Alkaline-earth Fluorohalides Research Articles

Cohesion energy simulation of inorganic layered alkaline-earth fluorohalides

Within the framework of an interionic potential model, electrostatic and repulsive energies, lattice self-potentials for distinct ions, and the Madelung constant were calculated for several technologically important layered alkaline-earth fluorohalide scintillators belonging to the matlockite family and crystallizing with the PbFCl-type structure. The Coulombic term was computed using the conventional Ewald method, where the formulas were adjusted to optimize the computer calculation, and the overlap repulsive term was computed by means of an empirical approach based on the compressible ionic theory. The dispersive contribution was quantified using well-known formulations, and the polarization contribution was determined purely by geometric considerations based on both the size and anisotropic coordination of the highly polarizable halogen atom. In general, the obtained results were found to be in close agreement with the available data, except for the contribution of short-range interactions in the lattice. A quantitative explanation has been proposed to elucidate the differences detected. It was pointed out that the structural stability of these lamellar structures can be understood in terms of the anisotropic coordination of halogen anions, especially the high coordination of metal cations combined with their sizes. Our calculations led to an accurate evaluation of the cohesive energy, which, to the best of our knowledge, has never been measured before. Finally, the corresponding results will be useful for a better understanding of the chemical bonds and structural behavior of PbFCl-type compounds at high pressures.

Investigation of structural, electronic and elastic properties of matlockite-type SrFI under high pressure: An Ab-initio study

In this study, Matlockite (PbFCl) type SrFI compound with a two-dimensional layered structure has been studied using density functional theory (DFT) and generalized gradient approximation (GGA) under high hydrostatic pressure. Calculations were made using the Siesta Method. In ambient conditions, matlockite type structures crystallize in tetragonal structure with space group P4/nmm. In nature, these structures are known as alkaline earth fluoro-halides. When gradually increasing pressures were applied to this structure, the phase transition was observed to the orthorhombic structure with space group Pmmn. In addition to these studies, some physical properties of SrFI, such as lattice parameters, bulk modulus, the derivative of the bulk modulus, and volume values were also calculated. On the other hand, total energy and enthalpy calculations were made to obtain the phase transition pressure values that are in good agreement with the experimental results. According to these calculations, phase transition pressure was predicted at about 44.8 GPa. Besides, how electronic and elastic properties of SrFI compound change with applied pressure were also examined. In the electronic structure calculations, about 5.09 eV band gap value was obtained for P4/nmm phase at 0 GPa pressure. Under increasing pressure, the 0.16 eV band gap value was calculated for the Pmmn phase of SrFI. According to these results, the stable phases obtained of the SrFI compound have semiconductor characters since the both phases have a conceivable band gap. In addition, elastic constants were calculated and two structures of SrFI were found to be mechanically stable.

Electronic and optical properties of PbFCl and PbFI monolayers using density functional theory and beyond

In the two years, the TMDs materials perform a hugely great role in that they represent other resources of getting a material with bandgap energy and stable dynamically for electronics purposes. Many subjects have confirmed that TMDs mono or bi-layer could offer properties superior to those of graphene. Astonishingly, few of them have synthesized. Based on the density functional theory and beyond, we study the electronic, vibrational, and optical properties of new 2D materials, i.e., the alkaline-earth fluoro-halides monolayers such as PbFCl and PbFI. By computing the phonon spectra of PbFCl and PbFI monolayers, we have shown that they are dynamically stable. We obtained the bandgaps of these monolayers as achieved with the GW approximation are 6.38 and 6.64 eV. Also, we found that the strength of the electric field decreases the bandgap energy of our compounds and modifies the location of CBM and VBM. Further, we obtained by employing the picture marks seen in the calculated STM images and their accompanying geometries as creating slabs it is probable to build these 2D monolayers.

Multicolor Emission in Chemically and Structurally Tunable Er:Yb:SrFX (X = Cl, Br) Upconverting Nanocrystals

The synthesis, structural analysis, and NIR-to-visible upconversion luminescence of color-tunable Er:Yb:SrFX (X = Cl, Br) nanocrystals are reported. Green, orange, and red-emitting upconverters were achieved by employing chemically and structurally tunable alkaline-earth fluorohalides as hosts for Er3+–Yb3+ activator–sensitizer pairs. Solution-phase thermolysis of metal trifluoroacetates and of the corresponding trihaloacetic acid was utilized to synthesize Er:Yb:SrFCl and Er:Yb:SrFBr nanocrystals with average sizes of ≈15 and 50 nm, respectively. X-ray total scattering and Sr K edge absorption spectroscopy showed that a bulk-type crystallographic model provided an adequate description of the average and local structures of Er:Yb:SrFBr but failed to describe the local structure of Er:Yb:SrFCl nanocrystals, as deviations from the model were observed in the second coordination shell of strontium. Steady-state and time-resolved spectrofluorometric analyses showed that the red-to-green ratio of the upconverted emission could be compositionally tuned between 1 (green emitter) and 50 (red emitter). Single-band, red-emitting Er:Yb:SrFCl nanocrystals with Er3+ excited state lifetimes in the 100–160 μs range were thus achieved.

