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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Synthesis, crystal structure and optical properties of the new lead fluoride borate-Pb{sub 2}BO{sub 3}F

A new compound, Pb{sub 2}BO{sub 3}F, has been grown by high temperature solution method from the PbO-PbF{sub 2}-B{sub 2}O{sub 3} system for the first time. The crystal structure of this compound has been identified by single crystal X-ray diffraction analysis. It crystallizes in the hexagonal system, space group P6{sub 3}/m (No. 176) with unit-cell parameters a=7.2460(3) A, c=14.5521(17) A, Z=6, V=661.69(9) A{sup 3}. Its structure was solved by the direct methods and refined to R{sub 1}=0.0163 and wR{sub 2}=0.0367. The structure of Pb{sub 2}BO{sub 3}F consists of the distorted PbO{sub 3}F{sub 2} groups and BO{sub 3} triangles, which are all symmetric with each other in the gestalt structure to the extent that the Pb{sub 2}BO{sub 3}F compound crystallizes in the symmetric space group. The powder X-ray diffraction pattern of the Pb{sub 2}BO{sub 3}F has been measured. The BO{sub 3} functional groups presented in the sample were identified by FTIR spectrum. The DTA curve of Pb{sub 2}BO{sub 3}F suggests that Pb{sub 2}BO{sub 3}F melts congruently at 448 deg. C. - Graphical Abstract: The Pb atoms are in the five coordination environments bonded to three O atoms and two F atoms to make up the distortional PbO{sub 3}F{sub 2} polyhedra. Every six PbO{sub 3}F{sub 2} polyhedra are connected by one F(1) atom to form the symmetrical structure, and then the distortion of the PbO{sub 3}F{sub 2} polyhedra is offset. Highlights: > A new compound, Pb{sub 2}BO{sub 3}F, has been grown by high temperature solution method from the PbO-PbF{sub 2}-B{sub 2}O{sub 3} system for the first time. > It crystallizes in the hexagonal system, space group P6{sub 3}/m (No. 176) with unit-cell parameters a=7.2460(3) A, c=14.5521(17) A, Z=6, V=661.69(9) A{sup 3}. > UV-vis-NIR Diffuse-Reflectance spectrum shows that its ultraviolet cutoff edge is about 288 nm. > DTA curve of Pb{sub 2}BO{sub 3}F suggests that Pb{sub 2}BO{sub 3}F melts congruently at 448 deg. C.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

High pressure phase transitions in scheelite structured fluoride: ErLiF{sub 4}

Our synchrotron based angle dispersive x-ray diffraction studies on scheelite structured ErLiF{sub 4} show that it undergoes two phase transitions, at ~11.5 and ~15.5 GPa to lower symmetry monoclinic phases, before becoming (irreversibly) amorphous at ~28 GPa. The first high pressure phase transformation to the fergusonite structure (space group I2/a) is found to be of thermodynamically second order. The second high pressure phase could be fitted to the P2/c space group, but detailed analysis rules out the wolframite structure (P2/c space group), common to many scheelite compounds under high pressures. We also suggest that despite the ionic character of the LiF{sub 4} tetrahedra, the compressibility of LnLiF{sub 4} (Ln=Eu–Lu) kind of scheelites is more affected by the LnF{sub 8} dodecahedra than the LiF{sub 4} tetrahedra. - Graphical abstract: Volume per formula unit of the scheelite and high pressure phases of ErLiF{sub 4} as a function of pressure. - Highlights: • ErLiF{sub 4} transforms to fergusonite and P2/c phase at high pressure. • Polyhedra of LnF{sub 8} affects compressibility of LnLiF{sub 4} (Ln=Eu–Lu) more than LiF{sub 4}. • Amorphization pressure varies inversely in LnLiF{sub 4} with ionic size of Ln cation. • In ErLiF{sub 4}a/c ratio reduces with pressure in contrast to reported increase in YLiF{sub 4}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Frequency non-degenerate sequential excitation of the impurity trapped exciton in strontium fluoride crystals doped with ytterbium

