine atom arranged in a “super-dense” sphere packing at higher pressures. The zero pressure bulk modulus of FeF₃ and GaF₃ was determined as 12(2) and 37(3) GPa, respectively, and a scaling relation between the zero pressure bulk modulus and unit cell volumes was found for TiF₃, CrF₃, FeF₃ and GaF₃.     

Ab initio density functional study on changes in local structure in perovskite compound,LixLa1/3NbO3

Present paper describes the changes in local structure of perovskite type electrode material, Li_xLa_1/3NbO₃, by using ab initio density functional theory calculations. Although the lithium insertion leads to the elongation of Nb–O bonds due to reduction of Nb, no marked change in cell volume is indicated in the calculated results as well as experimental observation previously reported. (M. Nakayama et al. J. Phys. Chem. B 109 (2005) 14648) The distribution of bond angles of Nb–O–Nb deviated from 180° by lithium insertion. Such behaviors was ascribed to the tilt of the NbO₆ octahedron induced by lithium insertion.     

Elastic,electronic structure,and optical properties of orthorhombic Na<Subscript>3</Subscript>AlF<Subscript>6</Subscript>: a first-principles study

First-principles investigation of elastic, electronic, and optical properties of orthorhombic Na₃AlF₆ has been carried out by DFT using plane-wave pseudo-potentials within the LDA and GGA. Calculated lattice parameters agree well with experimental results. From calculated elastic constants, Na₃AlF₆ is a mechanically stable anisotropic and behaves in a ductile manner. Electronic structure analysis indicates that Na₃AlF₆ behaves as an insulator with a direct band gap of 6.065 eV in LDA and 5.868–5.949 eV in GGA. DOS, population analysis, and charge densities difference indicate that Al-F bonds are mainly ionic as well as partially covalent due to the hybridization of F-2p and Al-3s (3p) states. Moreover, the imaginary part of calculated dielectric function ε₂(ω) shows three prominent peaks due to the inter band transitions F 2p states→Na 3s states. From calculated ε (ω), other optical properties such as reflectivity and refractive index are also obtained up to the photon energy range of 40 eV.     

Infrared spectra of AlF3 in solid argon

Three infrared absorptions are assigned to AlF₃ isolated in solid argon: ν₂ = 286.2 cm^?1, ν₃ = 909.4 cm^?1, and ν₄ = 276.9 cm^?1. The multiplet pattern previously reported near 950 cm^?1 is not present in carefully isolated AlF₃. Our spectra do not conclusively rule out a pyramidal structure for AlF₃; however, no infrared absorptions can be confidently assigned to ν₁, the symmetric stretch.     

Infrared-to-ultraviolet up-conversion luminescence from AlF3: 0.2%Tm3+, 10%Yb3+ particles prepared by pulsed laser ablation

AlF₃: 0.2%Tm³⁺, 10%Yb³⁺ particles were prepared by pulsed laser ablation (PLA). Under a 978 nm laser diode excitation, intense ultraviolet and weak blue up-conversion emissions were observed in the AlF₃: 0.2%Tm³⁺, 10%Yb³⁺ particles. Intensity dependence of the up-conversion emissions on the pump power was measured. The results show that the population of the states ¹I₆, ¹D₂ and ¹G₄ may come from a five-photon, four-photon and three-photon energy transfer up-conversion process.     

Local tetragonal distortions of the (CrF6)3− and (FeF6)3− octahedral clusters in Cr3+- and Fe3+-doped tetragonal Rb2KGaF6 crystals

The local tetragonal distortions (α???α₀) (where α is the angle defined as tgα?=?R_⊥/R_//, R_⊥ and R_// are the metal–ligand distances parallel with and normal to the C₄ axis, α₀?=?45° is the same angle in cubic symmetry) of (CrF₆)^3? and (FeF₆)^3? octahedral clusters in the tetragonal Rb₂KGaF₆ crystals are estimated by analyzing their electron paramagnetic resonance (EPR) zero-field splittings D. The results indicate that the two impurity octahedra and hence the host (GaF₆)^3? octahedra are tetragonally elongated. The distortion (α???α₀) in magnitude differs from impurity to impurity because of the different sizes and natures of these impurities. These results are analogous to those in ABX₃ and doped ABX₃ perovskite crystals where the cubic-to-tetragonal phase transition is due to the rotation of BX₆ octahedra associated with the release or elongation of B–X bond along the C₄ rotational axis.     

Electronic transition dipole moment functions for transitions among the twenty-six lowest-lying states of Li2

Ab initio electronic transition dipole moment functions are calculated for all dipole-allowed transitions among the 26 states of Li₂ treated by D. D. Konowalow and J. L. Fish (Chem. Phys. 77, 435–448 (1983); Chem. Phys. 84, 463–475 (1984)). Many of the moments exhibit interesting behavior due to charge transfer or ion pair character. Comparisons are made with existing empirical and theoretical determinations of the 1¹Σ_u⁺-1¹Σ_g⁺ and 1¹Π_u-1¹Σ_g⁺ transition moments. The positions of a number of satellite bands are predicted from the potential energy difference curves.     