Infrared to-visible upconversion luminescence in Er:Yb:SrFBr nanocrystals

The ability of SrFBr nanocrystals codoped with ∼1 mol% of Er3+ and Yb3+ to perform infrared-to-visible light upconversion is demonstrated. Excitation of Yb3+ at 980 nm results in two-photon upconversion and green-yellow luminescence from Er3+, highlighting the potential of alkaline-earth fluorohalides as hosts for Yb3+–Er3+ sensitizer–activator pairs.

Mechanochemical preparation of nanocrystalline BaFCl doped with samarium in the 2+ oxidation state.

We report a facile mechanochemical preparation method for nanocrystalline BaFCl doped with samarium in the 2+ oxidation state by ball milling BaCl2, BaF2, and SmI2 under a nitrogen atmosphere. The resulting phosphors were characterized by powder X-ray diffraction; electron microscopy, X-ray photoelectron spectroscopy; and photoluminescence, photoexcitation, cathodoluminescence, and diffuse reflectance spectroscopy. This is the first report of a direct preparation method of Sm(2+) doped alkaline earth fluorohalides at room temperature and points to a significant potential for the preparation of a wide range of related X-ray storage phosphors containing rare earth ions in divalent and trivalent cationic states by mechanochemical methods.

Effects of Postannealing on the Photoluminescence Properties of Coprecipitated Nanocrystalline BaFCl:Sm3+

Nanocrystalline BaFCl:Sm(3+), as prepared by coprecipitation from aqueous solutions, is an efficient photoluminescent X-ray storage phosphor. In the present study, we report effects on its photoluminescence properties resulting from postannealing treatment in air in the temperature range between 100 to 900 °C. Interestingly, upon annealing at temperatures from 200 to 600 °C in air, a small fraction of the Sm(3+) ions in nanocrystalline BaFCl can be reduced to Sm(2+) ions. In addition to the creation of Sm(2+) ions, two different sites of Sm(3+) ions, denoted as sites A and B, are observed when the nanocrystalline BaFCl:Sm(3+) is annealed between 500 to 900 °C. The temperature dependence of photoluminescence properties of the two different sites in the 500 °C annealed sample reveals that the Sm(3+) ions at site A are possibly located at or near the crystallite surface, whereas site B is situated in a very ordered environment. To the best of our knowledge, this is the first report on the reduction of Sm(3+) ions doped in alkaline-earth fluorohalides to Sm(2+) ions by annealing in air.

Optical properties of alkaline-earth fluorohalides BaFX (X = Cl, Br, I) compounds

The ab initio calculations of the optical properties for the alkaline-earth fluorohalides BaFX (X = Cl, Br, I) compounds in its matlockite-type structure have been reported here by using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed generalized gradient approximation (GGA), which is based on exchange–correlation energy optimization to calculate the total energy. Also we have used the Engel–Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our calculations show that these compounds have a direct energy band gap (Γ–Γ). We have calculated the frequency-dependent complex dielectric function ε( ω) and its zero-frequency limit ε 1(0). We find that the values of ε 1(0) increases with decreasing the energy gap. The reflectivity spectra and absorption coefficient have been calculated and compared with the available experimental data. The optical properties are analyzed and the origin of some of the peaks in the spectra is discussed in terms of the calculated electronic structure. A considerable anisotropy is found between the parallel and perpendicular components of the frequency-dependent optical properties.

The mechanism of photobleaching in Sm 2+-doped alkaline-earth fluorohalides

The fluorescence intensity of divalent samarium (Sm 2+) ions doped in alkaline-earth fluorohalides were found to decrease under the optical excitation from a blue laser, a green laser or even a red laser. This photobleaching effect has a mysterious and unknown mechanism. After the bleaching effect happened, no Sm 3+ ions generation, no relative spectral change in Sm 2+ emissions and no lifetime decrease for the levels of Sm 2+ ions could be observed. Under blue- or green-laser irradiation, the decrease of Sm 2+ fluorescence showed a perfect biexponential function; however, when the exciting photon energy was lower than that of 5D 0→ 7F 0 transition, the photobleaching effect could not be observed anymore. An identical mechanism in the photobleaching with that in the spectral hole burning was proposed.