We model the dynamic behaviour observed for impurity-trapped excitons in SrF{sub 2}:Yb{sup 2+} using transient photoluminescence enhancement induced via a two-frequency, sequential excitation process employing an UV optical parametric amplifier synchronized to an IR free electron laser (FEL). We observe sharp transitions interpreted as a change of state of the localized hole and broad bands interpreted as a change of state of the delocalized electron. Our modeling indicates that the 4f crystal-field interaction is 25% smaller than in CaF{sub 2}. The photoluminescence enhancement transients are analyzed across a range of excitation frequencies using a system of rate equations. The temporal behavior is explained in terms of intra-excitonic relaxation, local lattice heating by the FEL, and liberation of electrons from trap states.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Atomic-scale study of the adsorption of calcium fluoride on Si(100) at low-coverage regime

We investigate, experimentally and theoretically, the initial stage of the formation of Ca/Si and Si/F structures that occurs during the adsorption of CaF{sub 2} molecules onto a bare Si(100) surface heated to 1000 K in a low-coverage regime (0.3 monolayer). A low-temperature (5 K) scanning tunneling microscope (STM) is used to observe the topographies and the electronic properties of the exposed silicon surfaces. Our atomic-scale study reveals that several chemical reactions arise during CaF{sub 2} deposition, such as dissociation of the CaF{sub 2} molecules and etching of the surface silicon dimers. The experimental and calculated STM topographies are compared using the density functional theory, and this comparison enables us to identify two types of reacted structures on the Si(100) surface. The first type of observed complex surface structure consists of large islands formed with a semiperiodic sequence of 3 x 2 unit cells. The second one is made of isolated Ca adatoms adsorbed at specific sites on the Si(100)-2 x 1 surface.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Kinetics of Separations of Volatile Fluorides from Used Fuel Using NF3

The study of the fluorination of UO2 with various fluorinating agents using thermogravimetric analysis (TGA) is well documented. Additional research has examined the fluorination of other metals and metal oxides. Seeing the possible benefit of using analogous fluorination technologies to affect separations in used nuclear fuel, we have expanded the existing research to an investigation of the reaction of NF3 with UO2 and NpO2. The conversion fractions gathered from experimental results have been compared with those predicted by existing models for various rate-limiting phenomena.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

The Role of Electrolyte Composition in Enabling Li Metal‐Iron Fluoride Full‐Cell Batteries

Abstract FeF 3 conversion cathodes, paired with Li metal, are promising for use in next‐generation secondary batteries and offer a remarkable theoretical energy density of 1947 Wh kg −1 compared to 690 Wh kg −1 for LiNi 0.5 Mn 1.5 O 4 ; however, many successful studies on FeF 3 cathodes are performed in cells with a large (>90‐fold) excess of Li that disguises the effects of tested variables on the anode and decreases the practical energy density of the battery. Herein, it is demonstrated that for full‐cell compatibility, the electrolyte must produce both a protective solid‐electrolyte interphase and cathode‐electrolyte interphase and that an electrolyte composed of 1:1.3:3 (m/m) LiFSI, 1,2‐dimethoxyethane, and 1,1,2,2‐tetrafluoroethyl‐2,2,3,3‐tetrafluoropropyl ether fulfills both these requirements. This work demonstrates the importance of verifying electrode level solutions on the full‐cell level when developing new battery chemistries and represents the first full cell demonstration of a Li/FeF 3 cell, with both limited Li and high capacity FeF 3 utilization.

25 ENERGY STORAGE↗
DOE OSTI2016

Lithium Fluoride Additives for Stable Cycling of Lithium Batteries at High Current Densities

The addition of LiF salt to a lithium battery electrolyte improves the surface diffusion of ions on the electrode surface, which leads to smooth electrodeposition. It also forms a protective layer on the electrode that prevents degradation of electrolyte by side reactions. This LiF additive can be effective in improving the capacity and lifetime of a commercial lithium metal battery.

25 ENERGY STORAGE↗
DOE OSTI2019

Uncovering Multiple Metal-Metal Bonding in a Tetranuclear Fluoride Rhenium Cluster or the Curious Case of {[Ni(H 2 O) 6 ](NH 4 ) 4 }[Re 4 F 18 ]·4H 2 O

