Isochoric temperature coefficient of surface tension andS o-parameter of quasi-spherical molecular liquids

The Sharma parameterS _o which is characteristic of molten alkali halides and polymers has also been shown to be characteristic of a wide variety of liquids. Calculated data using the volume expansivity of the liquid establish the constancy of theS _o-parameter which retains, on an average, a constant value of 1·11 for quasi-spherical molecular liquids. It is shown thatS _o-parameter is also related to volume expansivity of the surface layer of the liquid, temperature coefficient of surface tension of liquids and describes the temperature and volume or pressure dependence of thermodynamic Grüneisen parameter and isochoric heat capacity with significant contribution for influencing the thermoacoustic and surface tension properties of liquids.     

Temperature invariance ofS 0-parameter of polymers

The parameterS ₀ which is characteristic of molten alkali halides and liquids has also been shown to be characteristic of a wide variety of polymers. Calculated data using the volume expansivity of the polymer establish the temperature invariance and constancy of theS ₀-parameter which retains, on an average, a constant value of 1.11 for polymers. Further understanding of the significance of fractional free volume andS ₀-parameter in describing various thermoacoustic properties and the anharmonic behaviour in polymers, has been developed.     

Modified grüneisen and anderson-grüneisen relations for quasispherical molecular liquids

The validity of the expression for the Grüneisen parameter of liquids has been tested by obtaining expressions for the heat capacity ratio, isothermal and adiabatic Anderson-Grüneisen parameters,C ₁-parameter, Rao’s acoustical parameter, Beyer’s non-linearity parameter, and relate them to the Grüneisen parameter. The calculated values for five liquefied gases comparising of quasi-spherical molecules are reasonably satisfactory and explain the experimental results for the variation of heat capacity ratio, Beyer’s nonlinearity parameter andC ₁-parameter with temperature for liquid state. It is shown that the isochoric temperature derivative of the sound speed, specified heat ratio and the compressibility are dominant factors with significant contribution for influencing the thermo-acoustic properties of liquids.     

Cathodo-luminescence spectra of alkali halides

Cathodo-luminescence spectra of fourteen alkali halides and of some mixed crystals of NaCl and KCl have been studied. The spectra show broad structureless bands. In some ten halides it is possible to find a strong band whose peak position shifts towards longer wavelengths as the inter-atomic distance increases. For pottasium halides the relationship λ_max=1380d, where “d” is the inter-atomic distance, holds approximately but is not generally true. A mechanism of cathodo-luminescence in alkali halides has been postulated, and an interpretation of the above band and of some NaCl bands has been made on that basis.     

Thermally stimulated depolarization of Eu2+-cation vacancy dipoles in alkali halide single crystals

Ionic thermocurrent (ITC) measurements have been performed on eight alkali halide single crystals doped with divalent europium. In all cases, the observed ITC peaks were fitted with a mono-energetic model without to appeal to any dipole-dipole interaction. Values for the reorientation parameters have been calculated. The relationship T_M1nτ^?1 α E previously found for I–V complexes in alkali halides has been found to be very well obeyed for the experimental data obtained in this investigation. It is also reported that the logarithm of the experimentally determined energies for free dipole reorientation shows a linear dependence on the interaction distance between the Eu²⁺ ion and the surrounding halogen ions in the distorted cubic site occupied by this impurity in the alkali halides.     

Lattice model calculation of hugoniot curves and the grüneisen parameter at high pressure for the alkali halides

A seven parameter shell model of the interatomic forces in the NaCl lattice is used to make a detailed lattice dynamics calculation at arbitrary volume, for fourteen alkali halides. The calculated normal mode spectrum gives an explicit vibrational contribution to the pressure and the elastic constants in the quasiharmonic approximation. The seven parameters are chosen to fit low pressure ultrasonic data and the low and high frequency dielectric constants. Prediction of the Grüneisen parameter γ, (?lnγ/?lnV), and δ_s = (?1/βB_s)(dB_s/dT) are in reasonable agreement with experiment. The calculated γ decreases monotonically with volume. Calculated Hugoniots are in good agreement with experiment for NaCl, NaBr and NaI, and in fair agreement for LiBr, LiI and NaF.     

Modes of vibrations due to (U-center + additive cation impurities) in alkali halides

Local and resonant modes due to hydride ions in various alkali halides containing additive cation impurities have been computed by the Green function technique. Local vibrations due to U-centers in alkali halides have O_h symmetry. When one of the six nearest neighbour cations is replaced by an additive impurity, the site symmetry of the system is lowered from O_h to C_4v. The phonon Green functions matríx is analysed according to the irreducible representation of the point group symmetry pertaining to the substitutional impurity. We have considered the vibrations of the hydride ion and all its six nearest neighbours. Analytical expressions have been derived for various modes of vibrations. Using group theory the 21 × 21 matrix has been block diagonalized into various irreducible representations. The effect of mass changes and the changes in short-range force constants have been taken into account. The computed results of the localized modes have been compared with the available experimental results. Good agreement has been found. Theoretical results on resonant modes are also displayed, which will be of use in future experiments on these systems.     

