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From Phase Separation to Nanocrystallization in Fluorosilicate Glasses: Structural Design of Highly Luminescent Glass-Ceramics Junjie Zhao, Ronghua Ma, Xinkai Chen, Binbin Kang, Xvsheng Qiao, Jincheng Du, Xianping Fan, Ulrich Ross, Claire Roiland, Andriy Lotnyk, Lorenz Kienle, and Xiang-Hua Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05796 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 16, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

From Phase Separation to Nanocrystallization in Fluorosilicate Glasses: Structural Design of Highly Luminescent Glass-ceramics Junjie Zhao,a Ronghua Ma,a Xinkai Chen, a Binbin Kang, a Xvsheng Qiao,a,* Jincheng Du,b,* Xianping Fan a, Ulrich Ross, c Claire Roiland, d Andriy Lotnyk c, Lorenz Kienle e

a

and Xianghua Zhang d

State Key Laboratory of Silicon Materials & School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

b

Department of Materials Science and Engineering, University of North Texas, Denton, Texas 76203-5017, U.S.

c

Leibniz Institute of Surface Modification (IOM,) Permoserstr. 15, D-04318 Leipzig, Germany d

Institute for Material Science, Synthesis and Real Structure, Christian Albrechts University Kiel, Kaiserstr. 2, 24143 Kiel, Germany

e

Laboratory of Glasses and Ceramics, Institute of Chemical Science UMR CNRS 6226, University of Rennes 1, 35042 Rennes, France

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ACS Paragon Plus Environment


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ABSTACT: Tremendous enhancement of optical emission efficiency was achieved in fluorosilicate glasses by growing lanthanide doped fluoride nano-crystals embedded in oxide glass matrix. The formation mechanism of the microstructure were elucidated by combining solid state NMR, scanning TEM, EDX map and large scale molecular dynamics simulations. The results reveal that the growth of fluoride nano-crystals in fluoroslicacte glass was originated from fluoride phase separation. Atomic level structures of phase separation of fluoride rich regions in oxyfluoride glasses matrix were observed from both EDX maps and MD simulations and it was found that, while silicon exclusively coordinated by oxygen and alkali earth ions and lanthanide mainly coordinated by fluorine, aluminum played the role of linking the two fluoride glass and oxide glass regions by bonding to both oxygen and fluoride ions.

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1.

INTRODUCTION

Rare earth doped fluoride nano-crystals have low phonon energy and superb optical emission efficiency and, when embedded in an oxide glass matrix that provides high optical transparency and chemical/thermal stability, they become an optimum system for luminescence and laser applications.1,

2

Oxyfluoride

silicate and aluminosilicate glass-ceramics are such kind of material prepared by carefully annealing precursor fluorosilicate glasses, in which large quantities of single phase fluoride nano-crystals are homogenously precipitated in the glass matrix while maintaining their optical transparency. In addition, lanthanide ions can be enriched in the fluoride nano-crystals to achieve high luminescence efficiencies. Various types of fluoride nano-crystals have been successfully crystallized in fluorosilicate glass-ceramics, including MF23-5, RF36-8, M2RF79-11 and AMF412, where M is alkaline earth metal (e.g. Ca, Sr and Ba), R is rare earth metal (such as Y, La and Gd), and A is alkali metal (such as Li, Na and K). Despite wide interests and research on this new type of functional nanomaterials, the fundamental understanding of the detailed process of nano-crystal formation is still very limited and, as a result, the design and processing of these glass-ceramics rely on a trial-and-error approach. In this paper, we aim to combine advanced characterizations techniques such as solids state 19F nuclear magnetic resonance (NMR) and high resolution TEM, together with large scale molecular dynamics (MD) simulations that employ efficient partial charge potentials in order to provide insights into intriguing

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crystallization process by focusing on two series of glass compositions: 50SiO2-(22-x)Al2O3-(28+x)BaF2 and 50SiO2-20Al2O3-(30-x)BaO-xBaF2with gradually increasing fluoride concentrations.The barium based aluminosilicate oxyfloride glasses were chosenin this study due to theirwider glass forming range and lower melting temperature as compared to other alkali earth containing glasses. These characteristics ensure rooms of fine-tuning of glass compositions for suitable glass-ceramic formations. The two glass series were selected to gain insight on alumina/barium fluoride and barium oxide/barium fluoride substitution on the structure and nanocrystalline formation.

2.

EXPERIMENTAL AND COMPUTATIONAL DETAILS

The glass samples listed in Table 1 and Table 2 were prepared by melting batches of 50 g raw materials (in the forms of SiO2, Al2O3, BaF2, BaCO3, ErF3 and EuF3) at 1450 °C for 45 min in partially covered alumina crucibles in air. Then, the melts were cast onto a copper plate and pressed to form a glass sheet with another copper plate. In order to study crystallization behaviours of the investigated glasses, differential Thermal Analyse (DTA) of the glasses were carried out on a CDR-1 Differential Thermal Analyser. The crystallization peak temperatures were then determined as shown in Table 1, and the glasses were subsequently anealed at the crystallization temperatures to form the glass-ceramics containing certain crystalline phase. To identify such crystalline phase, powder X-ray diffraction (XRD) was performed on a Shimadzu