The compound {[Ni(H 2 O) 6 ](NH 4 ) 4 }[Re 4 F 18 ]·4H 2 O (1) has been isolated and characterized by single-crystal X-ray diffraction, Raman spectroscopy and computational methods. The compound 1 crystallizes in the triclinic space group P 1 (a = 6.6620(9) Å, b = 9.7121(13) Å, c = 11.2007(15) Å, α = 67.644(2)°, β = 83.827(2)°, γ = 84.366(2)°). In 1, the [Re 4 F 18 ] 6– anion is composed of four Re(III) atoms bridged by two µ-bridging fluorine atoms and two di-µ-bridging fluorine atoms. Furthermore, in [Re 4 F 18 ] 6– , the shortest Re–Re distance (i.e., 2.3631(6) Å) is characteristic of the presence of a Re=Re double bond; the Re–F distances range from 1.854(6) to 2.008(5) Å. Theoretical calculations confirm the presence of a weak Re=Re double bond; the Mayer–Mulliken bond order of the doubly bridged Re atoms is 1.8097 and the natural localized molecular orbital bond order is 1.8751. The Raman spectrum of 1 has been measured and shows a series of bands in the region 167–1449 cm –1 that have been attributed to Re–F and Ni–O stretches, Re 4 F 18 ring breathing and NH 4 + bending and libration modes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

{sup 18}F-sodium fluoride positron emission tomography assessed microcalcifications in culprit and non-culprit human carotid plaques

Background: {sup 18}F-NaF positron emission tomography (PET) targets microcalcifications. We compared in vitro microPET assessed {sup 18}F-NaF uptake between culprit and non-culprit human carotid plaques. Furthermore, we compared {sup 18}F-NaF uptake with calcification visualized on microcomputed tomography (microCT). Methods: Carotid plaques from stroke patients undergoing surgery were incubated in {sup 18}F-NaF and scanned using a microPET and a microCT scan. The average PET assessed {sup 18}F-NaF uptake was expressed as percentage of the incubation dose per gram (%Inc/g). {sup 18}F-NaF PET volume of interest (VOI) was compared with CT calcification VOI. Results: 23 carotid plaques (17 culprit, 6 non-culprit) were included. The average {sup 18}F-NaF uptake in culprit carotid plaques was comparable with the uptake in non-culprit carotid plaques (median 2.32 %Inc/g [IQR 1.98 to 2.81] vs. median 2.35 %Inc/g [IQR 1.77 to 3.00], P = 0.916). Only a median of 10% (IQR 4 to 25) of CT calcification VOI showed increased {sup 18}F-NaF uptake, while merely a median of 35% (IQR 6 to 42) of {sup 18}F-NaF PET VOI showed calcification on CT. Conclusions: {sup 18}F-NaF PET represents a different stage in the calcification process than CT. We observed a similar PET assessed {sup 18}F-NaF uptake and pattern in culprit and non-culprit plaques of high-risk patients, indicating that this method may be of more value in early atherosclerotic stenosis development. [French] French Abstract: Contexte: La tomographie par émission de positons (TEP) au {sup 18}F-NaF marque les micro-calcifications. Nous avons comparé in vitro les plaques carotidiennes humaines responsable d’accident vasculaire cérébrales marquées au {sup 18}F-NaF avec une microTEP et leur calcification au moyen d’un microCT Méthodes: Les plaques carotidiennes de patients victimes d’un accident vasculaire cérébral qui ont bénéficié d’ une intervention chirurgicale ont été incubées dans du {sup 18}F-NaF et imagées avec une microTEP et un microCT. Le pourcentage d’absorption moyen de {sup 18}F-NaF par gr de