Thermal Properties of Bafx (X=CL, BR) Single Crystals: Experiments and Molecular Dynamics Simulations (original) (raw)

Time-resolved study and molecular dynamics simulation of defect centers in BaFX (X=Cl, Br) crystals

Journal of Luminescence, 2000

Some new evidence to determine the origin of the F-aggregate centers located in the near-IR region in BaFX (X"Cl, Br) crystals is demonstrated through a series of measurements and molecular dynamics simulations. Moreover, a new method to determine the concentration of the F centers in the photostimulable materials is also demonstrated. 2000 Elsevier Science B.V. All rights reserved.

Temperature-Dependent Properties of Molten Li2BeF4 Salt Using Ab Initio Molecular Dynamics

ACS Omega

Molten lithium tetrafluoroberyllate (Li 2 BeF 4) salt, also known as FLiBe, with a 2:1 mixture of LiF and BeF 2 is being proposed as a coolant and solvent in advanced nuclear reactor designs, such as the molten salt reactor or the fluoride salt cooled high-temperature reactor. We present the results on the structure and properties of FLiBe over a wide range of temperatures, 0−2000 K, from high-throughput ab initio molecular dynamics simulation using a supercell model of 504 atoms. The variations in the local structures of solid and liquid FLiBe with temperature are discussed in terms of a pair distribution function, coordination number, and bond angle distribution. The temperature-dependent electronic structure and optical and mechanical properties of FLiBe are calculated. The optical and mechanical property results are reported for the first time. The results above and below the melting temperature (∼732 K) are compared with the experimental data and with data for crystalline FLiBe. The electronic structure and interatomic bonding results are discussed in correlation with the mechanical strength. A novel concept of total bond order density (TBOD), an important quantum mechanical parameter, is used to characterize the internal cohesion and strength in the simulated models. The results show a variation in the rate of change in properties in solid and liquid phases with anomalous behavior across the melting region. The observed trend is the decrease in mechanical strength, band gap, and TBOD in a nonlinear fashion as a function of temperature. The refractive index shows a surprising minimum at 850 K, among the tested temperatures, which lies above the melting point. These findings provide a new platform to understand the interplay between the temperature-dependent structures and properties of FLiBe salt.

Synthesis and Structure of Mixed Ba12F19Cl?Br5-? Crystals

Zeitschrift f�r anorganische und allgemeine Chemie, 1996

In the structure Ba,2F19C1S [hexagonal space group P62m] the two chlorides on the sites Cl(1) and Cl(2) can partially be replaced by bromide ions. Single crystals of the type BalzF19ClsBrs-s with a chloride to bromide ratio up to 2:3 could be obtained by cooling a flux of 75molVo BaF, and 25 mol% BaX, with X = C1, Br. The crystal quality decreases with increasing bromide concentration. Structural parameters of five selected single crystals with different chloride/bromide ratio were studied by single crystal X-ray diffraction methods. The refined total Cl-/Br-population ratio in the crystals is close to the one of the flux. The lattice parameters and interatomic distances change in various ways, when the smaller chloride ion is replaced by the bigger bromide ion. The refinements show a statistical disorder on the halide sites with preferential bromide substitution on site Cl(1).

Problems in the thermal investigation of the BaF2-YF3 system

Journal of Thermal Analysis and Calorimetry, 2009

The BaF 2-YF 3 system was partially investigated, with focus given to the BaY 2 F 8 compound and its neighboring phases. In this report, various difficulties that hinder the thermal analysis investigation of this binary fluoride system are described in detail. Samples of various compositions ranging from 58 to 79% YF 3 were prepared and subjected to thermal analysis (DTA, TG and DSC) and X-ray diffraction. Diffraction patterns were analyzed through the Rietveld method for the calculation of phase concentrations in samples and determination of the lattice parameters of monoclinic BaY 2 F 8. Thermal results were compared with data from the literature and discrepancies were found.

Doping dependence of the specific heat of single crystal BaFe2(As1-xPx)2

2011

We present specific heat measurements on a series of BaFe2(As1-xPx)2 single crystals with phosphorous doping ranging from x = 0.3 to 0.55. Our results reveal that BaFe2(As1-xPx)2 follows the scaling Delta_C/Tc ~ Tc^2 remarkably well. The clean-limit nature of this material imposes new restraints on theories aimed at explaining the scaling. Furthermore, we find that the Ginzburg-Landau parameter decreases significantly with doping whereas the superconducting anisotropy is gamma~2.6, independent of doping.

Inelastic neutron scattering and lattice dynamical studies of BaFCl

Applied Physics A: Materials Science & Processing, 2002

The matlockite-structured compounds (e.g. BaFCl) on dilute doping with rare-earth ions are of technological importance for their X-ray image storage properties. The knowledge of their thermodynamic properties would be useful in designing new and improved materials. We report the measurement of the phonon density of states for BaFCl using the triple-axis spectrometer at Trombay. A rigid ion lattice dynamical model is used for the interpretation of the experimental results. The model is then exploited for the calculation of high-pressure and high-temperature thermodynamic properties of BaFCl.

Ionic layered BaFCl and Ba_{1−x}Sr_{x}FCl compounds: Physical- and chemical-pressure effects

Physical Review B, 2010

The effect on crystal structure and vibrational frequencies of physical pressure in BaFCl and chemical pressure in Ba 1−x Sr x FCl solid solutions is studied using periodic density-functional theory ͑DFT͒ calculations performed within the local-density approximation ͑LDA͒ and the generalized gradient approximation ͑GGA͒. These results are compared with previously published experimental data for BaFCl in conjunction with new experimental data for Ba 1−x Sr x FCl and show overall a good agreement with experiment. The GGA method outperforms the LDA method for the description of BaFCl under pressure. However, the two DFT methods perform equally well for the description of the solid solutions, which have been studied within the virtualcrystal approximation. They also give consistent values of the energy of formation of Ba 1−x Sr x FCl, which can be correlated with the experimentally observed melting points. The comparison of the calculated mode Grüneisen parameters shows that, for the investigated systems, the effect of the chemical pressure and that of the physical pressure are not identical.

Lattice dynamics ofBaFe2X3(X=S,Se)compounds

Physical Review B, 2015

We present the Raman scattering spectra of the BaFe2X3 (X=S, Se) compounds in a temperature range between 20 K and 400 K. Although the crystal structures of these two compounds are both orthorhombic and very similar, they are not isostructural. The unit cell of BaFe2S3 (BaFe2Se3) is base-centered Cmcm (primitive P nma) giving totaly 18 (36) modes to be observed in the Raman scattering experiment. We have detected almost all Raman active modes, predicted by factor group analysis, which can be observed from the cleavage planes of these compounds. Assignment of the observed Raman modes of BaFe2S(Se)3 is supported by the lattice dynamics calculations. The antiferromagnetic long-range spin ordering in BaFe2Se3 below TN =255 K leaves a fingerprint both in the A1g and B3g phonon mode linewidth and energy.