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XRD-6000 X-ray diffractometer in the 2θ-range of 10° to 80° with a 4°/min scanning speed, and selected area electron diffraction (SAED) was performed using a Tecnai F30 S-Twin microscope (FEI company) operating at 300 kV. To distinguish different phase separation phase in the glasses, energy dispersive X-ray (EDX) maps were obtained using a probe Cs-corrected Titan 3 G2 60-300 microscope operating at 300 kV accelerating voltage and a beam current of 80 pA. The microscope is equipped with a Super-X EDX detector system. EDX maps were recorded in scanning TEM (STEM) mode using fast chemical mapping and subsequent averaging of the EDX maps. A probe forming aperture of 20 mrad was used in the experiments. To study the local coordination environments, the

19

F,

27

Al and

29

Si magic angle spinning (MAS) nuclear

magnetic resonance (NMR) spectra were recorded at room temperature on an Avance 300 Bruker spectrometer. The external reference used for the chemical shift of

19

F,

27

Al and

29

Si were CFCl3, aqueous aluminum nitrate, and

tetramethylsilane (TMS), respectively. Two glasses 3 mol % Er-doped G34BaF and 3 mol % Eu-doped G34BaF were prepared via the melt-and-quench method according to the compositions (in mol %) of 50SiO2 -16Al2O3 -34BaF2 -3ErF3 and 50SiO2 -16Al2O3 -34BaF2 -3EuF3 in order to to investigate the structural effect on luminescent behaviors of the glasses. The Er-doped and the Eu-doped G34BaF glasses were annealed for 1 hours at 660 °C to form the glass-ceramics containing BaF2 nanocrystals.The Er-doped and the Eu-doped G34BaF glass and glass-ceramics

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were polished to 1 mm thick for further spectroscopic characterization. The photoluminescence (PL) spectra were measured on an Edinburgh Instruments (EI) FLSP920 spectrophotometer. The pump source of up-conversion PL spectra was a 450 mW 980 nm laser diode (LD). The excitation source of down-conversion PL spectra is a 450 W Xe lamp. All the PL measurements were performed at room temperature. The structure of the oxyfluoride glasses were simulated by using molecular dynamics (MD) simulations by using the DL-POLY 2.20 package developed at Daresbury Laboratory in the UK.13 The effective partial charge pairwise potentials were chosen to simulate the oxyfloride glasses due to their computational efficiency and applicability in wide range of silicate, aluminosilicate and oxyfloride glass systems.14-16 The potential has a long range Coulombic interaction and short range interaction in the Buckingham form V୧୨ ሺrሻ =

୯౟ ୯ౠ ௥

+ ‫ܣ‬௜௝ ∙ ݁ ି஻೔ೕ ௥ −

஼೔ೕ ௥ల

,

(1)

where qi and qj are partial atomic charges, for example -1.2 for oxygen, -0.6 for fluorine, 2.4 for silicon, 1.8 for aluminium, and 1.2 for barium, Aij, Bij and Cij are parameters and their values have been reported earlier.14, 15, 17 Correction of the Buckingham potential was introduced at short distances to avoid unphysical fusion of atoms: V୧୨′ ሺrሻ = ‫ܣ‬௜௝∙ ‫ ݎ‬௡ + ‫ܥ‬௜௝ ∙ ‫ ݎ‬ଶ .

(2)

Details of the correction term can be found in references.17, 18 We chose to combine canonical (NVT), microcanonical (NVE) and constant temperature and pressure (NPT)

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ensembles to generate the structure of these glasses following a simulated melt and quench process. The cutoff distance used for the short-range interaction was 8 Å. Long-range Columbic interactions were calculated using the Ewald sum method with a relative precision of 1×10-6 and a cutoff of 10 Å. Integration of motion equations was carried out using the Verlet Leapfrog algorithm with a time step of 1 femto second (fs). Due to small amount of inevitable fluoride

evaporation, e.g. SiF4 or

AlF3, the final composition of the fluorosilicate glasses always deviate slightly from the nominal composition. In order to eliminate such deviation, the initial atom positions were generated randomly in a cubic simulation box with density empirically calculated from ideal mixing (Table S1).

After initial relaxation at 0 K and zero

pressure, the temperature was gradually increased to 6000 K through 2000 and 4000K using NVT ensemble for 100, 000 steps (100 ps), which was then followed by an NVE run for another 60,000 steps at each temperature. The glass melt was gradually cooled down to 300 K at a nominal cooling rate of about 4 K/ps. As a final step of glass structure generation, a MD run with the NPT at ambient pressure at 300 K for 100000 steps was performed to allow the system to relax to equilibrium volume and subsequently an NVE run for another 60,000 steps was performed and the trajectory at the final step was used for structural analysis. The total simulation time of each glass composition is around 2.1 nanoseconds. Equilibration at the melting temperature was confirmed by observing the average movement of each atom to be beyond half of the simulation cell size and a subsequent microcanomical (NVE) run without indication of large temperature fluctuation. The Berendsen thermostat and barostat

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were used to control temperature and pressure during NVT or NPT MD simulations. MD trajectories of the final glass at 300K under NVE ensemble were recorded every 50 steps for final structural analysis. Various structural analyses methods such as radial distribution function (RDF), bond angle distribution (BAD), coordination analysis and various visualization methods were used to gain insights of the simulated glass structures.

3.