tissu (% Inc/g) ont été comparées au volume d’intérêt (VOI) des calcifications mesurées par CT. Résultats: 23 plaques carotidiennes (17 responsables d’un AVC et 6 non responsables) ont été incluses. Le % moyen d’absorption de {sup 18}F-NaF dans les plaques carotidiennes responsables furent comparable à l’absorption dans les plaques non responsables (2,32% Inc/g vs 2,35% Inc/g , p = 0,916). Dix pourcent seulement des calcifications visualisées par CT démontrèrent une augmentation d’absorption de {sup 18}F-NaF et 35% des plaques montrant une absorption de {sup 18}F-NaF PET VOI démontrèrent des calcifications au CT. Conclusions: L’imagerie de l’absorption de {sup 18}F-NaF par tomographie à positrons représente un processus physiopathologique différent de l’imagerie des calcifications par CT. L’absorption du {sup 18}F-NaF par les plaques responsables et non responsables d’accidents vasculaire cérébraux pourrait s’avérer utile dans le diagnostic précoce des sténoses athéromateuses. [Spanish] Spanish Abstract: Antecedentes: La tomografía por emisión de positrones (PET) con {sup 18}F-floruro de sodio ({sup 18}F-NaF) demuestra microcalcificaciones. En este estudio, comparamos la captación de {sup 18}F-NaF entre placas carotideas culpables y no culpables valorada con microPET in vitro. Además, comparamos la captación de {sup 18}F-NAF con las calcificaciones detectadas por microtomografía computada (microCT). Métodos: Placas carotídeas de pacientes postoperados que sufrieron de un evento vascular cerebral fueron incubadas en 18-NaF y escaneadas usando microPET y microCT. El promedio de la captación fue expresado como porcentaje de la dosis de incubación por gramo (%Inc/gr). El volumen de interés (VOI) del PET {sup 18}F-NaF se comparó con el VOI de calcificación por CT. Resultados: Se incluyeron 23 placas carotídeas (17 culpables y 6 no culpables). El promedio de la captación de {sup 18}F-NaF en placas carotídeas culpable fue comparable al de las placas no culpables. (mediana 2.32%Inc/gr [IQR1.98-2.81] vs. mediana 2.35%Inc/gr [IQR1.77-3], p=.916). Solo una mediana de 10% (IQR 4-25) del VOI en CT mostró un aumento en la captación de {sup 18}F-NaF, mientras una mediana de 35% (IQR 6-42) del VOI de PET {sup 18}F-NaF mostró calcificación en CT. Conclusiones: La tomografía por emisión de positrones con {sup 18}F-NaF evalúa una etapa distinta en el proceso de calcificación que la CT. El patrón de captación de {sup 18}F-NaF que observamos en este estudio fue similar entre placas culpables y no culpables de pacientes de alto riesgo, lo cual indica que este método puede ser de mayor valor en el desarrollo temprano de la estenosis ateroesclerótica. [Chinese] Chinese Abstract: 背景: F18 氟化钠正电子发射断层扫描 (PET) 可用于检测微钙化。我们使用微型 PET 来评估病人体外 F18 氟化钠在颈动脉罪犯和非罪犯斑块之间的的摄取区别。此外, 我们还将 F18 氟化钠的摄取与 microCT 扫描检测的钙化进行对比。 方法: 将接受手术的中风患者的颈动脉斑块在 F18 氟化钠中培养并用 microPE 和 microCT 进行扫描。PET 对 F18 氟化钠的平均摄取率表示为每克培养剂量的百分比。F18 氟化钠 PET 中的感兴趣区域和 CT 钙化图像中的感兴趣区域 (VOI) 进行比较。 结果: 一共有 23个颈动脉斑块, 其中 17个罪犯斑块和 6个非罪犯斑块。罪犯和非罪犯颈动脉斑块的 F18 氟化钠的平均摄取率无明显差异 (中值 2.32 % Inc/g [IQR 1.98-2.81] vs. 中值2.35 % Inc/g [IQR 1.77-3.00], p = 0.916)。仅中值为10% [IQR 4-25] 的 CT 钙化 VOI 显示有F18氟化钠的摄取增加, 中值为 35%[IQR 6-42] 的 F18 氟化钠 PET VOI 显示有CT 上的钙化。 结论: 相较于 CT, F18 氟化钠 PET 展现了钙化过程中的一个不同阶段。在高危病人中, 我们观察到罪犯和非罪犯斑块具有类似的 PET F18 氟化钠摄取和模式, 表明该方法可能在早期动脉粥样硬化狭窄发展中具有更高的价值。.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2022