The potential function and rotation-vibration energy levels of the methyl radical CH3

The equilibrium bond length and the shape of the complete potential energy curve for the methyl radical CH₃ are determined. This is done by fitting the experimental data [mainly from C. Yamada, E. Hirota, and K. Kawaguchi, J. Chem. Phys.75, 5256–5264 (1981)] using the nonrigid invertor Hamiltonian and a model anharmonic potential function. As a result the v₂ (out-of-plane bending) dependence of the rotational constants is explained and the v₂ dependence of the spin-rotation coupling constants is modeled. In addition, some of the vibrational energies and rotational, centrifugal distortion, and spin-rotation constants are predicted for the ¹³CH₃, ¹²CD₃, and ¹²CT₃ isotopes.     

The ν2, ν5 Raman band system of CH3F

The anisotropic and isotropic components of the ν₂, ν₅ rotation-vibrational Raman bands of ¹³CH₃F were obtained separately. The two upper states are coupled by a strong second-order Coriolis resonance. The anisotropic spectrum was analyzed by means of a program system due to R. Escribano. A contour simulation and a least-squares fit of 233 assigned transitions yielded values for ν₅, ΔA₅, ΔA₂, and Aζ_{5a, 5b^(z)}. The ¹³C shifts of ν₂ and ν₅ were obtained from the isotropic spectrum.     

Fluoride glasses in the InF3-GaF3-YF3-PbF2-CaF2-ZnF2 system

New glasses have been synthesized in a multicomponent system based on indium fluoride. Samples of a few mm in thickness were obtained. They are transparent and homogeneous. Main physical properties such as density, characteristic temperatures, density, thermal expansion and refractive index have been measured. The evolution versus composition is reported for samples with the formula: (35−x) InF₃-xGaF₃-10YF₃-25PbF₂-15CaF₂-15ZnF₂. T_g lies between 260 and 296 °C while melting starts around 480 °C. Glass samples are stable at room temperature. By comparison with other standard fluoride glasses, they exhibit higher refractive index and density.     

Transmission of polar substituent effects across the cubane ring system: 19F substituent chemical shifts (SCS) of 4‐substituted (X) cub‐1‐yl fluorides revisited

¹⁹F NMR shieldings of 4‐substituted (X) cub‐1‐yl fluorides ( 4 ) for a set of substituents (X?H, NO₂, CN, NC, CF₃, COOH, F, Cl, HO, NH₂, CH₃, Si(CH₃)₃, Li, O^? and NH) covering a wide range of electronic substituent effects were calculated using the DFT‐GIAO theoretical model. The level of theory, B3LYP/6‐311+G(2d,p), provided ¹⁹F substituent chemical shifts (SCS) in good agreement with experimental values where known. By means of NBO analysis, various molecular parameters were obtained from the optimized geometries. Linear regression analysis was employed to explore the relationship between the calculated ¹⁹F SCS and polar field, resonance and group electronegativity substituent constants (σ_F, σ_R and σ_x, respectively) and also the NBO derived molecular parameters (fluorine natural charges (Q_n), electron occupancies on fluorine of lone pairs (n_F) and occupation number of the C? F antibonding orbital (σ_CF*)). The key determining parameters appear to be n_F and σ_CF*(occup). Both factors are a function of the electrostatic field influence of the substituent (σ_F effect) but are counteractive in their influence on the shifts. No evidence for a significant resonance effect influence on the shifts could be identified. Copyright © 2009 John Wiley & Sons, Ltd.     

Mössbauer study of various fluoride glasses containing iron fluoride

(PbF₂, ZnF₂, AlF₃, or ZrF₄) - (BaF₂ and CaF₂) - FeF₃ glasses with 20 or 30 mol%FeF₃ contents were studied by ⁵⁷Fe Mössbauer spectroscopy. The Mössbauer spectrum at room temperature for the PbF₂-based glass is composed of one doublet due to Fe³⁺, whereas those for the ZnF₂?, AlF₃?, and ZrF₄-based glasses, two doublets due to Fe³⁺ and Fe²⁺. Both the iron ions have octahedral F coordination in the high spin states. The Fe²⁺/(Fe²⁺ + Fe³⁺) value in glass increases with the basicity of base glass.     

Calculation of the Kirkwood–Frohlich correlation factor and dielectric constant of methanol using a statistical model and density functional theory

The geometries of methanol monomer and methanol clusters, (CH₃OH) _m , m = 2–10, were optimized using the DFT/B3LYP/6-31++G(d,p) method. For each m > 2, a number of conformers were found to satisfy the optimization condition, showing no imaginary frequency in their calculated IR spectra. With increasing m, five- and six-membered rings begin to appear with open chain branches and the calculated IR spectra approach the experimentally observed IR spectrum of liquid methanol. Using the average energy of formation of one hydrogen bond and a statistical model, the Kirkwood–Frohlich (K–F) correlation factor (g) and dielectric constant (ε) were calculated for each methanol cluster. From a plot of ε versus cluster size (m), the bulk dielectric constant was obtained by extrapolation to m→∞. The value of g averaged over all conformers is in almost complete agreement with the g value obtained in an earlier molecular dynamics simulation study by Fonseca and Ladanyi [J. Chem. Phys. 93, 8148 (1990)]. Using this value of g in the K–F equation, the dielectric constant (ε) of methanol was calculated and found to be in fair agreement with (～17% lower than) the experimental value and also with an earlier molecular dynamics simulation [Mol. Phys. 94, 435 (1998)]. The calculated ε follows the same trend in variation with temperature as the experimental ε in the range 288–318 K.