Analysis of thermoelastic behaviour of MgO at high temperatures and high pressures

The thermoelastic behaviour of MgO has been studied for the temperature range (300-3000 K) under different compressions down to V/V₀=0.3. It has been shown that a comprehensive study of the thermoelastic properties of MgO can be made with the help of the Anderson-Isaak equation for thermal expansivity and the Vinet equation of state taken together. We have estimated the values of thermal expansivity α, isothermal bulk modulus K_T, their variations with pressure and temperature, the Anderson-Gruneisen parameter and the change in entropy with compression for MgO along isotherms at different temperatures. The results have been discussed and compared with the corresponding values reported in the recent literature.     

Study of strain optical constants of mixed lithium halides

The theory of photo-elasticity of mixed alkali halides has been used to analyse the strain optical constants and volume derivative of the electronic dielectric constant of the mixed crystal, LiF_0.5Cl_0.5.     

Formation time of F centers in alkali halides

It is argued that in most alkali halides an appreciable fraction of the self-trapped excitons may undergo non-radiative transitions from vibrationally excited states of B_3u to the A_1g state during the relaxation of the self-trapped excitons created by ionizing radiation. Numerical calculations show that the non-radiative transition probabilities, from vibrational levels of B_3u near or above the crossing point of the A_1g and B_3u potential curves to the A_1g state, are consistent with the observed formation times of F centers in various alkali halides. The exceptional case of KI is also discussed.     

Third-order elastic constants for 2:2 chalcide crystals possessing the sodium chloride structure

Third-order elastic constants of 45 chalcide crystals having the sodium chloride structure are reported using Born-Mayer potential model. We have considered repulsive interaction up to second nearest neighbours. The temperature coefficients of the third-order elastic constants have also been computed for these crystals. As is the case for NaCl-type alkali halides we find that C₁₁₁, C₁₁₂, C₁₆₆ are negative and C₁₄₄ are positive for 2:2 chalcide crystals possessing the NaCl-type structure. We have found that a₁₂₃, a₄₅₆ and a₁₄₄ are negative whereas a₁₁₂ and a₁₆₆ are positive, once again in agreement with the situation found for the alkali halides a₁₁₁ values are positive for alkali halides whereas they are both positive and negative depending upon the interionic distance for the chalcide crystals. We have found that the nature of the variation of C⁰_αβγ with interionic separation is similar for alkali halides and for the 2:2 chalcides having the NaCl-structure. We have also computed the values of the pressure derivatives of second-order elastic constants for MgO, CaO, and SrO which agree well with the experimental values indicating the satisfactory nature of our computed data for C_αβγ.     

Charged-soft-sphere potentials for trivalent metal halides

Summary Octahedral-type coordination by halogens in the liquid state has been reported for a number of trivalent metal ions from diffraction and Raman scattering experiments on their molten trihalides and from Raman scattering spectroscopy of liquid mixtures of trihalides with alkali halides. We analyse the available data on bond lengths and Raman frequencies by treating an isolated (MX₆)³⁻ species within a model which adopts charged-soft-sphere interionic potentials supplemented by an account of ionic polarization. The trivalent metal ions that we consider are M=La, Ce, Pr, Nd, Sm, Gd, Dy and Y for X=Cl and M=Al for X=F. The main result of the analysis is the prediction of trends in the soft-sphere repulsive parameters for the trivalent metal ions, leading to estimates of all the vibrational frequencies and the binding energy of such octahedral species. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

Mechanisms of spin-lattice relaxation of vk and related centers

An experimental and theoretical investigation of the spin-lattice relaxation (SLR) mechanisms of V_K-centers in alkali and alkaline earth halides has been carried out. It has been shown that at low temperatures the main role in SLR is played by rapidly relaxing centers (RRC). In some cases, however, an intrinsic one-phonon SLR process due to modulation of both the isotropic and anisotropic hyperfine (HF) interaction seems to be important. At higher temperatures SLR can be explained by an anharmonic Raman process with the participation of the resonant molecular vibrational (RMV) mode of the V_K-center. In this SLR process, similarly to the low-temperature intrinsic process, the spin-phonon coupling is due to the modulation of the HF interaction. Deviations from the Debye phonon spectrum and the local anharmonicity of V_K-centers turn out to be important. The SLR of several other hole and interstitial type centers in alkali halides has also been investigated and their SLR mechanisms are discussed.     

Model for evaluating and predicting thermal expansivity at high temperatures for geophysical minerals

In the present communication, a new relationship for temperature dependence of thermal expansivity at high temperatures has been reported. The relation is acquired on the ground of high-temperature experimental data collected by Anderson for six geophysical minerals, viz. NaCl, KCl, MgO, CaO,Mg₂SiO₄ and Grossular Garnet. According to the present study, the thermal expansivity varies exponentially with temperature above the Debye temperature. The model projected in the present study is valuable to predict data on thermal expansivity in the high-temperature domain where experimental data have not been found so far.     