RESULTS AND DISCUSSION

3.1 Luminescence enhancement by nano-crystallization The crystallization procedure study of the 50SiO2-(22–x)Al2O3-(28+x)BaF2 (x=0; 3; 6; 9; 12 mol %) glasses (Table 1 and Figure S1-S6) shows that an increasing substitution of Al2O3 by BaF2 lower the crystallization peak temperature (Tc1 or Tc2) and shift the co-precipitated oxyfluoride phases (BaAlF5+BaAl2Si2O8) into the singly precipitated fluoride phase (BaF2 or BaAlF5).19 Excess of

BaF2 produces a loss in the transparencies of glasses

(G37BaF and G40BaF). Consequently, the “G34BaF” glass is chosen as the base glass to prepare 3 mol % Er- or Eu-doped luminescent glass-ceramics. The photoluminescence (PL) of the lanthanide (Ln, such as Er3+ or Eu2+) doped oxyfluoride glass-ceramics can be highly enhanced via selective enrichment of RE ions into the precipitated BaF2 nanocrytallites. By annealing G34BaF glass between Tg(590 °C) and Tc1 (698 °C), BaF2 gradually precipitates in the glass and Ln ions could be easily enriched into the newly formed fluoride nano-phase

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to form Ba1-xLnxF2+x. Both up- and down-conversion PL (Figure 1) are largely enhanced because the precipitated BaF2 nanocrytallites provide Ln ions an ideal luminescent environment (Ba1-xLnxF2+x) to achieve extremely suppressed non-radiative rate related with low maximum lattice phonon vibration energy (ħωmax), or it stabilize Ln ions

at lower valence (such as Eu2+) to present

broad band emission with high quantum efficiencies. For example, 3 mol % Er3+-doped G34BaF glass-ceramics display visible up-conversion PL more than 200 times stronger than that of the precursor glass (Figure 1(a)), and the ultraviolet (UV) excited down-conversion PL of 3 mol % Eu2+-doped G34BaF glass-ceramics is also enhanced by factor of 6 (Figure 1(b)). The up-conversion PL is originated from Er3+: 2H11/2→4I15/2 (522 nm), 4S3/2→4I15/2 (545 nm) and 4

F9/2→4I15/2 (665 nm) transitions under 980 nm laser diode (LD) excitation,

primarily depending on the multi-photon mechanisms of excited state absorption (ESA) or energy transfer (ET) between two excited ions, thus it in favour of lower

is

ħωmax of BaF2 (~346 cm-1) than that of Si-O (1100 cm-1) in

glass matrix to acquire high quantum yields (QY). In contrast, the down-conversion PL is assigned to Eu2+: 4f65d1→4f7(8S7/2) transition and mainly

promoted

by

the

defect

reaction

and

reduction

of

୆ୟ୊మ

2+ ′× × EuFଷ ሱۛۛሮ Eu∙୆ୟ + F୧′ + 2F୊× → Eu× in ୆ୟ + F୧ + 2F୊ to generate more Eu

quantity. Here Eu is introduced by EuF3 and is easily reduced into Eu2+ by occupying Ba site.20

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3.2 Fluoride phase separation in the glasses TEM (Figure 2) evidences that fluorine-rich phases are segregating in the G34BaF glass. Further annealing induces a self-arranging into an ordered lattice and growth into BaF2 nano-crystals with typical dimensions around 60 nm. The glass state of G34BaF was confirmed by XRD and SAED patterns and STEM observation, proving the absence of any characteristic for crystalline phases (Figure 2(a-b)). However, the EDX maps (Figure 2(c) and Figure S7-S9) reveal that the composing elements Ba, F, O are distributed somewhat inhomogeneously. Barium (red) and fluorine (green) are segregating in the same

domains

while

oxygen

(blue)

is

dispersed

in-between.

Such

nano-segregation of phases is confined to areas of several nano-meters at most. Thus, fluoride rich nano-phase has been separated from the glass. After annealing at Tc1, the glass (G34BaF) transforms into the glass-ceramics (GC34BaF) containing the cubic BaF2 crystalline phase. XRD and SAED patterns (Figure 2(a-d)) show sharp Bragg intensities and relatively pronounced poly-crystalline electron diffraction rings

emerging on an amorphous

background. The high angle annular dark field (HAADF) STEM image (Figure 2(b-e)) shows

bright contrast for the BaF2 nano-crystals sized around 60 nm

and embedded in the darker glass host (Figure 2(d)). The precipitated BaF2 nano-crystalline phase is well identified by matching XRD peaks with JCPDS card No. 04-0452. EDX maps of Ba, O and F (Figure 2(f)), as well as Si and Al (Figure S10-S11), further reveal the elemental distribution differences between

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the glassy phase and the crystalline phase. The nano-crystalline region includes much higher content of Ba and F, unlike the glassy region which contains more Si and O. This verifies the BaF2 crystalline nature of the precipitated phase. Thus, the separated BaF2-rich glass nano-phases in the precursor glass practically act as nucleation centres of the BaF2 nano-crystals in the glass-ceramics. The observed coordinations of fluorine around barium, and the formation of the Al-F-Ba and Al-F-Al linkages, as well as six-coordinated Al3+, from nuclear magnetic resonance (NMR) spectra (Figure 3) are typical features of fluoride separated phases that are devitrified to BaF2 or BaAlF5crystals in the oxyfluoride glasses.21-25In these glasses, the coordination and linkage ranges are n≤ 4 for F-Ba(n) and n≤ 3 for Al-F-Ba(n), where n stands for coordination number (CN). In crystals n has a fixed value but in glasses n is usually a range due to the amorphous nature. On

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F magic angle spinning (MAS) NMR

spectra (Figure 3(a)), the resonance at 10 ppm, -120 ppm and

-139 ppm

chemical shift are assigned to F-Ba(n), Al-F-Ba(n) and Al-F-Al coordination, respectively.