Separations of U/Pu and Np/Pu using fluoride volatility

Here the importance of uranium/plutonium and neptunium/plutonium separations at large scale or at environmental levels is such that the reactivity of nitrogen trifluoride (NF 3 ) with respect to other fluorinating reagents warranted investigation. We show that NF 3 does not convert PuO 2 or PuF 4 to PuF 6 , during separations of U/Pu and Np/Pu, even under forcing conditions. Herein, this is analytically demonstrated to trace level. This rather exclusive behavior has been reported for only one other reagent: BrF 3 . Confidence in this result now allows for rapid and selective separations of volatile UF 6 and NpF 6 from plutonium. Preliminary data concerning the separation of 233 Pa from 237 Np is also described.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Pulsed laser deposition of transparent fluoride glass

Here, the presence of reduced zirconium in fluorozirconate (FZ) glasses is highly unfavorable due to its detrimental effect on glass quality. Previous researchers have relied upon the use of a fluorine-containing processing gas to prevent the reduction of zirconium in FZ glasses deposited as thin films by pulsed laser deposition (PLD). However, the use of a fluorine-containing processing gas as an oxidizing agent is disadvantageous, due to its toxicity. Eliminating the need for the processing gas would lead to a significantly safer and simpler process. Our approach is to incorporate indium, which is multivalent, into the PLD ablation target in order to stabilize the zirconium and remove the need for a processing gas. A ZLANI glass, based on the composition 60.24ZrF 4 -4.13LaF 3 -3.54AlF 3 -31.49NaF-0.59InF 3 (values are in mol%), was synthesized for use as an ablation target. Using the ZLANI target, FZ glass films were successfully deposited on fused silica substrates by PLD, without the need for any processing gas. Multiple depositions were performed to observe the effects of deposition duration on growth rate and on film roughness. The deposited films are transparent with a brownish coloration. The source of this coloration is postulated to be color centers that are created during synthesis. Thermal annealing increases light transmission through the films up to 400%, depending upon wavelength, in the UV and visible spectrum (350–600 nm), after a series of heat treatments culminating at 300 °C. The fact that the transparency of the films can be correlated to annealing temperature suggests that the films may be of use as passive temperature sensors.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Thermal properties and phase transition in the fluoride, (NH{sub 4}){sub 3}SnF{sub 7}

Calorimetric, dilatometric and differential thermal analysis studies were performed on (NH{sub 4}){sub 3}SnF{sub 7} for a wide range of temperatures and pressures. Large entropy (δS{sub 0}=22 J/mol K) and elastic deformation (δ(ΔV/V){sub 0}=0.89%) jumps have proven that the Pa-3↔Pm-3m phase transition is a strong first order structural transformation. A total entropy change of ΔS{sub 0}=32.5 J/mol K is characteristic for the order–disorder phase transition, and is equal to the sum of entropy changes in the related material, (NH{sub 4}){sub 3}TiF{sub 7}, undergoing transformation between the two cubic phases through the intermediate phases. Hydrostatic pressure decreases the stability of the high temperature Pm-3m phase in (NH{sub 4}){sub 3}SnF{sub 7}, contrary to (NH{sub 4}){sub 3}TiF{sub 7}, characterised by a negative baric coefficient. The effect of experimental conditions on the chemical stability of (NH{sub 4}){sub 3}SnF{sub 7} was observed. - Graphical abstract: Strong first order structural transformation Pa-3↔Pm-3m in (NH{sub 4}){sub 3}SnF{sub 7} is associated with very large total entropy change of ΔS{sub 0}=32.5 J/mol K characteristic for the ordering processes and equal to the sum of entropy changes in the related (NH{sub 4}){sub 3}TiF{sub 7} undergoing transformation between the same two cubic phases through the intermediate phases. - Highlights: • (NH{sub 4}){sub 3}SnF{sub 7} undergoes strong first order Pa-3↔Pm-3m phase transition. • Anomalous behaviour of ΔC{sub p} and ΔV/V exists far below phase transition temperature. • Structural distortions are accompanied by huge total entropy change ΔS≈Rln50. • High pressure strongly increases the stability of Pa-3 phase in (NH{sub 4}){sub 3}SnF{sub 7}. • Entropy of the Pa-3↔Pm-3m phase transition does not depend on pressure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Effect of anodization parameters on the structural morphology of titanium in fluoride containing electrolytes

In the present work, formation of titania nanopores on titanium substrate was investigated in aqueous (sulphuric acid) and neutral organic (glycerol) electrolytes containing different concentrations of HF (0.1-0.2 M). The surface morphology, topography and phase structures were analyzed using scanning electron microscopy, atomic force microscopy and X-ray diffraction techniques, respectively. Under optimized electrolyte conditions, titania nanopore is obtained with the average pore diameter of 64 nm and 84 nm for aqueous and organic electrolytes, respectively. Atomic force microscope investigation shows that the porous layer forms under a competition of titania formation and oxide dissolution up to a limiting thickness of {approx} 75 nm and 100 nm respectively for aqueous and organic electrolytes. The porous structure was observed at 0.15 M HF concentration. It was found that depending on the electrolytes the pore diameter alters and also its concentration effected the porous structures formation. Compared to aqueous electrolytes, organic electrolytes showed larger diameter pores. From the X-ray diffraction measurement it is evident that the as prepared titania nanopores are amorphous in nature, whereas the anodized heat treated titania nanopores are in anatase phase. - Highlights: Black-Right-Pointing-Pointer Aqueous and organic electrolytes were used to synthesize nanoporous titania. Black-Right-Pointing-Pointer The surface morphology varies with various concentrations of HF. Black-Right-Pointing-Pointer Nanopores were formed at optimum HF concentration of 0.15 M. Black-Right-Pointing-Pointer Nanopore diameter was found to be affected by the electrolyte used.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Synthesis, crystal structure and properties of a new lead fluoride borate, Pb{sub 3}OBO{sub 3}F