Electronic structure of ionic PbFCl-type compounds under pressure

The electronic structures of alkaline-earth fluoro-halides—SrFBr, SrFI, andCaFBr, which crystallize in the PbFCl-type structure—have been studied usingthe tight-binding linear muffin-tin orbital method within the local densityapproximation. The total energies were calculated using the atomic sphereapproximation and were used to determine the ground state properties ofthese systems. The calculated ground state properties agree fairly wellwith the experimental results. These systems were found to be directband gap insulators. The pressure dependence of the band gap was alsostudied. The band gap closes at high pressures leading to band overlap. Apossible reason for the metallization in these compounds is discussed.

Structural and electronic properties of alkaline-earth fluorohalides under pressure

Structural and electronic properties of alkaline-earth fluorohalides under pressure

Samarium-doped thin films of the Matlockite structure: Design, luminescence, and hole-burning experiments

The growth of thin films made from Samarium-doped alkaline earth fluoro halides (AEFH) of composition SrxCa1−xFCl:Sm2+ (0⩽x⩽1) is presented and the possibilities are studied to increase significantly the inhomogeneous width of the Sm2+ optical zero phonon transitions. The best films were obtained when grown with a molecular beam deposition (MBD) method involving two separate molecular beams: one for the alkaline earth fluoride, the other one for the alkaline earth halide (Cl or Br). The results demonstrate that the double beam MBD technique employed is able to produce pure and mixed Matlockite films with targeted composition. The results of mainly optical studies of the samarium f–f transitions and of other complementary techniques are used to assess the composition and homogeneity of the films. With the aid of a model the composition dependence of the positions of specific optical f–f emission lines is established. Their inhomogeneous linewidth is compared with that of corresponding emission lines obtained from bulk samples of the same chemical composition. The linewidths of the films are only slightly larger (∼1.5–2 times). Thus, the film morphology cannot be exploited to increase substantially the inhomogeneous broadening of the luminescence lines. A novel approach to increase this broadening was devised, theoretically modeled and successfully tested by using multilayered sandwich-type thin films in conjunction with interdiffusion. Films with cation disorder of composition SrxCa1−xFCl (x=0.5 /0/ 0.5/ 0/..) were grown. The D15→F07 Sm2+ emission linewidth is thereby increased to 70 cm−1 full width half maximum. A width of 100 cm−1 may be obtained within the composition range x=0, x=1. This represents an enhancement by a factor of 3–5 in comparison with the largest values obtained in appropriate mixed bulk AEFH of constant composition. A factor >50 is gained in comparison with pure bulk AEFH hosts. The room temperature (RT) homogeneous linewidths, on the other hand, are similar to those found in bulk mixed crystals of constant composition. The intrafilm host cation diffusion during film growth of the sandwich structures was further studied. A diffusion constant of 2⋅10−19 m2 s−1 for the Sr and Ca ions was deduced from this observation. These films are among the most promising materials for optical mass data storage through RT hole burning.

Evidence for D-band emission from Pb 2+ in alkaline-earth fluorohalides with the PbFCl structure

The luminescence properties of Pb 2+ in alkaline-earth fluorohalides have been studied and were found to originate from two types of excited state. Next to a normal ultraviolet emission ( 3 P 0.1 → 1 S 0 ) a visible emission was found and ascribed to a transition from a higher level, probably of charge-transfer nature (D transition).

Two types of luminescence from Pb2+ in alkaline-earth fluorohalides with the PbFCl structure

The luminescence of the Pb2+ ion in the alkaline-earth fluorohalides with the PbFCl structure is reported. These systems show an ultraviolet emission at low temperatures, and a blue to yellowish emission at higher temperatures. This is ascribed to the (co-) existence of two different excited states, one typical for Pb2+, and the other probably of charge-transfer nature.

Luminescence of ytterbium(II) in alkaline earth fluorohalides

The luminescence properties of divalent ytterbium in alkaline earth fluorohalides are reported. In the Ca and Sr fluorohalides an emission is observed in the 400-nm region. The excitation spectrum of CaFBr: Yb 2+ is different from those of the others, which is ascribed to a different coordination of the metal ion in this compound. The luminescence of YbFBr is also studied. In this compound energy migration to Sm 2+ occurs. In BaFCl and BaFBr no Yb 2+ luminescence is observed. From thermoluminescence measurements it is concluded that the Ca and Sr compounds act as storage phosphors and that the storage and recombination mechanisms are similar to those of the Eu 2+-activated compositions.