21

On

27

Al MAS NMR spectra (Figure 3(a)), the resonance of

4-coordinated Al(IV), 5-coordinated Al(V) and 6-coordinated Al(VI) are at 54 ppm, 25 ppm and -1 ppm, respectively.22 Only one resonance band appears on 29

Si MAS NMR spectra (Figure 3(c)) and corresponds to Si-O tetrahedral

Q4(3Al) species at -90 ppm.22,

23

At 28 mol % BaF2, most F- ions are

coordinated with Al3+ or Ba2+ to form Al-F-Al, minority to form Al-F-Ba(n)

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coordinations and rarely form F-Ba(n) coordinations. Meanwhile, Al3+ ions are primarily in [Al(O,F)4] tetrahedra with CN = 4, seldom in [Al(O,F)5] semi-octahedra with CN = 5 or in [Al(O,F)6] octahedra with CN = 6. In contrast, at 40 mol % BaF2, a large portion of F- ions are coordinated with Ba2+ and Al3+ to form F-Ba(n) cluster and Al-F-Ba(n) links, while the sixfold coordinations of Al(VI) increases, indicating the formation of more

[AlF6] octahedra in the

glass. But the coordination of Si4+ has no obvious change accompanied with a continuous introduction of BaF2, mainly as four bridging O2- coordinated tetrahedra and 3 next nearest neighboring Al3+ in the form of Q4(3Al). Thus, a continuous introduction of BaF2 into the 50SiO2-(22-x)Al2O3 -(28+x)BaF2/BaO (in mol %) glass system preferentially produces Al-F and Ba-F coordinations with next nearest neighboring Al3+ in majority and Ba2+ in minority, then generates coordinations of Ba-F thoroughly with next nearest neighboring Ba2+. Considering crystallization behaviors of the glasses (Table 1), the domains of Al-F-Al and Al-F-Ba(n) have potential to be crystallized as BaAlF5 (e.g., Tc1 of G28BaF and Tc2 of G40BaF), while those of F-Ba(n) maybe converted into pure BaF2 nano-crystals. Reasonably, fluoride-rich glass phase composed of Al-F-Al,

Al-F-Ba(n) and F-Ba(n) has separated from the glass as the

preliminary stage of the crystallization of BaAlF5 and BaF2, observed as yellow (red (Ba) + green (F)) colored region on the EDX map (Figure 2(c)).

3.3 MD simulation of the glass strucures

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By deploying a molecular dynamic simulation on the system of 50SiO2 -(22-x)Al2O3 -(28+x)BaF2/BaO (in mol %) (Table 2), the separated fluoride glass nano-phases are vividly observed for the first time. Pure silicate glass, G30BaO (Figure 4(a)) has a uniform matrix without any phase separations, while the fluorosilicate glasses, G30BaF and G34BaF (Figure 4(b-c)), have glass phase separation only composed of fluorides. The separated phase of G30BaF includes [AlF6] coordination octahedra and Ba2+ and F- connected by ionic bonds. In comparison, the separated phase of 34BaF is primarily formed by Ba2+ and F-. Upon the simulated glass phase separation structures, a further step can be made to predict crystallization of fluorosilicate glasses. Statistic of separated phase (Figure 4(f)) in the simulated G34BaF structure shows Ba2+ is coordinated with 8 F- by a majority and F- is coordinated with 4 Ba2+ or Al3+. That is similar to the situation in the cubic BaF2 stucture (Figure 4(i)). So we could reasonably predict that such fluoride phase separation region is able to convert into cubic BaF2 crystalline phase by an ordered arrangement of atoms under suitable thermodynamic condition, such as annealing. And it should be the first crystallization phase of the glass, because fluoride glasses generally have much lower glass transition, crystallization and melting points than silicate glasses. Similar but not the same, the fluoride separated phase (Figure 4(e)) in the simulated G30BaF structure includes a number of [AlF6] and [BaF8] coordination polyhedra. That is similar to the situation in the lattice formed by AlF3 and BaF2, such as BaAlF5 (Figure 4(h)). So we can predict that the first

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crystallization phase of G30BaF should be expressed as BamAlnF2m+3n. Additionally, the simulated structure of GBaO does not include any phase separation phase (Figure 4(d)), so its crystallization phase is most likely to be ternary BaO·Al2O3·SiO2 (e.g., BaAl2Si2O8 shown as Figure 4 (g)), other than binary BaO·SiO2(e.g., BaSiO3) or BaO·Al2O3(e.g., BaAl2O4). 26, 27 Compositional and structural features of the silicate-rich phase, as well as the separated fluoride phase, are also revealed with statistical surveys on the simulation structures of the fuorosilicate glasses. In association with an augmentation of BaF2 in the 50SiO2-(22-x)Al2O3 -(28+x)BaF2/BaO (in mol %)， the percentage of Ba-F is enlarged notably, the ratio of Al-F coordination also gets higher moderately, thus Ba-O and Al-O coordinations become less (Figure 5(a)). The extraction of Ba-F and Al-F into separated F-rich phase bring on a distinct evolution of bridging oxygen related Qn species of Al3+ (Figure 5(b)), where the gradient introduction of BaF2 leads to a decreasing quantity of QIV, a stable quantity of QIII, and an increasing quantity of QI,II. This is due to the loss of bridging O2- on Qn(Al3+) species, because F- could compete to form covalent bond with Al3+. But in Qn species of Si4+ there appear no conspicuous changes, because Si4+ is hardly coordinated with F- and insensitive to fluorine addition. Those can also be evidenced from the radial distribution function (RDF) statistic of G34BaF simulation structure (Figure 5(c)), for O2-, Si4+, Al3+ and Ba2+ could be found at the first nearest neighbor of O2-.