Graphical abstract: The structure of Pb{sub 3}OBO{sub 3}F consists of two distortional Pb-centered tetrahedra and BO{sub 3} triangles which are all symmetrical with each other respectively in the gestalt structure to the extent that the Pb{sub 3}OBO{sub 3}F compound crystallizes in the symmetrical space group. Highlights: Black-Right-Pointing-Pointer Pb{sub 3}OBO{sub 3}F has been grown from PbO-PbF{sub 2}-B{sub 2}O{sub 3} system for the first time. Black-Right-Pointing-Pointer It crystallizes in the orthorhombic system, space group Pbcm. Black-Right-Pointing-Pointer Pb{sub 3}OBO{sub 3}F consists of Pb(1)O{sub 3}F tetrahedra, Pb(2)O{sub 4} tetrahedra and BO{sub 3} triangles. -- Abstract: A new compound, Pb{sub 3}OBO{sub 3}F, has been grown by the high temperature solution method from the PbO-PbF{sub 2}-B{sub 2}O{sub 3} system. It crystallizes in the orthorhombic system, space group Pbcm with unit-cell parameters a = 7.6313(14) Angstrom-Sign , b = 6.5229(12) Angstrom-Sign , c = 11.906(2) Angstrom-Sign , Z = 4, volume = 592.66(19) Angstrom-Sign {sup 3}. The structure of the compound is solved by the direct methods and refined to R{sub 1} = 0.0528 and wR{sub 2} = 0.1400. Pb{sub 3}OBO{sub 3}F consists of Pb(1)O{sub 3}F tetrahedra, Pb(2)O{sub 4} tetrahedra and BO{sub 3} triangles which build up the symmetrical chains extended along the c-axis. The powder X-ray diffraction pattern of the Pb{sub 3}OBO{sub 3}F has been measured. Functional groups presented in the sample were identified by Fourier transform infrared spectrum.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Microwave-assisted synthesis and characterization of hierarchically structured calcium fluoride

Graphical abstract: CaF{sub 2} with different morphologies and hierarchical structure such as dendrites, spherical and cube was prepared under microwave-assisted condition by a simple solvothermal synthesis route using CaCl{sub 2} and [BMIM]BF{sub 4} as initial reagents, respectively. It was found that the species of precursor was of vital importance for the formation of CaF{sub 2} crystals. That is, precursors itself also acted as structure directing agent. The outstanding features of the approach to obtain hierarchical structure CaF{sub 2} were its simplicity, effectiveness and ease of assembly. On the basis of the experimental results, a possible growth mechanism of the CaF{sub 2} crystals was proposed. Display Omitted Highlights: ► CaF{sub 2} with hierarchical structure was synthesized under microwave-assisted conditions. ► Characteristic morphologies with cube, dendrite, hierarchical sphere, etc. were observed. ► Thin cubes assembled into a spherical structure. ► Ionic liquid of [BMIM]BF{sub 4} was fluorine source. ► Possible growth mechanisms of CaF{sub 2} crystals were discussed. -- Abstract: CaF{sub 2} with different morphologies and hierarchical structures was prepared under microwave-assisted condition by a simple hydrothermal route using CaCl{sub 2} and ionic liquid [BMIM]BF{sub 4} as initial reagents, respectively. It was found that both the precursor and pH values played an important role in the formation of CaF{sub 2} crystals with different morphologies. The results suggested that the CaF{sub 2} with cube and spherical structures was obtained at different pH values (4, 8, 11), while the CaF{sub 2} with dendrite shape was formed through an oriented self-assembly growth using (NH{sub 4}){sub 2}HPO{sub 4} as a structure directing agent. Scanning electron microscope observation showed that the as-prepared CaF{sub 2} was of three-dimensional eight-horn-shaped dendritic structure. The influence of the pH values for the reaction solution and precursors has been discussed in detail. On the basis of the experimental results, possible growth mechanisms of the CaF{sub 2} microcrystals were proposed.

36 MATERIALS SCIENCE↗