Electron spin resonance in the relaxed excited state of F-centres detected via saturated optical pumping

A fairly simple and effective method is described to detect spin resonances in the relaxed excited state (RES) of F-and similar color centres in alkali halides. It has been applied to F-centers in CsCl and CsBr, yielding g-factors of 1.77(4) and 1.54(4), respectively, for the RES.     

Evaluation of the spectroscopic constants of the modified rittner potential by the Simon-Parr-Finlan technique

A critical examination is shown of the modified Rittner potential by use of the Simon-Parr- Finlan (SPF) technique, including contributions from the polarisability of ions and the van der Waals term. We develop an equation for the SPF constants, which are related to spectroscopic constants and the polarisability of the constituent ions. Our analysis indicates excellent agreement for the dissociation energies of 20 alkali halides with available experimental data. We also cite theoretical values of the SPF coefficients Y₂₀ and Y₁₁ for the 20 alkali halides; these also compare favourably with experimental data.     

Investigation on the electric properties in molten quaternary systems by MD simulation

For the pyrochemical reprocessing of spent metallic fuels in molten salt, it is of importance to estimate the enrichment degree of Cs. The molecular dynamics simulation has been carried out on molten quaternary systems (Li, Na, K, Cs)Cl at 625K and (Li, Na, K, Cs)F at 727K for the various compositions in order to investigate the electric properties, i.e., the electric conductivity, self-diffusion coefficient, self-exchange velocity and the relative differences in the internal cation mobilities of Cs in molten LiCl-NaCl-KCl eutectic mixtures and in the FLINAK melts. These results allow us to conclude that the electric conductivities, self-diffusion coefficients and self-exchange velocities of Li⁺, Na⁺, K⁺ and Cs⁺ with reference to Cl⁻ and F⁻ have almost similar tendencies for each composition. We found it possible to enrich at up to χ_Cs = 0.38 in molten LiCl-NaCl-KCl eutectic as well as LiCl-KCl system and up to χ_Cs = 0.42 in the FLINAK melts as well as in molten FLINA system. In addition, the sequence of the simulated electric conductivity in molten quaternary alkali chloride and fluoride systems was in a fair agreement with that of the current simulated self-exchange velocities and self-diffusion coefficients.     

The elastic constants and their temperature and pressure derivatives of AgBr-AgCl mixed crystals

The three independent second-order elastic constants and their temperature and pressure derivatives have been measured for four AgBr-AgCl mixed crystals, with 19.5, 39.1, 56.6 and 78.7 mole % AgCl, using the ultrasonic pulse-echo technique at room temperature. The explicit temperature dependence of the elastic constants is calculated and is found to be much larger than that of other NaCl structure crystals. The violation of the Cauchy relation C₁₂ = C₄₄ is found to be significant and increases between AgBr and AgCl. The high temperature limit of the Gruneisen parameter is calculated from the elastic data. A comparison is made between the elastic properties of the silver halides and the alkali halides.     

Optical transitions of Ag− centers in alkali halides

Ag^? centers in alkali halides give rise to a strong absorption band in the 300 nm region (formerly called “B band”). Its resolved triplet structure in CsCl suggests that it corresponds to the C band of the isoelectronic In⁺ center. Two very weak bands are found in several alkali halides in the 400 nm region. These new bands are assigned to the A and B transitions of the In⁺-type centers. This is supported by the doublet structure in the A band, and by the temperature dependence of the oscillator strength of the B band. In KCl∶Ag^? the ratios of the oscillator strengths are found to bef _c /f _A =610 andf _c /f _B =3,400 at low temperatures. The energy parameters of Ag^? centers are computed and compared with those of others ²-type centers. The electron-lattice coupling parameters are estimated from the Jahn-Teller splitting of the C band in CsCl and of the A band in KC1. The temperature dependence of the lifetime of the visible fluorescence suggests that a metastable state is involved in the emission process after a C band excitation.     

UV absorption edge shift by doping alkali fluorides in fluoroaluminate glass

The effect on transmittance and reflectance in the vacuum ultraviolet (VUV) region by doping alkali fluorides (LiF, NaF and KF) has been investigated in a ternary fluoroaluminate (18BaF₂-37CaF₂-45AlF₃) glass. The absorption edge of the glass obeys the Urbach rule and was shifted monotonically towards higher energies by increasing the concentration of each alkali fluoride. The VUV reflection peak at 11.3 eV was not sensitive to the change of the concentration of dopants. The magnitude of the edge shift was 10-20 times larger than that expected from the additive law on the magnitude of absorption coefficients of the glass and alkali fluorides. An excitonic interaction similar to that observed in mixed crystals of alkali halides is suggested from the monotonic shift toward the absorption edges of the dopants. The weak sensitivity of the reflection peak upon doping supports that the excitonic state lies predominantly around the edge energies.