For F-, only Al3+ and

Ba2+ could be identified as the first nearest neighbour of F-, while Si4+ is located

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too far away (> 4 Å) from F- to form Si-F bonds. The bond angle distribution (BAD) statistic (Figure 5(d)) sketches the contours of the internal and the interfacial feature of fluoride phase separation. The linkages of Ba-F-Ba, Al-F-Ba, F-Al-O and F-Ba-O are recognized in large amounts, but Al-F-Al and F-Si-O are hardly observed.

Accordingly，BaF2 can self-aggregate, but AlF3

cannot self-aggregate, to form BaF2 or BaF2-AlF3 phase separation, surrounding which F-Al-O and F-Ba-O act as the interface between the silicate phase and the fluoride phase.

3.4 Comparison between experiments and simulation The above coordination structural units can be vividly observed within boundary regions (black dash line circled in Figure 6(a) and magnified as Figure 6(b)) between fluoride and silicate phases of G34BaF, and match well with the result of NMR (Figure 6(c-h)). Structures of F-Ba(n) have been found at the separated fluoride glass phase, where quantities of F-Ba(n) and Ba-F(n) are aggregated to each other to form pure BaF2 “cluster” region, showing potential to be transformed into BaF2 crystalline phase. Structures of Al-F-Ba(n) have been found at the interface of fluoride and silicate region to connect [Al(O,F)4] tetrahedra and F-Ba(n) coordinated structures, or at separated fluoride phases to link [AlO6] octahedra and F-Ba(n) coordinated structures. Structures of Al-F-Al have been predominantly found at the separated fluoride glass phases, partially acting as

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glass former to support the fluoride matrix under certain condition. Al(IV) are predominantly distributed in the silicate phase in [Al(O,F)4] tetrahedra, while Al(VI) is mainly in the fluoride phase in [AlF6] octahedra. A minority of Al(IV) and Al(VI) can be found as [AlO3F] tetrahedra and [AlO5F] octahedra at the interface region to connect fluoride and silicate phases. And Al(V) appears as [AlO3F2] or [AlO2F3] coordination polyhedra also at the interface region to connect fluoride and silicate phases. The resonance band on

29

Si NMR spectra

can be assigned to Q4(3Al) specie. It indicates that the silicate matrix is mainly constructed by the interconnection of [SiO4], [AlO4] and/or [AlO3F] tetrahedra by sharing bridging oxygens.

3.5 Design of glass-ceramics containing fluoride nano-crystals The MD simulations and comparative experiments also allow us to achieve the detailed composition-structure relationship to design glass-ceramics containing fluoride nano-crystals. It is verified that the system of BaF2 ≥ 34 mol% could produce fluoride glass separation formed by aggregation of BaF2 and AlF3. The maximum ratios of Ba-F and Al-F coordination even reach about 75% and 25%, respectively. Consequently, fluoride phases become a majority of Ba2+, Al3+ and F-. However, investigation on crystallization procedure shows only the glass of 34 mol % BaF2 could be converted into transparent glass-ceramics singly containing BaF2 nano-crystals via heat treatment at the first crystallization temperature (Tc1). Less BaF2 leads to the co-crystallization of

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BaAlF5 and BaAl2Si2O8, and more BaF2 make the glass-ceramics loss transparency. Thus, BaF2 and AlF3 should have played different role and should have critical concentration to ensure the glass-ceramics to singly contain fluoride nano-crystals and simultaneously keep transparency. Ultimately, the dissection of the glass of 34 mol % BaF2 provide several rules to design a precursor

glass

in

order

to

get

glass-ceramics

containing

fluoride

nano-crystalline phase: (1) The fluosilicate glass should be primarily considered as an immiscible system at microscopic scale constructed by both silicate glassy nano-phases and fluoride glassy nano-phases. The amorphous nature of the separated fluoride nano-phases has been doubly proved by experimental results (Figure 2(a-c)) and MD simulation (Figure 4(b-c)). And it has also been statistically confirmed by highly similar RDF and BAD with the pure silicate and fluoride glasses (Figure 4(c-d)). 28-31 (2) Network former (such as Be2+)

32, 33

of bulk fluoride glass is not necessary

for the separated fluoride glassy nano-phase, but is helpful to keep the stability of the whole glass system. This is firstly because the formation of fluoride glass depend more on the multi-component effect rather than network former; secondly, nano-size fluoride glass formation is very different from normal situations. In this case, both Ba2+ and Al3+ act not as network former but as modifier and intermediator of fluoride glass, respectively.

17



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(3) Network intermediator (such as Al3+, Zn2+ and Pb2+) of bulk fluoride glass plays significant roles to link fluoride and silicate glassy phases at interface positions. Most of the similar fluorosilicate glass-ceramics previously reported 34

have high [Al3+] concentration, because they can act as network

intermediator for both silicate and fluoride glass and bridge from silicate to fluoride easily. In some cases, the introduction of intermediator could make a self-limitation to prevent uncontrollable crystallization of fluoride phase. 35, 36 (4) Network modifier (such as Na+, K+, Ca2+, Sr2+, Ba2+, Y3+ and La3+) of bulk fluoride glass should be elementary component to be firstly crystallized from the separated fluoride nano-phase. But too much modifier content (e.g. G37BaF and G40BaF) will lead to uncontrollable crystallization, so those cases usually require the introduction of more intermediator ions to stably link fluoride and silicate phase and play a self-limitation effect on the crystallization.

4.

CONCLUSION

In conclusion, introduction of fluoride in the aluminosilicate glasses leads to the formation of fluoride phase separation regions that act as precursor of nucleation for the fluoride nano-crystals as confirmed by STEM, EDX maps and MD simulations. The atomic structure details of these glasses were elucidated by

19

F,

29

Si, and

27

Al solid state NMR.

The results show that

increasing fluoride concentration leads to large structural changes of fluorine and aluminium while silicon local environments remain largely unchanged.

18


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Aluminium changes from a mainly 4-coordinated states to mixed 4-, 5- and 6-coordination with increasing fluoride content. MD simulation results support the coordination change of aluminium and silicon and found that silicon ions are mainly coordinated by oxygen. They do not participate in the fluoride rich regions and, as a result, their coordination does not change with composition. Aluminium ions, on the other hand, bond to both oxygen and fluorine. The coordination number of aluminium increases with fluoride concentration due to their existence in the fluoride region and, interestingly, aluminium ions were found enriched at the fluoride and oxide interfaces. These results suggest that the formation of fluoride glass phase separation is the precursor for the formation of fluoride nano-crystals in fluoroslicate glass. In this process, intermediator ions such as Al3+ at the interface between silicate and fluoride regions play a significant role to maintain stability and transparency of the mixed anion

glass system. This detailed structural understanding paves the

way to efficiently design and optimize the fluorosilicate glass-ceramics for various luminescent and energy applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at http://pubs.acs.org. Theoretical and experimental densities of the samples: Table S1; Crystallization procedure investigation results, including DTA and XRD

19



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data, of the samples: Figure S1-S7; HAADF STEM images, EDX maps and EDX quantitative analysis of the glass G34BaF and the glass-ceramics GC34BaF: Figure S7-S11.

AUTHOR INFORMATION *Corresponding Author. Dr. Xvsheng Qiao: E-mail: [email protected]; Tel.: +86-571-87951234. Dr. Jincheng Du: E-mail: [email protected]; Tel.: +1- 940-369-8184. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from the Program for International S&T Cooperation Projects of China (No. 2014DFB50100), Zhejiang Provincial Natural Science Foundation of China(No. LY16E020003), the Fundamental Research Funds for the Central Universities (No. 2016QNA4005; No. 2016FZA4007). J. Du acknowledges financial support from US National Science Foundation (NSF) DMR Ceramics program (No. 1508001 and 1105219). X. Qiao gratefully acknowledges Dr. UlrichSchürmann for his help on electron microscopy measurments.

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Ph.D.

Dissertation, Zhejiang University (in Chinese), Hangzhou, 2007. (20)Kaminskii, A. A. Laser Crystals; Springer: Berlin Heidelberg, Germany, 1990.

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P MAS–NMR

Spectroscopy. Journal of Non-Crystalline Solids 2005, 351, 3289-3295. (23)Pedone,

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C.

The

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Fluoride-Containing Bioactive Glasses: New Insights from First-Principles Calculations and Solid State NMR Spectroscopy. Journal of Materials Chemistry 2012, 22, 12599. (24)Youngman, R. E.; Dejneka, M. J. NMR Studies of Fluorine in Aluminosilicate-Lanthanum Fluoride Glasses and Glass-Ceramics. Journal of the American Ceramic Society 2002, 85, 1077-1082. (25)Xue, Z.; Edwards, T. G.; Sen, S. Structural Evolution During Precipitation of Alkaline-Earth Fluoride Nanocrystals in Oxyfluoride Glasses: A Multinuclear Nuclear Magnetic Resonance Spectroscopic Study. Journal of the American Ceramic Society 2011, 94, 2092-2098. (26)Da Silva, M. J.; Bartolomé, J. F.; De Aza, A. H.; Mello-Castanho, S. Glass Ceramic Sealants Belonging to BAS (BaO–Al2O3–SiO2) Ternary System Modified with B2O3 Addition: A Different Approach to Access the SOFC Seal Issue. Journal of the European Ceramic Society 2016, 36, 631-644.

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(27)Eichler,

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Otschik,

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BAS

(BaO–Al2O3–SiO2)-Glasses for High Temperature Applications. Journal of the European Ceramic Society 1999, 19, 1101-1104. (28)Christie, J. K.; Ainsworth, R. I.; de Leeuw, N. H. Ab Initio Molecular Dynamics

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Incorporation of Fluorine in Bioactive Phosphate Glasses. Biomaterials 2014, 35, 6164-71. (29)Xiang, Y.; Du, J.; Smedskjaer, M. M.; Mauro, J. C. Structure and Properties of Sodium Aluminosilicate Glasses from Molecular Dynamics Simulations. J Chem Phys 2013, 139, 044507. (30)Xiang, Y.; Du, J.; Skinner, L. B.; Benmore, C. J.; Wren, A. W.; Boyd, D. J.; Towler, M. R. Structure and Diffusion of ZnO–SrO–CaO–Na2O–SiO2 Bioactive Glasses: A Combined High Energy X-Ray Diffraction and Molecular Dynamics Simulations Study. RSC Advances 2013, 3, 5966-5978. (31)Tilocca, A.; Cormack, A. N. Surface Signatures of Bioactivity: MD Simulations of 45S and 65S Silicate Glasses. Langmuir: the ACS journal of surfaces and colloids 2010, 26, 545-551. (32)Wright, A. F.; Fitch, A. N.; Wright, A. C. The preparation and structure of the α- and β-quartz polymorphs of beryllium fluoride. Journal of Solid State Chemistry 1988, 73, 298-304. (33)Vogel, W.; Kreidl, N.; Barreto M. L. Glass Chemistry; Springer: Berlin Heidelberg, Germany, 2012.

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(34)Fedorov, P. P.; Luginina, A. A.; Popov, A. I., Transparent Oxyfluoride Glass Ceramics. Journal of Fluorine Chemistry 2015, 172, 22-50. (35)Bhattacharyya, S.; Bocker, C.; Heil, T.; Jinschek, J. R.; Hoche, T.; Russel, C.; Kohl, H. Experimental Evidence of Self-Limited Growth of Nanocrystals in Glass. Nano letters 2009, 9 (6), 2493-2496. (36)Lin, C.; Bocker, C.; Russel, C. Nanocrystallization in Oxyfluoride Glasses Controlled by Amorphous Phase Separation. Nano letters 2015, 15, 6764-6769.

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Table 1 Crystallization behaviours of the 50SiO2-(22–x)Al2O3-(28+x)BaF2 (x=0; 3; 6; 9; 12 mol %) glass.10 Crystalline phase @Tc1

Crystalline phase@Tc2

G28BaF 0 BaAlF5+BaAl2Si2O8@704 °C

—

G31BaF 3 BaAlF5+BaAl2Si2O8@700 °C

—

G34BaF 6

BaF2@698 °C

Ba5AlF13+ Ba2Al2Si2O8@785 °C

G37BaF* 9

BaF2@688 °C

Ba5AlF13@827 °C

G40BaF*12

BaF2@686 °C

Ba5AlF13@813 °C

Glass

*

x

Glass is translucent.

Table 2 Glass composition, F/O ratio, final simulation sizes and glass density. Composition (mol %)

Number of

Cell size

Density

SiO2 Al2O3 BaO

BaF2

atoms

(Å)

(g/cm3)

Glass

G30BaO 50

20

30

0

9300

51.440

3.594

G30BaF

50

20

0

30

10200

52.934

3.488

G34BaF

50

16

0

34

10800

52.994

3.538

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1 (a) Upconversion luminescence spectra of 3 mol % Er3+ doped 50SiO2 -16Al2O3 -34BaF2 glass and glass-ceramics; (b) photoluminescence (PL) excitation and emission spectra of 3 mol % Eu2+ doped 50SiO2 -16Al2O3 -34BaF2 glass and glass-ceramics. The glass-ceramics were obtained via annealing the precursor glasses at 698 °C for 1 hour.

28


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Figure 2 XRD pattern profile (a), high angle annular dark field (HAADF) STEM image (b) and EDX map of glass G34BaF(c), where the inset of (a) shows a SAED pattern of the glass; XRD pattern profile (d), HAADF STEM image (e) and EDX map of the GC34BaF glass-ceramics containing BaF2 nano-crystals, where the inset of (d) shows a SAED pattern of the glass-ceramics.

29



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(a)

(b)

Al-F-Ba(n)

40 mol% BaF2

Al-F-Al

(c)

Al(IV) Al(V) Al(VI)

F-Ba(n)

Page 30 of 41

Q4(3Al) 40 mol% BaF2

40 mol% BaF2 *

34 mol% BaF2 34 mol% BaF2

F-Ba(n)

28 mol% BaF2

28 mol% BaF2

28 mol% BaF2

F-Ba(n)

100 0 -100 -200 -300 F Chemical shift (ppm)

19

Figure 3

19

F (a),

34 mol% BaF2

27

Al (b) and

100 50 0 -50 -100 0 -60 -120 -180 29 Al Chemical shift (ppm) Si Chemical shift (ppm)

27

29

Si (c) magic angle spinning (MAS) NMR spectra of

the 50SiO2-(22–x)Al2O3-(28+x)BaF2 (x=0; 6; 12 mol %) glasses.

30


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Figure 4 Snapshots of MD simulation structure of G30BaO (a), G30BaF (b) and G34BaF (c), and the magnification of local structure of G30BaO (d), the fluoride separated phase in G30BaF (e) and G34BaF (f). The local structures (d-f) possibly covert into crystalline phases of monoclinic BaAl1.94 Si2.06O8 (g), β-BaAlF5 (h) and cubic BaF2 (i). Here the standard lattice structures are built according to ICSD files No. 27528 (monoclinic BaAl1.94 Si2.06O8), No. 80564 (β-BaAlF5) and No. 64717 (cubic BaF2). Red ball: O; Cyan: F; Yellow ball: Si; Magenta: Al; Green: Ba. Line stands for covalent bond.

31



(a)

(b)

100

Al-O Al-F

50 25 0 100

Ba-O Ba-F

75 50

4+

Qn(Si )

100 75

Percentage (%)

Percentage (%)

75

50

Q4 Q3 Q2 Q1

25 0 4+

Qn(Al )

100

25

75 50

QIV QIII QII QI

25

0

28%

31%

34%

37%

20 10 0 25 20 15 10 5 0

31%

Si-F Al-F Ba-F

34%

37%

40%

BaF2 (mol %) 0.02

Bond Angle Distribution

30

28%

(d)

Si-O Al-O Ba-O

40

0

40%

BaF2 (mol %)

(c) 50 Radial Distribution Function

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 41

Si-O-Si Si-O-Al Si-O-Ba Ba-O-Ba Al-O-Ba Al-O-Al

0.01

0.00 0.20

Al-F-Al Ba-F-Ba Al-F-Ba F-Ba-O F-Al-O

0.10

0.00 0

1

2

3

4

5

6

7

8

9 10

0

Distance (Angstrom)

30

60

90

120 150 180

Bond Angle (°)

Figure 5 (a) Percentage of coordinated Al-O and Al-F as well as Ba-O and Ba-F, and (b) percentage of Qn (n stands for the number of bridging O2- and values from 1, 2, 3 and 4)

for Si4+ and Al3+ in the simulation structure of

50SiO2·(22–x)Al2O3·(28+x)BaF2 (x=0; 3; 6; 9; 12 mol %). (c) radial distribution function of

Si-O, Al-O, Ba-O, Si-F, Al-F and Ba-F, and (d) bond angle

distribution of Si-O-Si, Si-O-Al, Si-O-Ba, Ba-O-Ba, Al-O-Ba, Al-O-Al, Al-F-Al, Ba-F-Ba, Al-F-Ba, F-Ba-O and F-Al-O, determined from the simulation structure of G34BaF.

32


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Figure 6

The MD simulation structure of G34BaF(a), where the rectangle

dash line selected region at the boundary between silicate and fluoride phases is depicted as the magnification (b). Red ball: O; Cyan: F; Yellow ball: Si; Magenta: Al; Green: Ba. Stick stands for covalent bond and dash line stands for ionic bond (partially labbelled), from which different coordination structures (c-e) are extracted and corresponded to

19

F,

27

Al and

(f-h).

33


29

Si MAS NMR bands


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Table of Contents Graphic

34


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Figure 1 (a) Upconversion luminescence spectra of 3 mol % Er3+ doped 50SiO2 -16Al2O3 -34BaF2 glass and glass-ceramics; (b) photoluminescence (PL) excitation and emission spectra of 3 mol % Eu2+ doped 50SiO2 16Al2O3 -34BaF2 glass and glass-ceramics. The glass-ceramics were obtained via annealing the precursor glasses at 698 °C for 1 hour. Figure 1 121x54mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2 XRD pattern profile (a), high angle annular dark field (HAADF) STEM image (b) and EDX map of glass G34BaF(c), where the inset of (a) shows a SAED pattern of the glass; XRD pattern profile (d), high angle annular dark field (HAADF) STEM image (e) and EDX map of the GC34BaF glass-ceramics containing BaF2 nano-crystals, where the inset of (d) shows a SAED pattern of the glass-ceramics. Figure 2 121x79mm (300 x 300 DPI)


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Figure 3

19

F (a),

27

Al (b) and

29

Si (c) magic angle spinning (MAS) NMR spectra of the 50SiO2-(22–x)Al2O3(28+x)BaF2 (x=0; 6; 12 mol %) glasses. Figure 3 46x27mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4 Snapshots of MD simulation structure of G30BaO (a), G30BaF (b) and G34BaF (c), and the magnification of local structure of G30BaO (d), the fluoride separated phase in G30BaF (e) and G34BaF (f). The local structures (d-f) possibly covert into crystalline phases of monoclinic BaAl1.94 Si2.06O8 (g), β-BaAlF5 (h) and cubic BaF2 (i). Here the standard lattice structures are built according to ICSD files No. 27528 (monoclinic BaAl1.94 Si2.06O8), No. 80564 (β-BaAlF5) and No. 64717 (cubic BaF2). Red ball: O; Cyan: F; Yellow ball: Si; Magenta: Al; Green: Ba. Line stands for covalent bond. Figure 4 99x87mm (300 x 300 DPI)


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Figure 5 (a) Percentage of coordinated Al-O and Al-F as well as Ba-O and Ba-F, and (b) percentage of Qn (n stands for the number of bridging O2- and values from 1, 2, 3 and 4) for Si4+ and Al3+ in the simulation structure of 50SiO2∙(22–x)Al2O3∙(28+x)BaF2 (x=0; 3; 6; 9; 12 mol %). (c) radial distribution function of SiO, Al-O, Ba-O, Si-F, Al-F and Ba-F, and (d) bond angle distribution of Si-O-Si, Si-O-Al, Si-O-Ba, Ba-O-Ba, AlO-Ba, Al-O-Al, Al-F-Al, Ba-F-Ba, Al-F-Ba, F-Ba-O and F-Al-O, determined from the simulation structure of G34BaF. Figure 5 98x89mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6 The MD simulation structure of G34BaF(a), where the rectangle dash line selected region at the boundary between silicate and fluoride phases is depicted as the magnification (b). Red ball: O; Cyan: F; Yellow ball: Si; Magenta: Al; Green: Ba. Stick stands for covalent bond and dash line stands for ionic bond (partially labbelled), from which different coordination structures (c-e) are extracted and corresponded to 19 F, 27Al and 29Si MAS NMR bands (f-h). Figure 6 80x72mm (300 x 300 DPI)


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Table of Contents Graphic Table of Contents Graphic 85x27mm (300 x 300 DPI)


From Phase Separation to Nanocrystallization in ... - ACS Publications

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